Proteins

The 411

• Proteins account for more than 50% of the dry mass of most cells

• Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances

• Most important, protein enzymes function as catalysts in cells, regulating metabolism by selectively accelerating chemical reactions without being consumed.

Amino Acids

• Small – molecules (20 variants)– 1 Amino group– 1 Carboxyl Group– 1 “R” Group– Amino acids differ in their properties due

to differing side chains, called R groups

• Joined by Peptide Bonds to form Polypeptides– A protein consists of one or more

polypeptides– Each polypeptide has a unique linear

sequence of amino acids

Fig. 5-UN1

Aminogroup

Carboxylgroup

Alpha carbon

http://jaxspotpictures.com/images/Digital%20Art/gene-transcription.jpg

DNA T A C C G C T C C G C C G T C G A C A A T A C C A C T

mRNA ____________________________________________________________________________

tRNA ____________________________________________________________________________

AA ____________________________________________________________________________

How to Make Proteins!

Fig. 5-17a

Nonpolar

Glycine (Gly or G)

Alanine (Ala or A)

Valine (Val or V)

Leucine (Leu or L)

Isoleucine (Ile or I)

Methionine (Met or M)

Phenylalanine (Phe or F)

Tryptophan (Trp or W)

Proline (Pro or P)

What makes all Amino Acids different?

Fig. 5-17b

Polar

Asparagine (Asn or N)

Glutamine (Gln or Q)

Serine (Ser or S)

Threonine (Thr or T)

Cysteine (Cys or C)

Tyrosine (Tyr or Y)

Fig. 5-17c

Acidic

Arginine (Arg or R)

Histidine (His or H)

Aspartic acid (Asp or D)

Glutamic acid (Glu or E)

Lysine (Lys or K)

Basic

Electricallycharged

Peptidebond

Fig. 5-18

Amino end(N-terminus)

Peptidebond

Side chains

Backbone

Carboxyl end(C-terminus)

(a)

(b)

Polypeptide Formation

Structure

GrooveGroove

Levels of Structure

• The primary structure – Unique sequence of amino acids

• Secondary structure– Found in most proteins, consists of coils and folds

in the polypeptide chain due to Hydrogen Bonding

• Tertiary structure– Determined by interactions among various side

chains (R groups)– Fully Folded

• Quaternary structure– Results when a protein consists of multiple

polypeptide chains

Amino acidsubunits

+H3N Amino end

25

20

15

10

5

1

Primary Structure Primary

• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word

• Primary structure is determined by inherited genetic information

Fig. 5-21c

Secondary Structure

Beta pleated sheet

Examples ofamino acidsubunits

alpha helix

• The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone

• Typical secondary structures are a coil called an helix and a folded structure called a pleated sheet

Fig. 5-21f

Polypeptidebackbone

Hydrophobicinteractions andvan der Waalsinteractions

Disulfide bridge

Ionic bond

Hydrogenbond

Tertiary

Quaternary

• Quaternary structure results when two or more polypeptide chains form one macromolecule

• Collagen is a fibrous protein consisting of three polypeptides coiled like a rope

• Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains

Fig. 5-21g

Polypeptidechain

Chains

HemeIron

Chains

CollagenHemoglobin

Fig. 5-22

Primarystructure

Secondaryand tertiarystructures

Quaternarystructure

Normalhemoglobin(top view)

Primarystructure

Secondaryand tertiarystructures

Quaternarystructure

Function Function

subunit

Molecules donot associatewith oneanother; eachcarries oxygen.

Red bloodcell shape

Normal red bloodcells are full ofindividualhemoglobinmoledules, eachcarrying oxygen.

10 µm

Normal hemoglobin

1 2 3 4 5 6 7Val His Leu Thr Pro Glu Glu

Red bloodcell shape

subunit

Exposedhydrophobicregion

Sickle-cellhemoglobin

Moleculesinteract withone another andcrystallize intoa fiber; capacityto carry oxygenis greatly reduced.

Fibers of abnormalhemoglobin deformred blood cell intosickle shape.

10 µm

Sickle-cell hemoglobin

GluProThrLeuHisVal Val

1 2 3 4 5 6 7