Section 2 Biochemical Building Blocks. Chapter 5 Amino Acids, Peptides, & Proteins.

Section 2

Biochemical Building Blocks

Chapter 5

Amino Acids, Peptides, & Proteins

Section 5.1: Amino Acids

Proteins are molecular toolsThey are a diverse and complex group of macromolecules

Figure 5.1 Protein Diversity


Proteins can be distinguished by the number, composition, and sequence of amino acid residues

Amino acid polymers of 50 or less are peptides; polymers greater than 50 are proteins or polypeptides

There are 20 standard amino acids

Section 5.1: Amino Acids 19 have the same general

structure: central (a) carbon, an amino group, carboxylate group, hydrogen atom, and an R group (proline is the exception)

At pH 7, the carboxyl group is in its conjugate base form (-COO-) while the amino group is its conjugate acid form (-NH3

+); therefore, it is amphoteric

Molecules that have both positive and negative charges on different atoms are zwitterions and have no net charge at pH 7

The R group is what gives the amino acid its unique properties

Figure 5.3 General Structure of the a-Amino Acids


Amino Acid Classes Classified by their ability to interact with water Nonpolar amino acids contain hydrocarbon

groups with no charge

Figure 5.2 The Standard Amino Acids


Amino Acid Classes Continued Polar amino acids have functional groups that

can easily interact with water through hydrogen bonding

Contain a hydroxyl group (serine, threonine, and tyrosine) or amide group (asparagine)



Amino Acid Classes ContinuedAcidic amino acids have side chains with a carboxylate group that ionizes at physiological pH

Basic amino acids bear a positive charge at physiological pH

At physiological pH, lysine is its conjugate acid (-NH3

+), arginine is permanently protonated, and histidine is a weak base, because it is only partly ionized



Biologically Active Amino Acids Amino acids can have other

biological roles1. Some amino acids or derivatives can act as chemical messengers

Neurotransmitters (tryptophan- derivative serotonin) and hormones (tyrosine-derivative thyroxine)

Figure 5.5 Some Derivatives of Amino Acids


2. Act as precursors for other molecules (nucleotides and heme)3. Metabolic intermediates (arginine, ornithine, and citrulline in the urea cycle)

Figure 5.6 Citruline and Ornithine


Modified Amino Acids in Proteins Some proteins have amino acids that are modified

after synthesis Serine, threonine, and tyrosine can be

phosphorylated g-Carboxyglutamate (prothtrombin), 4-

hydroxyproline (collagen), and 5-hydroxylysine (collagen)

Figure 5.7 Modified Amino Acid Residues Found in Polypeptides

Section 5.1: Amino AcidsAmino Acid Stereoisomers

Because the a-carbon (chiral carbon) is attached to four different groups, they can exist as stereoisomers

Except glycine, which is the only nonchiral standard amino acid The molecules are mirror

images of one another, or enantiomers

They cannot be superimposed over one another and rotate plane, polarized light in opposite directions (optical isomers)

Figure 5.8 Two Enantiomers


Molecules are designated as D or L (glyceraldehyde is the reference compound for optical isomers)

D or L is used to indicate the similarity of the arrangement of atoms around the molecule’s asymmetric carbon to the asymmetric carbon of the glyceraldehyde isomers

Chirality has a profound effect on the structure and function of proteins

Figure 5.9 D- and L-Glyceraldehyde


Titration of Amino Acids Free amino acids contain ionizable groups The ionic form depends on the pH When amino acids have no net charge due to

ionization of both groups, this is known as the isoelectric point (pI) and can be calculated using:

pK1 + pK2pI = 2

This formula only works if there is no pKR. If there is a pKR, then you will need to determine which pK values are on either side of zero net charge!


Alanine is a simple amino acid with two ionizable groups

Alanine loses two protons in a stepwise fashion upon titration with NaOH

Isoelectric point is reached with deprotonation of the carboxyl group

Figure 5.10 Titration of Two Amino Acids: Alanine


Amino acids with ionizable side chains have more complex titration curves

Glutamic acid is a good example, because it has a carboxyl side chain group

Glutamic acid has a +1 charge at low pH

Glutamic acid’s isoelectric point as base is added and the a-carboxyl group loses a proton

As more base is added, it loses protons to a final net charge of -2

Figure 5.10 Titration of Two Amino Acids: Glutamic Acid

+10

-1

-2

pK1+pKR= pKI

2


Amino Acid Reactions Amino acids, with their

carboxyl, amino, and various R groups, can undergo many chemical reactions

Peptide bond and disulfide bridge are of special interest because of the effect they have on structure

Figure 5.11 Formation of a Dipeptide


Peptide Bond Formation: polypeptides are linear polymers of amino acids linked by peptide bonds

Peptide bonds are amide linkages formed by nucleophilic acyl substitution

Dehydration reaction Linkage of two amino acids is

a dipeptide

Figure 5.11 Formation of a Dipeptide


Linus Pauling was the first to characterize the peptide bond as rigid and flat

Found that C-N bonds between two amino acids are shorter than other C-N bonds

Gives them partial double-bond characteristics (they are resonance hybrids)

Because of the rigidity, one-third of the bonds in a polypeptide backbone cannot rotate freely

Limits the number of conformational possibilitiesFigure 5.12 The

Peptide Bond


Cysteine oxidation leads to a reversible disulfide bond

A disulfide bridge forms when two cysteine residues form this bond

Helps stabilize polypeptides and proteins

Figure 5.13 Oxidation of Two Cysteine Molecules to Form Cystine

Section 5.2: Peptides

Less structurally complex than larger proteins, peptides still have biologically important functions Glutathione is a tripeptide found in most all

organisms and is involved in protein and DNA synthesis, toxic substance metabolism, and amino acid transport

Vasopressin is an antidiuretic hormone that regulates water balance, appetite, and body temperature

Oxytocin is a peptide that aids in uterine contraction and lactation

From McKee and McKee, Biochemistry, 5th Edition, © 2011 by Oxford University Press

Section 5.3: Proteins

Of all the molecules in a living organism, proteins have the most diverse set of functions: Catalysis (enzymes) Structure (cell and organismal) Movement (amoeboid movement) Defense (antibodies) Regulation (insulin is a peptide hormone) Transport (membrane transporters) Storage (ovalbumin in bird eggs) Stress Response (heat shock proteins)


Due to recent research, numerous multifunction proteins have been identified

Proteins are categorized into families based on sequence and three-dimensional shape Superfamilies are more distantly related

proteins (e.g., hemoglobin and myoglobin to neuroglobin)

Proteins are also classified by shape: globular and fibrous

Proteins can be classified by composition: simple (contain only amino acids) or conjugated

Conjugated proteins have a protein and nonprotein component (i.e., lipoprotein or glycoprotein)


Protein Structure Proteins are extraordinarily

complex; therefore, simpler images highlighting specific features are useful

Space-filling and ribbon models

Levels of protein structure are primary, secondary, tertiary, and quaternary

Figure 5.15 The Enzyme Adenylate Kinase


Primary Structure is the specific amino acid sequence of a protein

Homologous proteins share a similar sequence and arose from the same ancestor gene

When comparing amino acid sequences of a protein between species, those that are identical are invariant and presumed to be essential for function

Figure 5.16 Segments of b-chain in HbA and HbS


Figure 5.18 The a-Helix

Secondary Structure: Polypeptide secondary structure has a variety of repeating structures

Most common include the a-helix and b-pleated sheet

Both structures are stabilized by hydrogen bonding between the carbonyl and the N-H groups of the polypeptide’s backbone

The a-helix is a rigid, rod-like structure formed by a right-handed helical turn

a-Helix is stabilized by N-H hydrogen bonding with a carbonyl four amino acids away

Glycine and proline do not foster a-helical formation


Figure 5.19 b-Pleated Sheet

The b-pleated sheets form when two or more polypeptide chain segments line up, side by side


Each b strand is fully extended and stabilized by hydrogen bonding between N-H and carbonyl groups of adjacent strands

Parallel sheets are much less stable than antiparallel sheets

Figure 5.19 b-Pleated Sheet


Many proteins form supersecondary structures (motifs) with patterns of a-helix and b-sheet structures

(a) bab unit(b) b-meander(c) aa unit(d) b-barrel(e) Greek key

Figure 5.20 Selected Supersecondary Structures

Section 5.3: Proteins Tertiary Structure refers to unique three-

dimensional structures formed by globular proteins

Also prosthetic groups Protein folding is the process by which a

nascent molecule acquires a highly organized structure

Information for folding is contained within the amino acid sequence

Interactions of the side chains are stabilized by electrostatic forces

Tertiary structure has several important features1. Many polypeptides fold in a way to bring distant amino acids into close proximity2. Globular proteins are compact because of efficient packing

Section 5.3: Proteins Tertiary structure has several important features

1. Many polypeptides fold in a way to bring distant amino acids into close proximity2. Globular proteins are compact because of efficient packing3. Large globular proteins (200+ amino acids) often contain several domains

Domains are structurally independent segments that have specific functions

Core structural element of a domain is called a fold 4. A number of proteins called mosaic or modular proteins consist of repeated domains

Fibronectin has three repeated domains (F1, F2, and F3) Domain modules are coded for by genetic sequences

created by gene duplications


Figure 5.21 Selected Domains Found in Large Numbers of Proteins


Interactions that stabilize tertiary structure are hydrophobic interactions, electrostatic interactions (salt bridges), hydrogen bonds, covalent bonds, and hydration

Figure 5.23 Interactions That Maintain Tertiary Structure


Quaternary structure: a protein that is composed of several polypeptide chains (subunits)

Multisubunit proteins may be composed, at least in part, of identical subunits and are referred to as oligomers (composed of protomers)

Figure 5.25 Structure of Immunoglobulin G


Reasons for common occurrence of multisubunit proteins:

1. Synthesis of subunits may be more efficient2. In supramolecular complexes replacement of worn-out components can be handled more effectively 3. Biological function may be regulated by complex interactions of multiple subunitsFigure 5.25

Structure of Immunoglobulin G


Polypeptide subunits held together with noncovalent interactions

Covalent interactions like disulfide bridges (immunoglobulins) are less common

Other covalent interactions include desmosine and lysinonorleucine linkages

Figure 5.26 Desmosine and Lysinonorleucine linkages


Interactions between subunits are often affected by ligand binding

An example of this is allostery, which controls protein function by ligand binding

Can change protein conformation and function (allosteric transition)

Ligands triggering these transitions are effectors and modulators


Unstructured proteins: Some proteins are partially or completely unstructured

Unstructured proteins referred to as intrinsically unstructured proteins (IUPs) or natively unfolded proteins

Often these proteins are involved in searching out binding partners (i.e., KID domain of CREB)

Figure 5.27 Disordered Protein Binding

Loss of Protein Structure: Because of small differences between the free energy of folded and unfolded proteins, they are susceptible to changes in environmental factors

Disruption of protein structure is denaturation (reverse is renaturation)

Denaturation does not disrupt primary protein structure

Figure 5.28 The Anfinsen Experiment


The Folding Problem The direct relationship between a protein’s

primary sequence and its final three-dimensional conformation is among the most important assumptions in biochemistry

Painstaking work has been done to be able to predict structure by understanding the physical and chemical properties of amino acids

X-ray crystallography, NMR spectroscopy, and site-directed mutagenesis


Important advances have been made by biochemists in protein-folding research

This research led to the understanding that it is not a single pathway

A funnel shape best describes how an unfolded protein negotiates its way to a low-energy, folded state

Numerous routes and intermediates Figure 5.29 The Energy

Landscape for Protein Folding


Small polypeptides (<100 amino acids) often form with no intermediates

Larger polypeptides often require several intermediates (molten globules)

Many proteins use molecular chaperones to help with folding and targeting

Figure 5.30 Protein Folding


Molecular chaperones assist protein folding in two ways:

Preventing inappropriate protein-protein interactions

Helping folding occur rapidly and precisely

Two major classes: Hsp70s and Hsp60s (chaperonins)

Figure 5.31 Space-Filling Model of the E. Coli Chaperonin, called the GroES-GroEL Complex


Hsp70s are a family of chaperones that bind and stabilize proteins during the early stages of folding

Hsp60s (chaperonins) mediate protein folding after the protein is released by Hsp70

Increases speed and efficiency of the folding process

Both use ATP hydrolysis Both are also involved in

refolding proteins If refolding is not possible,

molecular chaperones promote protein degradation

Figure 5.32 The Molecular Chaperones


Fibrous Proteins Typically contain high

proportions of a-helices and b-pleated sheets

Often have structural rather than dynamic roles and are water insoluble

Keratin (a-helices) and silk fibroin (b-sheets)Figure 5.33 a-

Keratin


Globular Proteins Biological functions often

include precise binding of ligands

Myoglobin and hemoglobin

Both have a specialized heme prosthetic group used for reversible oxygen binding

Figure 5.36 Heme


Myoglobin: found in high concentrations in cardiac and skeletal muscle

The protein component of myoglobin, globin, is a single protein with eight a-helices

Encloses a heme [Fe2+] that has a high affinity for O2

Figure 5.37 Myoglobin


Hemoglobin is a roughly spherical protein found in red blood cells

Primary function is to transport oxygen from the lungs to tissues

HbA molecule is composed of 2 a-chains and 2 b-chains (a2b2)

2% of hemoglobin contains d- chains instead of b-chains (HbA2)

Embryonic and fetal hemoglobin have e- and g-chains that have a higher affinity for O2

Figure 5.38 The Oxygen-Binding Site of Heme Created by a Folded Globin Chain


Comparison of myoglobin and hemoglobin identified several invariant residues, most having to do with oxygen binding

Four chains of hemoglobin arranged as two identical ab dimers

Figure 5.39 Hemoglobin


Hemoglobin shows a sigmoidal oxygen dissociation curve due to cooperative binding

Binding of first O2 changes hemoglobin’s conformation making binding of additional O2 easier

Myoglobin dissociation curve is a hyperbolic simpler binding pattern

Figure 5.41 Equilibrium Curves Measure the Affinity of Hemoglobin and Myoglobin for Oxygen


Binding of ligands other than oxygen affects hemoglobin’s oxygen-binding properties

pH decrease enhances oxygen release from hemoglobin (Bohr effect)

The waste product CO2 also enhances oxygen release by increasing H+ concentration

Binding of H+ to several ionizable groups on hemoglobin shifts it to its T state


2,3-Bisphosphoglycerate (BPG) is also an important regulator of hemoglobin function

Red blood cells have a high concentration of BPG, which lowers hemoglobin’s affinity for O2

In the lungs, these processes reverse

Figure 5.42 The Effect of 2,3-Bisphosphoglycerate (BPG) on the Affinity Between Oxygen and Hemoglobin


Molecular Machines Purposeful movement is a hallmark of living

things This behavior takes a myriad of forms Biological machines are responsible for these

behaviors Usually ATP or GTP driven

Motor proteins fall into the following categories:1. Classical motors (myosins, dyneins, and

kinesin)2. Timing devices (EF-Tu in translation)3. Microprocessing switching devices (G

proteins)4. Assembly and disassembly factors

(cytoskeleton assembly and disassembly)

Section 5.4: Molecular Machines

Chapter 7

Carbohydrates

Carbohydrates are the most abundant biomolecule in nature Have a wide variety of cellular functions: energy,

structure, communication, and precursors for other biomolecules

They are a direct link between solar energy and chemical bond energy

Chapter 7: Overview

Section 7.1: Monosaccharides

Monosaccharides, or simple sugars, are polyhydroxy aldehydes or ketones Sugars with an aldehyde functional group are aldoses

Sugars with an ketone functional group are ketoses

Figure 7.1 General Formulas for the Aldose and Ketose Forms of Monosaccharides

Section 7.1: MonosaccharidesMonosaccharide

Stereoisomers An increase in the number of

chiral carbons increases the number of possible optical isomers

2n where n is the number of chiral carbons

Almost all naturally occurring monosaccharides are the D form

All can be considered to be derived from D-glyceraldehyde or nonchiral dihydroxyacetone

Figure 7.3 The D Family of Aldoses


Cyclic Structure of Monosaccharides Sugars with four or more carbons exist primarily

in cyclic forms Ring formation occurs because aldehyde and

ketone groups react reversibly with hydroxyl groups in an aqueous solution to form hemiacetals and hemiketals

Figure 7.5 Formation of Hemiacetals and Hemiketals


The two possible diastereomers that form because of cyclization are called anomers

Hydroxyl group on hemiacetal occurs on carbon 1 and can be in the up position (above ring) or down position (below ring)

In the D-sugar form, because the anomeric carbon is chiral, two stereoisomers of the aldose can form the a-anomer or b-anomer

Figure 7.6 Monosaccharide Structure


Haworth Structures—these structures more accurately depict bond angle and length in ring structures than the original Fischer structures

In the D-sugar form, when the anomer hydroxyl is up it gives a b-anomeric form (left in Fischer projection) while down gives the a-anomeric form (right)

Figure 7.7 Haworth Structures of the Anomers of D-Glucose


Five-membered rings are called furanoses and six-membered rings are pyranoses

Cyclic form of fructose is fructofuranose, while glucose in the pyranose form is glucopyranose

Figure 7.8 Furan and Pyran

Figure 7.9 Fischer and Haworth Representations of D-Fructose


Reaction of Monosaccharides The carbonyl and hydroxyl groups can undergo

several chemical reactions Most important include oxidation, reduction,

isomerization, esterification, glycoside formation, and glycosylation reactions


Glycoside Formation—hemiacetals and hemiketals react with alcohols to form the corresponding acetal and ketal

When the cyclic hemiacetal or hemiketal form of the monosaccharide reacts with an alcohol, the new linkage is a glycosidic linkage and the compound a glycoside

Figure 7.17 Formation of Acetals and Ketals


Naming of glycosides specifies the sugar component

Acetals of glucose and fructose are glucoside and fructoside

Figure 7.18 Methyl Glucoside Formation


If an acetal linkage is formed between the hemiacetal hydroxyl of one monosaccharide and the hydroxyl of another, this forms a disaccharide

In polysaccharides, large numbers of monosaccharides are linked together through acetal linkages


Glycosylation Reactions attach sugars or glycans (sugar polymers) to proteins or lipids

Catalyzed by glycosyl transferases, glycosidic bonds are formed between anomeric carbons in certain glycans and oxygen or nitrogen of other types of molecules, resulting in N- or O-glycosidic bonds


Glycation is the reaction of reducing sugars with nucleophilic nitrogen atoms in a nonenzymatic reaction

Most researched example of the glycation reaction is the nonenzymatic glycation of protein (Maillard reaction)

The Schiff base that forms rearranges to a stable ketoamine, called the Amadori product

Can further react to form advanced glycation end products (AGEs)

Promote inflammatory processes and involved in age-related diseases


Figure 7.20 The Maillard Reaction


Important Monosaccharides Glucose (D-Glucose) —originally called

dextrose, it is found in large quantities throughout the natural world

The primary fuel for living cells Preferred energy source for brain cells and cells

without mitochondria (erythrocytes)

Figure 7.21 a-D-glucopyranose


Fructose (D-Fructose) is often referred to as fruit sugar, because of its high content in fruit

On a per-gram basis, it is twice as sweet as sucrose; therefore, it is often used as a sweetening agent in processed food

Figure 7.22 b-D-fructofuranose


Galactose is necessary to synthesize a variety of important biomolecules

Important biomolecules include lactose, glycolipids, phospholipids, proetoglycan, and glycoproteins

Galactosemia is a genetic disorder resulting from a missing enzyme in galactose metabolism

Figure 7.23 a-D-galactopyranose

Section 7.2: Disaccharides

DisaccharidesTwo monosaccharides linked by a glycosidic bond Linkages are named by a- or b-conformation and

by which carbons are connected (e.g., a(1,4) or b(1,4))

Figure 7.27 Glycosidic Bonds


Disaccharides Continued Lactose (milk sugar) is the

disaccharide found in milk One molecule of galactose linked

to one molecule of glucose (b(1,4) linkage)

It is common to have a deficiency in the enzyme that breaks down lactose (lactase)

Lactose is a reducing sugar

Figure 7.28 a- and b-lactose


Disaccharides Continued Sucrose is common table sugar

(cane or beet sugar) produced in the leaves and stems of plants

One molecule of glucose linked to one molecule of fructose, linked by an a,b(1,2) glycosidic bond

Glycosidic bond occurs between both anomeric carbons

Sucrose is a nonreducing sugar

Figure 7.31 Sucrose

Section 7.3: Polysaccharides

Polysaccharides (glycans) are composed of large numbers of monosaccharides connected by glycosidic linkages Smaller glycans made of 10 to 15 monomers

called oligosaccharides, most often attached to polypeptides as glycoproteins

Two broad classes: N- and O-linked oligosaccharides


O-Glycosidic linkages attach glycans to the side chain hydroxyl of serine or threonine residues or the hydroxyl oxygens of membrane lipids

Figure 7.32 Oligosaccharides Linked to Polypeptides

N-linked oligosaccharides are attached to polypeptides by an N-glycosidic bond with the side chain amide nitrogen from the amino acid asparagine

Three major types of asparagine-linked oligosaccharides: high mannose, hybrid, and complex


Homoglycans Have one type of monosaccharide and are found

in starch, glycogen, cellulose, and chitin (glucose monomer)

Starch and glycogen are energy storage molecules while chitin and cellulose are structural

Chitin is part of the cell wall of fungi and arthropod exoskeleton

Cellulose is the primary component of plant cell walls

No fixed molecular weight, because the size is a reflection of the metabolic state of the cell producing them


Starch—the energy reservoir of plant cells and a significant source of carbohydrate in the human diet

Two polysaccharides occur together in starch: amylose and amylopectin

Amylose is composed of long, unbranched chains of D-glucose with a(1,4) linkages between them

Figure 7.33 Amylose


Amylose typically contains thousands of glucose monomers and a molecular weight from 150,000 to 600,000 Da

The other form is amylopectin, which is a branched polymer containing both a(1,6) and a(1,4) linkages

Branch points occur every 20 to 25 residues

Figure 7.33 Amylose


Glycogen is the carbohydrate storage molecule in vertebrates found in greatest abundance in the liver and muscle cells

Up to 8–10% of the wet weight of liver cells and 2–3% in muscle cells

Similar in structure to amylopectin, with more branch points

More compact and easily mobilized than other polysaccharides


Figure 7.34 (a) Amylopectin and (b) Glycogen


Cellulose is a polymer of D-glucopyranosides linked by b(1,4) glycosidic bonds

It is the most important structural polysaccharide of plants (most abundant organic substance on earth)

Figure 7.35 The Disaccharide Repeating Unit of Cellulose


Pairs of unbranched cellulose molecules (12,000 glucose units each) are held together by hydrogen bonding to form sheetlike strips, or microfibrils

Each microfibril bundle is tough and inflexible with a tensile strength comparable to that of steel wire

Important for dietary fiber, wood, paper, and textiles

Figure 7.36 Cellulose Microfibrils


Heteroglycans High-molecular-weight carbohydrate polymers

that contain more than one type of monosaccharide

Major types: N- and O-linked glycosaminoglycans (glycans), glycosaminoglycans, glycan components of glycolipids, and GPI (glycosylphosphatidylinositol) anchors

GPI anchors and glycolipids will be discussed in Chapter 11

Section 7.3: PolysaccharidesHeteroglycans Continued

N- and O-Glycans—many proteins have N- and O-linked oligosacchaarides

N-linked (N-glycans) are linked via a b-glycosidic bond

O-linked (O-glycans) have a disaccharide core of galactosyl-b-(1,3)-N-acetylgalactosamine linked via an a-glycosidic bond to the hydroxyl of serine or threonine residues

Glycosaminoglycans (GAGs) are linear polymers with disaccharide repeating units

Five classes: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and heparin sulfate, and keratin sulfate

Varying uses based on repeating unit

Section 7.4: Glycoconjugates

Glycoconjugates result from carbohydrates being linked to proteins and lipids

Proteoglycans Distinguished from other

glycoproteins by their high carbohydrate content (about 95%)

Occur on cell surfaces or are secreted to the extracellular matrix

Figure 7.38 Proteoglycan Aggregate From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press


Glycoproteins Commonly defined as proteins that are covalently

linked to carbohydrates through N- and O-linkages Several addition reactions in the lumen of the

endoplasmic reticulum and Golgi complex are responsible for final N-linked oligosaccharide structure

O-glycan synthesis occurs later, probably initiating in the Golgi complex

Carbohydrate could be 1%–85% of total weight Glycoprotein Functions occur in cells as soluble

and membrane-bound forms and are nearly ubiquitous in living organisms

Vertebrate animals are particularly rich in glycoproteins


Figure 7.39 The Glycocalyx

Section 7.5: The Sugar Code

Living organisms require large coding capacities for information transfer Profound complexity of functioning systems To succeed as a coding mechanism, a class of

molecules must have a large capacity for variation

Glycosylation is the most important posttranslational modification in terms of coding capacity

More possibilities with hexasaccharides than hexapeptides


In addition to their immense combinatorial possibilities they are also relatively inflexible, which makes them perfect for precise ligand binding

Lectins Lectins, or carbohydrate-binding proteins, are

involved in translating the sugar code Bind specifically to carbohydrates via hydrogen

bonding, van der Waals forces, and hydrophobic interactions


Figure 7.40 Role of Oligosaccharides in Biological Recognition

Lectins Continued Biological processes

include binding to microorganisms, binding to toxins, and involved in leukocyte rolling


The Glycome Total set of sugars and glycans in a cell or

organism is the glycome Constantly in flux depending on the cell’s

response to environment There is no template for glycan biosynthesis; it is

done in a stepwise process Glycoforms can result based upon slight

variations in glycan composition of each glycoprotein

Chapter 11

Lipids and Membranes

Fatty Acids Monocarboxylic acids that typically contain

hydrocarbon chains of variable lengths (12 to 20 or more carbons)

Numbered from the carboxylate end, and the a-carbon is adjacent to the carboxylate group

Terminal methyl carbon is denoted the omega (w) carbon

Important in triacylglycerols and phospholipids

Figure 11.1 Fatty Acid Structure

Section 11.1: Lipid Classes

Most naturally occurring fatty acids have an even number of carbons in an unbranched chain

Fatty acids that contain only single carbon-carbon bonds are saturated

Fatty acids that contain one or more double bonds are unsaturated

Can occur in two isomeric forms: cis (like groups on the same side) and trans (like groups are on opposite sides)

Figure 11.2 Isomeric Forms of Unsaturated Molecules


The double bonds in most naturally occurring fatty acids are cis and cause a kink in the fatty acid chain

Unsaturated fatty acids are liquid at room temperature; saturated fatty acids are usually solid

Monounsaturated fatty acids have one double bond while polyunsaturated fats have two or more

Figure 11.3 Space-Filling and Conformational Models


Plants and bacteria can synthesize all fatty acids they require from acetyl-CoA

Animals acquire most of theirs from dietary sources

Nonessential fatty acids can be synthesized while essential fatty acids must be acquired from the diet

Omega-3 fatty acids (i.e., a-linolenic acid and its derivatives) may promote cardiovascular health

Certain fatty acids attach to proteins called acylated proteins; the groups (acyl groups) help facilitate interactions with the environment

Myristoylation and palmitoylation


Eicosanoids A diverse group of powerful, hormone-like

(generally autocrine) molecules produced in most mammalian tissues

Include prostaglandins, thromboxanes, and leukotrienes

Mediate a wide variety of physiological processes: smooth muscle contraction, inflammation, pain perception, and blood flow regulation

Figure 11.4a Eicosanoids


Eicosonoids are often derived from arachidonic acid or eicosapentaenoic acid (EPA)

Prostaglandins contain a cyclopentane ring and hydroxyl groups at C-11 and C-15

Prostaglandins are involved in inflammation, digestion, and reproduction

Figure 11.4a Eicosanoids


Figure 11.4b Eicosanoids


Thromboxanes differ structurally from other eicosanoids in that they have a cyclic ether

Synthesized by polymorphonuclear lymphocytes Involved in platelet aggregation and

vasoconstriction following tissue injury

Leukotrienes were named from their discovery in white blood cells and triene group in their structure

LTC4, LTD4, and LTE4 have been identified as components of slow-reacting substance of anaphylaxis

Other effects of leukotrienes: blood vessel fluid leakage, white blood cell chemoattractant, vasoconstriction, edema, and bronchoconstriction

Figure 11.4c Eicosanoids


Triacylglycerols Triacylglycerols are esters of glycerol with three

fatty acids Neutral fats because they have no charge Contain fatty acids of varying lengths and can be

a mixture of saturated and unsaturated

Figure 11.5 Triacylglycerol


Depending on fatty acid composition, can be termed fats or oils

Fats are solid at room temperature and have a high saturated fatty acid composition

Oils are liquid at room temperature and have a high unsaturated fatty acid composition

Figure 11.6 Space-Filling and Conformational Models of a Triacylglycerol


Roles in animals: energy storage (also in plants), insulation at low temperatures, and water repellent for some animals’ feathers and fur

Better storage form of energy for two reasons:1. Hydrophobic and coalesce into droplets; store an equivalent amount of energy in about one-eighth the space2. More reduced and thus can release more electrons per molecule when oxidized

Figure 11.5 Triacylglycerol


Wax Esters Waxes are complex mixtures of nonpolar lipids Protective coatings on the leaves, stems, and

fruits of plants and on the skin and fur of animals Wax esters composed of long-chain fatty acids

and long-chain alcohols are prominent constituents of most waxes

Examples include carnuba (melissyl cerotate) and beeswax

Figure 11.8 The Wax Ester Melissyl Cerotate


Phospholipids Amphipathic with a polar head group (phosphate

and other polar or charged groups) and hydrophobic fatty acids

Act in membrane formation, emulsification, and as a surfactant

Spontaneously rearrange into ordered structures when suspended in water

Figure 11.9 Phospholipid Molecules in Aqueous Solution


Two types of phospholipids: phosphoglycerides and sphingomyelins

Sphingomyelins contain sphingosine instead of glycerol (also classified as sphingolipids)

Phosphoglycerides contain a glycerol, fatty acids, phosphate, and an alcohol

Simplest phosphoglyceride is phosphatidic acid composed of glycerol-3-phosphate and two fatty acids

Phosphatidylcholine (lecithin) is an example of alcohol esterified to the phosphate group as choline


Another phosphoglyceride, phosphatidylinositol, is an important structural component of glycosyl phosphatidylinositol (GPI) anchors

GPI anchors attach certain proteins to the membrane surface

Proteins are attached via an amide linkage

Figure 11.10 GPI Anchor


Phospholipases Hydrolyze ester bonds in glycerophospholipid

molecules Three major functions: membrane remodeling,

signal transduction, and digestion Membrane remodeling—removal of fatty

acids to adjust the ratio of saturated to unsaturated or repair a damaged fatty acid

Figure 11.11 Phospholipases


Phospholipases Continued Signal Transduction—phospholipid

hydrolysis initiates the signal transduction by numerous hormones

Digestion—pancreatic phospholipases degrade dietary phospholipids in the small intestine

Toxic Phospholipases—various organisms use membrane-degrading phospholipases as a means of inflicting damage

Bacterial a-toxin and necrosis from snake venom (PLA2)


Sphingolipids Important components of animal and plant

membranes Sphingosine (long-chain amino alcohol) and

ceramide in animal cells

Figure 11.12 Sphingolipid Components


Sphingomyelin is found in most cell membranes, but is most abundant in the myelin sheath of nerve cells

Figure 11.13 Space-Filling and Conformational Models of Sphingolmyelin


The ceramides are also precursors of glycolipids A monosaccharide, disacchaaride, or

oligosaccharide attached to a ceramide through an O-glycosidic bond

Most important classes are cerebrosides, sulfatides, and gangliosides (may bind bacteria and their toxins)

Figure 11.14a Selected Glycolipids


Cerebrosides have a monosaccharide for their head group

Galactocerebroside is found in brain cell membranes

Sulfatides are negatively charged at physiological pH

Gangliosides possess oligosaccharide groups; occur in most animal tissues and GM2 is involved in Tay-Sachs disease

Figure 11.14b Selected Glycolipids


Isoprenoids Vast array of biomolecules containing repeating

five-carbon structural units, or isoprene units Isoprenoids consist of terpenes and steroids Terpenes are classified by the number of

isoprene units they have Monoterpenes (used in perfumes), sesquiterpines

(e.g., citronella), tetraterpenes (e.g., carotenoids)

Figure 11.15 Isoprene


Carotenoids are the orange pigments found in plants

Mixed terpenoids consist of a nonterpene group attached to the isoprenoid group (prenyl groups)

Include vitamin K and vitamin E

Figure 11.16 Vitamin K, a Mixed Terpenoid


A variety of proteins are covalently attached to prenyl groups (prenylation): farnesyl and geranylgeranyl groups

Unknown function, but may be involved in cell growth

Figure 11.17 Prenylated Proteins


Steroids are derivatives of triterpenes with four fused rings (e.g., cholesterol)

Found in all eukaryotes and some bacteria Differentiated by double-bond placement and

various substituents

Figure 11.18 Structure of Cholesterol


Cholesterol is an important molecule in animal cells that is classified as a sterol, because C-3 is oxidized to a hydroxyl group

Essential in animal membranes; a precursor of all steroid hormones, vitamin D, and bile salts

Usually stored in cells as a fatty acid ester The term steroid is commonly used to describe all

derivatives of the steroid ring structure


Figure 11.19 Animal Steroids


Lipoproteins Term most often applied to a

group of molecular complexes found in the blood plasma of mammals

Transport lipid molecules through the bloodstream from organ to organ

Protein components (apolipoproteins) for lipoproteins are synthesized in the liver or intestine

Figure 11.21 Plasma Lipoproteins


Lipoproteins are classified according to their density:

Chylomicrons are large lipoproteins of extremely low density that transport triacylglycerol and cholesteryl esters (synthesized in the intestines)

Very low density lipoproteins (VLDL) are synthesized in the liver and transport lipids to the tissues

Low density lipoproteins (LDL) are principle transporters of cholesterol and cholesteryl esters to tissues

High density lipoprotein (HDL) is a protein-rich particle produced in the liver and intestine that seems to be a scavenger of excess cholesterol from membranes


A membrane is a noncovalent heteropolymer of lipid bilayer and associated proteins (fluid mosaic model)

Membrane Structure Proportions of lipid, protein, and carbohydrate vary

considerably among cell types and organelles

Section 11.2: Membranes

Membrane lipids: phospholipids form bimolecular layers at relatively low concentrations; this is the basis of membrane structure

Membrane lipids are largely responsible for many membrane properties

Membrane fluidity refers to the viscosity of the lipid bilayer

Rapid lateral movement is apparently responsible for normal membrane function

Figure 11.25 Lateral Diffusion in Biological Membranes


The movement of molecules from one side of a membrane to the other requires a flipase

Membrane fluidity largely depends on the percentage of unsaturated fatty acids and cholesterol

Cholesterol contributes to stability with its rigid ring system and fluidity with its flexible hydrocarbon tail

Figure 11.24 Diagrammatic View of a Lipid Bilayer


Selective permeability is provided by the hydrophobic chains of the lipid bilayer, which is impermeable to most all molecules (except small nonpolar molecules)

Membrane proteins help regulate the movement of ionic and polar substances

Small nonpolar substances may diffuse down their concentration gradient

Self-sealing is a result of the lateral flow of lipid molecules after a small disruption

Asymmetry of biological membranes is necessary for their function

The lipid composition on each side of the membrane is different


Membrane Proteins—most functions associated with the membrane require membrane proteins

Classified by their relationship with the membrane: peripheral or integral

Figure 11.26 Integral and Peripheral Membrane Proteins


Integral proteins embed in or pass through the membrane

Red blood cell anion exchanger

Peripheral proteins are bound to the membrane primarily through noncovalent interactions

Can be linked covalently through myristic, palmitic, or prenyl groups

GPI anchors link a wide variety of proteins to the membrane

Figure 11.27 Red Blood Cell Integral Membrane Proteins


Membrane Microdomains—lipids and proteins in membranes are not uniformly distributed

Specialized microdomains like “lipid rafts” can be found in the external leaflet of the plasma membrane

Figure 11.28 Lipid Rafts


Lipid rafts often include cholesterol, sphingolipids, and certain proteins

Lipid molecules are more ordered (less fluid) than non- raft regions

Lipid rafts have been implicated in a number of processes: exocytosis, endocytosis, and signal transduction

Figure 11.29 The Lipid Raft Environment


Membrane Function There are a vast array of membrane functions,

including transport of polar and charged substances and the relay of signals

Figure 11.30 Transport across Membranes


Membrane Transport—the mechanisms are vital to living organisms

Ions and molecules constantly move across the plasma membrane and membranes of organelles

Important for nutrient intake, waste excretion, and the regulation of ion concentration

Biological transport mechanisms are classified according to whether they require energy


In passive transport, there is no energy input, while in active transport, energy is required

Passive is exemplified by simple diffusion and facilitated diffusion (with the concentration gradient)

Active transport uses energy to transport molecules against a concentration gradient

Figure 11.30 Transport across Membranes


Simple diffusion involves the propulsion of each solute by random molecular motion from an area of high concentration to an area of low concentration

Diffusion of gases O2 and CO2 across membranes is proportional to their concentration gradients

Does not require a protein channel Facilitated diffusion uses channel proteins to

move large or charged molecules down their concentration gradient

Examples include chemically gated Na+ channel and voltage-gated K+ channel


Active transport has two forms: primary and secondary

In primary active transport, transmembrane ATP-hydrolyzing enzymes provide the energy to drive the transport of ions or molecules

Na+-K+ ATPase

Figure 11.31 The Na+-K+ ATPase and Glucose Transport


In secondary active transport, concentration gradients formed by primary active transport are used to move other substances across the membrane

Na+-K+ ATPase pump in the kidney drives the movement of D-glucose against its concentration gradient

Figure 11.31 The Na+-K+ ATPase and Glucose Transport


Membrane Receptors provide mechanisms by which cells monitor and respond to changes in their environment

Chemical signals bind to membrane receptors in multicellular organisms for intracellular communication

Other receptors are involved in cell-cell recognition

Binding of ligand to membrane receptor causes a conformational change and programmed response


Chapter 17

Nucleic Acids

Scientists have studied how organisms organize and process genetic information, revealing the following principles:

1. DNA directs the function of living cells and is transmitted to offspring

DNA is composed of two polydeoxynucleotide strands forming a double helix

Figure 17.2 Two Models of DNA Structure

Section 17.1: DNA

A gene is a DNA sequence that contains the base sequence information to code for a gene product, protein, or RNA

The complete DNA base sequence of an organism is its genome

DNA synthesis, referred to as replication, involves complementary base pairing between the parental and newly synthesized strand

Figure 17.2 Two Models of DNA Structure

Section 17.1: DNA

2. The synthesis of RNA begins the process of decoding genetic information

RNA synthesis is called transcription and involves complementary base pairing of ribonucleotides to DNA bases

Each new RNA is a transcript

The total RNA transcripts for an organism comprise its transcriptome

Figure 17.3a An Overview of Genetic Information Flow

Section 17.1: DNA

3. Several RNA molecules participate directly in the synthesis of protein, or translation

Messenger RNA (mRNA) specifies the primary protein sequence

Transfer RNA (tRNA) delivers the specific amino acid

Ribosomal RNA (rRNA) molecules are components of ribosomes

Figure 17.3b An Overview of Genetic Information Flow

Section 17.1: DNA

The proteome is the entire set of proteins synthesized

4. Gene expression is the process by which cells control the timing of gene product synthesis in response to environmental or developmental cues

Metabolome refers to the sum total of low molecular weight metabolites produced by the cell

Figure 17.3b An Overview of Genetic Information Flow

Section 17.1: DNA

The Central dogma schematically summarizes the previous information

Includes replication, transcription, and translation

The central dogma is generally how the flow of information works in all organisms, except some viruses have RNA genomes and use reverse transcriptase to make DNA (e.g., HIV)

Section 17.1: DNA

DNA RNA Protein

DNA consists of two polydeoxynucleotide strands that wind around each other to form a right-handed double helix Each DNA nucleotide

monomer is composed of a nitrogenous base, a deoxyribose sugar, and phosphate

Figure 17.4 DNA Strand Structure

Section 17.1: DNA

Nucleotides are linked by 3′,5′-phosphodiester bonds

These join the 3′-hydroxyl of one nucleotide to the 5′-phosphate of another

Figure 17.4 DNA Strand Structure

Section 17.1: DNA

The antiparallel nature of the two strands allows hydrogen bonds to form between the nitrogenous bases

Two types of base pair (bp) in DNA: (1) adenine (purine) pairs with thymine (pyrimidine) and (2) the purine guanine pairs with the pyrimidine cytosine

Figure 17.5 DNA Structure

Section 17.1: DNA

The dimensions of crystalline B-DNA have been precisely measured:

1. One turn of the double helix spans 3.32 nm and consists of 10.3 base pairs

Figure 17.6 DNA Structure: GC Base Pair Dimensions

Section 17.1: DNA

2. Diameter of the double helix is 2.37 nm, only suitable for base pairing a purine with a pyrimidine3. The distance between adjacent base pairs is 0.29-0.30 nm

Figure 17.6 DNA Structure: AT Base Pair Dimensions

Section 17.1: DNA

DNA is a relatively stable molecule with several noncovalent interactions adding to its stability

1. Hydrophobic interactions—internal base clustering2. Hydrogen bonds—formation of preferred bonds: three between CG base pairs and two between AT base pairs3. Base stacking—bases are nearly planar and stacked, allowing for weak van der Waals forces between the rings4. Hydration—water interacts with the structure of DNA to stabilize structure5. Electrostatic interactions—destabilization by negatively charged phosphates of sugar-phosphate backbone are minimized by the shielding effect of water on Mg2+

Section 17.1: DNA

Mutation types—The most common are small single base changes, also called point mutations

This results in transition or transversion mutations

Transition mutations, caused by deamination, lead to purine for purine or pyrimidine for pyrimidine substitutions

Transversion mutations, caused by alkylating agents or ionizing radiation, occur when a purine is substituted for a pyrimidine or vice versa

Section 17.1: DNA

Point mutations that occur in a population with any frequency are referred to as single nucleotide polymorphisms (SNPs)

Point mutations that occur within the coding portion of a gene can be classified according to their impact on structure and/or function:

Silent mutations have no discernable effect Missense mutations have an observable

effect Nonsense mutations changes a codon for an

amino acid to that of a premature stop codon

Section 17.1: DNA

Insertions and deletions, or indels, occur from one to thousands of bases

Indels that occur within the coding region that are not divisible by three cause a frameshift mutation

Genome rearrangements can cause disruptions in gene structure or regulation.

Occur as a result of double strand breaks and can lead to inversions, translocations, or duplications

Section 17.1: DNA

DNA Structure: The Genetic Material In the early decades of the twentieth century, life

scientists believed that of the two chromosome components (DNA and protein) that protein was most likely responsible for transmission of inherited traits

The work of several scientists would lead to another conclusion

Section 17.1: DNA

DNA Structure: Variations on a Theme Watson and Crick’s discovery

is referred to as B-DNA (sodium salt)

Another form is the A-DNA, which forms when RNA/DNA duplexes form

Z-DNA (zigzag conformation) is left-handed DNA that can form as a result of torsion during transcription

Figure 17.12 A-DNA, B-DNA, and Z-DNA

Section 17.1: DNA

DNA can form other structures, including cruciforms, which are cross-like structures, probably a result of palindromes (inverted repeats)

Packaging large DNA molecules to fit inside a cell or nucleus requires a process termed supercoiling

Section 17.1: DNA

DNA Supercoiling Facilitates several

biological processes: packaging of DNA, replication, and transcription

Linear and circular DNA can be in a relaxed or supercoiled shape

Figure 17.13 Linear and Circular DNA and DNA Winding

Section 17.1: DNA

Chromosomes and Chromatin DNA is packaged into

chromosomes Prokaryotic and eukaryotic

chromosomes differ significantly

Prokaryotes—the E. coli chromosome is a circular DNA molecule that is extensively looped and coiled

Supercoiled DNA complexed with a protein core

Figure 17.17 The E. coli Chromosome Removed from a Cell

Section 17.1: DNA

Eukaryotes have extraordinarily large genomes when compared to prokaryotes

Chromosome number and length can vary by species

Each eukaryotic chromosome consists of a single, linear DNA molecule complexed with histone proteins to form nucleohistone

Chromatin is the term used to describe this complex

Figure 17.18 Electron Micrograph of Chromatin

Section 17.1: DNA

Nucleosomes are formed by the binding of DNA and histone proteins

Nucleosomes have a beaded appearance when viewed by electron micrograph

Histone proteins have five major classes: H1, H2A, H2B, H3, and H4

A nucleosome is positively coiled DNA wrapped around a histone core (two copies each of H2A, H2B, H3, and H4)

Figure 17.18 Electron Micrograph of Chromatin

Section 17.1: DNA

Prokaryotic Genomes—Investigation of E. coli has revealed the following prokaryotic features:

1. Genome size—usually considerably less DNA and fewer genes (E. coli 4.6 megabases) than eukaryotic genomes2. Coding capacity—compact and continuous genes3. Gene expression—genes organized into operons

Prokaryotes often contain plasmids, which are usually small and circular DNA with additional genes (e.g., antibiotic resistance)

Section 17.1: DNA

Eukaryotic Genomes—Investigation has revealed the organization to be very complex

The following are unique eukaryotic genome features:

1. Genome size—eukaryotic genome size does not necessarily indicate complexity2. Coding capacity—enormous protein coding capacity, but the majority of DNA sequences do not have coding functions 3. Coding continuity—genes are interrupted by noncoding introns, which can be removed by splicing from the primary RNA transcript

Section 17.1: DNA

Existence of introns and exons allows eukaryotes to produce more than one polypeptide from each protein-coding gene

Alternative splicing allows for various combinations of exons to be joined to form different mRNAs

Intergenic sequences are those sequences that do not code for polypeptide primary sequence or RNAs

Section 17.1: DNA

Of the 3,200 Mb of the human genome, only 38% comprise genes and related sequence

Only 4% codes for gene products Humans have about 23,000 protein coding

genesand several ncRNA genes

Section 17.1: DNA

25% of known protein-coding genes are related to DNA synthesis and repair

21% signal transduction 17% general biochemical

functions 38% other activities

Over 60% of the human genome is intergenic sequences

Figure 17.24 Human Protein-Coding Genes

Section 17.1: DNA

Two classes: tandem repeats and interspersed genome-wide repeats

Tandem repeats (satellite DNA) are DNA sequences in which multiple copies are arranged next to each other

Certain tandem repeats play structural roles like centromeres and telomeres

Some are small, like microsatellites (1-4 bp) and minisatellites (10-100 bp)

Used as markers in genetic disease, forensic investigations, and kinship

Section 17.1: DNA

Interspersed genome-wide repeats are repetitive sequences scattered around the genome

Often involve mobile genetic elements that can duplicate and move around the genome

Transposons and retrotransposones LINEs (long interspersed nuclear elements)

and SINEs (short interspersed nuclear elements) are two types of transposons

Section 17.1: DNA

RNA is a versatile molecule, not only involved in protein synthesis, but plays structural and enzymatic roles as well

Differences between DNA and RNA primary structure: 1. Ribose sugar instead of

deoxyribose 2. Uracil nucleotide instead of

thymine

Figure 17.25 Secondary Structure of RNA

Section 17.2: RNA

3. RNA exists as a single strand that can form complex three- dimensional structures by base pairing with itself

4. Some RNA molecules have catalytic properties, or ribozymes (e.g., self-cleavages or cleave other RNA)

Figure 17.25 Secondary Structure of RNA

Section 17.2: RNA

Transfer RNA Transfer RNA (tRNA) molecules

transport amino acids to ribosomes for assembly (15% of cellular RNA)

Average length: 75 bases At least one tRNA for each

amino acid Structurally look like a warped

cloverleaf due to extensive intrachain base pairing

Figure 17.26a Transfer RNA

Section 17.2: RNA

Amino acids are attached via specific aminoacyl-tRNA synthetases to the end opposite the three nucleotide anticodon

Anticodon allows the tRNA to recognize the correct mRNA codon and properly align its amino acid for protein synthesis

The tRNA loops help facilitate interactions with the correct aminoacyl-tRNA synthetases

Section 17.2: RNA

Figure 17.26b Transfer RNA

Ribosomal RNA Ribosomal RNA (rRNA) is the most abundant

RNA in living cells with a complex secondary structure

Components of ribosomes (eukaryotes and prokaryotes)

Similar in shape and function, both have a small and large subunit, but differ in size and chemical composition

Eukaryotic are larger (80S) with a 60S and 40S subunit, while prokaryotic are smaller (70S) with 50S and 30S subunits

Section 17.2: RNA

rRNA plays a role in scaffolding as well as enzymatic functions

Ribosomes also have proteins that interact with rRNA for structure and function

Section 17.2: RNA

Figure 17.27 rRNA Structure

Messenger RNA Messenger RNA (mRNA) is the carrier of

genetic information from DNA to protein synthesis (approximately 5% of total RNA)

mRNA varies considerably in size Prokaryotic and eukaryotic mRNA differ in

several respects Prokaryotes are polycistronic while eukaryotes

are usually monocistronic mRNAs are processed differently; eukaryotic

mRNA requires 5′ capping, 3′ tailing, and splicing

Section 17.2: RNA

Noncoding RNA RNAs that do not directly code for polypeptides

are called noncoding RNAs (ncRNAs) Micro RNAs and small interfering RNAs are

among the shortest and involved in the RNA-induced silencing complex

Small Nucleolar RNAs (snoRNAs) facilitate chemical modifications to rRNA in the nucleolus

Section 17.2: RNA

Noncoding RNA Small interfering RNAs (siRNAs) are 21-23 nt

dsRNAs that play a crucial role in RNA interference (RNAi)

Small nuclear RNAs (snRNAs) combine with proteins to form small nuclear ribonucleoproteins (snRNPs) and are involved in splicing

Section 17.2: RNA

Viruses lack the properties that distinguish life from nonlife (e.g., no metabolism)

Once a virus has infected a cell, its nucleic acid can hijack the host’s nucleic acid and protein-synthesizing machinery The virus can then make copies of itself until it

ruptures the host cell or integrates into the host cell’s chromosome

Section 17.3: Viruses

A viral infection can provide biochemical insight, because it subverts the host cell’s function Viruses can cause numerous different diseases,

but have also been invaluable in the development of recombinant DNA technology

Human papillomavirus can cause cervical cancer

Section 17.3: Viruses

Section 2 Biochemical Building Blocks. Chapter 5 Amino Acids, Peptides, & Proteins.

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Transcript of Section 2 Biochemical Building Blocks. Chapter 5 Amino Acids, Peptides, & Proteins.