3Life and Chemistry:Large Molecules

3 Macromolecules: Giant Polymers

• There are four major types of biological macromolecules:

Proteins

Carbohydrates

Lipids

Nucleic acids

3 Macromolecules: Giant Polymers

• Macromolecules are giant polymers.

• Polymers are formed by covalent linkages of smaller units called monomers.

3 Condensation and Hydrolysis Reactions

• Macromolecules are made from smaller monomers by means of a condensation or dehydration reaction in which an OH from one monomer is linked to an H from another monomer.

• The reverse reaction, in which polymers are broken back into monomers, is a called a hydrolysis reaction.

Figure 3.3 Condensation and Hydrolysis of Polymers (Part 1)

Figure 3.3 Condensation and Hydrolysis of Polymers (Part 2)

3 Proteins: Polymers of Amino Acids

• Proteins are polymers of amino acids. They are molecules with diverse structures and functions.

3 Proteins: Polymers of Amino Acids

• An amino acid has four groups attached to a central carbon atom:

A hydrogen atom

An amino group (NH3+)

The acid is a carboxyl group (COO–).

Differences in amino acids come from the side chains, or the R groups.

Table 3.2 The Twenty Amino Acids Found in Proteins (Part 1)

Table 3.2 The Twenty Amino Acids Found in Proteins (Part 2)

Table 3.2 The Twenty Amino Acids Found in Proteins (Part 3)

3 Proteins: Polymers of Amino Acids

• Proteins are synthesized by condensation reactions between the amino group of one amino acid and the carboxyl group of another. This forms a peptide linkage.

• Forms a polypeptide.

Figure 3.5 Formation of Peptide Linkages

3 Proteins: Polymers of Amino Acids

• There are four levels of protein structure: primary, secondary, tertiary, and quaternary.

• The precise sequence of amino acids is called its primary structure.

• The peptide backbone consists of repeating units of atoms: N—C—C—N—C—C.

• Enormous numbers of different proteins are possible.

Figure 3.6 The Four Levels of Protein Structure (Part 1)

3 Proteins: Polymers of Amino Acids

• A protein’s secondary structure consists of regular, repeated patterns in different regions in the polypeptide chain.

• This shape is influenced primarily by hydrogen bonds arising from the amino acid sequence (the primary structure).

• The two common secondary structures are the helix and the pleated sheet.

3 Proteins: Polymers of Amino Acids

• The helix is a right-handed coil.

• The R groups point away from the peptide backbone.

Figure 3.6 The Four Levels of Protein Structure (Part 2)

β pleated sheets form from peptide regions that lie parallel to each other.

Stabilized by hydrogen bonds between N-H groups on one chain with the C=O group on the other.

3 Proteins: Polymers of Amino Acids

• Tertiary structure is the three-dimensional shape of the completed polypeptide.

• Interaction between R groups.

• Includes the location of disulfide bridges, which form between cysteine residues.

Figure 3.4 A Disulfide Bridge

3 Proteins: Polymers of Amino Acids

• Other factors determining tertiary structure:

The nature and location of secondary structures

Hydrophobic side-chain aggregation and van der Waals forces, which help stabilize them

The ionic interactions between the positive and negative charges deep in the protein, away from water

3 Proteins: Polymers of Amino Acids

• It is now possible to determine the complete description of a protein’s tertiary structure.

• The location of every atom in the molecule is specified in three-dimensional space.

3 Proteins: Polymers of Amino Acids

• Quaternary structure results from the ways in which multiple polypeptide subunits bind together and interact.

• This level of structure adds to the three-dimensional shape of the finished protein.

Figure 3.8 Quaternary Structure of a Protein

3 Proteins: Polymers of Amino Acids

• Shape is crucial to the functioning of some proteins.

• The combination of attractions, repulsions, and interactions determines the right fit.

Figure 3.9 Noncovalent Interactions between Proteins and Other Molecules

3 Proteins: Polymers of Amino Acids

• Changes in temperature, pH, salt concentrations, and oxidation or reduction conditions can change the shape of proteins.

• This loss of a protein’s normal three-dimensional structure is called denaturation.

Figure 3.11 Denaturation Is the Loss of Tertiary Protein Structure and Function

Figure 3.12 Chaperonins Protect Proteins from Inappropriate Folding

Chaperonins are specialized proteins that help keep other proteins from interacting inappropriately with

one another.

3 Carbohydrates: Sugars and Sugar Polymers

• Carbohydrates are carbon molecules with hydrogen and hydroxyl groups.

• They act as energy storage and transport molecules.

• They also serve as structural components.

3 Carbohydrates: Sugars and Sugar Polymers

• There are four major categories of carbohydrates:

Monosaccharides

Disaccharides, which consist of two monosaccharides

Oligosaccharides, which consist of between 3 and 20 monosaccharides

Polysaccharides, which are composed of hundreds to hundreds of thousands of monosaccharides

3 Carbohydrates: Sugars and Sugar Polymers

• The general formula for a carbohydrate monomer is multiples of CH2O, maintaining a ratio of 1 carbon to 2 hydrogens to 1 oxygen.

• During the polymerization, which is a condensation reaction, water is removed.

• Carbohydrate polymers have ratios of carbon, hydrogen, and oxygen that differ somewhat from the 1:2:1 ratios of the monomers.

3 Carbohydrates: Sugars and Sugar Polymers

• All living cells contain the monosaccharide glucose (C6H12O6).

• Glucose exists as a straight chain and a ring, with the ring form predominant.

• The two forms of the ring, -glucose and -glucose, exist in equilibrium when dissolved in water.

Figure 3.13 Glucose: From One Form to the Other

Figure 3.14 Monosaccharides Are Simple Sugars (Part 1)

Figure 3.14 Monosaccharides Are Simple Sugars (Part 2)

3 Carbohydrates: Sugars and Sugar Polymers

• Monosaccharides are bonded together covalently by condensation reactions. The bonds are called glycosidic linkages.

Figure 3.15 Disaccharides Are Formed by Glycosidic Linkages

3 Carbohydrates: Sugars and Sugar Polymers

• Oligosaccharides contain more than two monosaccharides.

• Many proteins found on the outer surface of cells have oligosaccharides attached to the R group of certain amino acids, or to lipids.

3 Carbohydrates: Sugars and Sugar Polymers

• Polysaccharides are giant polymers of monosaccharides connected by glycosidic linkages.

• Cellulose is a giant polymer of glucose joined by -1,4 linkages.

• Starch is a polysaccharide of glucose with -1,4 linkages.

Structure of cellulose as it occurs in a plant cell wall.

Cellulose Fibers from Print Paper (SEM x1,080).

Figure 3.16 Representative Polysaccharides (Part 1)

3 Carbohydrates: Sugars and Sugar Polymers

• Starches vary by amount of branching.

3 Carbohydrates: Sugars and Sugar Polymers

• Carbohydrates are modified by the addition of functional groups.

Figure 3.17 Chemically Modified Carbohydrates (Part 1)

Figure 3.17 Chemically Modified Carbohydrates (Part 2)

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• Nucleic acids, composed of many nucleotides, are polymers that are specialized for storage and transmission of information.

• Two types of nucleic acid are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• Nucleic acids are polymers of nucleotides.

• A nucleotide consists of a pentose sugar, a phosphate group, and a nitrogen-containing base.

• In DNA, the pentose sugar is deoxyribose; in RNA it is ribose.

Figure 3.24 Nucleotides Have Three Components

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• DNA typically is double-stranded.

• The two separate polymer chains are held together by hydrogen bonding between their nitrogenous bases.

• The base pairing is complementary.

• Purines have a double-ring structure – Adenine and Guanine.

• Pyrimidines have one ring – Cytosine and Thymine.

• A pairs with T, G pairs with C.

Figure 3.25 Distinguishing Characteristics of DNA and RNA

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• The linkages that hold the nucleotides in RNA and DNA are called phosphodiester linkages.

• These linkages are formed between carbon 3 of the sugar and a phosphate group that is associated with carbon 5 of the sugar.

• The backbone consists of alternating sugars and phosphates.

• In DNA, the two strands are antiparallel.

• The DNA strands form a double helix, a molecule with a right-hand twist.

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• Most RNA molecules consist of only a single polynucleotide chain.

• Instead of the base thymine, RNA uses the base uracil; DNA has deoxyribose sugar, RNA has ribose.

Figure 3.26 Hydrogen Bonding in RNA

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• DNA is an information molecule. The information is stored in the order of the four different bases.

• This order is transferred to RNA molecules, which are used to direct the order of the amino acids in proteins.

DNA RNA PROTEIN

3 Lipids: Water-Insoluble Molecules

• Lipids are insoluble in water.

• This insolubility results from the many nonpolar covalent bonds of hydrogen and carbon in lipids.

3 Lipids: Water-Insoluble Molecules

• Roles for lipids in organisms include:

Energy storage (fats and oils)

Cell membranes (phospholipids)

Capture of light energy (carotinoids)

Hormones and vitamins (steroids and modified fatty acids)

Thermal insulation

Electrical insulation of nerves

Water repellency (waxes and oils)

3 Lipids: Water-Insoluble Molecules

• Fats and oils store energy.

• Fats and oils are triglycerides, composed of three fatty acid molecules and one glycerol molecule.

Figure 3.18 Synthesis of a Triglyceride

3 Lipids: Water-Insoluble Molecules

• Saturated fatty acids have only single carbon-to-carbon bonds and are said to be saturated with hydrogens.

3 Lipids: Water-Insoluble Molecules

• Unsaturated fatty acids have at least one double-bonded carbon in one of the chains —the chain is not completely saturated with hydrogen atoms.

Figure 3.19 Saturated and Unsaturated Fatty Acids

3 Lipids: Water-Insoluble Molecules

• Phospholipids have two hydrophobic fatty acid tails and one hydrophilic phosphate group attached to the glycerol.

Figure 3.20 Phospholipid Structure

Figure 3.21 Phospholipids Form a Bilayer

3 Lipids: Water-Insoluble Molecules

• Carotenoids are light-absorbing pigments found in plants and animals.

Figure 3.22 –Carotene is the Source of Vitamin A

3 Lipids: Water-Insoluble Molecules

• Steroids are signaling molecules.

• Steroids are organic compounds with a series of fused rings.

Figure 3.23 All Steroids Have the Same Ring Structure

3 Lipids: Water-Insoluble Molecules

• Waxes are highly nonpolar molecules consisting of saturated long fatty acids bonded to long fatty alcohols.

• A fatty alcohol is similar to a fatty acid, except for the last carbon, which has an —OH group instead of a —COOH group.