The primary structure of a protein is the sequence of amino acids in the peptide chain

Ala-Leu-Cys-Met

19.6 Primary Structure

CH CH3

CH3

H3N CH C

O

N

H

CH C

O

N

H

CH C

O

N

H

CH C O-

OCH CH2

CH2

S

CH3

CH2

SH

CH3

Protein backbone

Insulin

Insulin: The first protein to have its

primary structure determined 51 residues 2 chains 2 disulfide bridges

The secondary structure of a protein

indicates the arrangement of the

polypeptide chains in orderly patterns.

1. Alpha helix

2. Beta-pleated sheet

19.7 Secondary Structure

Alpha Helix

The -helix is a three-dimensional arrangement of the polypeptide

chain that gives a corkscrew shape like a coiled telephone cord

The coiled shape of the alpha helix is held in place by hydrogen bonds

between the amide groups and the carbonyl groups of the amino acids

along the chain

Beta-Pleated Sheet

The -pleated sheet: Holds proteins in a parallel arrangement with hydrogen bonds Has R groups that extend above and below the sheet Is typical of fibrous proteins such as silk

Indicate the type of structure as:

1) primary 2) -helix 3) -pleated sheet

A. Polypeptide chains held side by side by H bonds.

B. Sequence of amino acids in a polypeptide chain.

C. Corkscrew shape with H bonds between amino acids.

Learning Check

Levels of Structure

So far… Primary Secondary

Next… Tertiary

The tertiary structure: Gives a specific overall shape to a protein Involves interactions and cross-links between R

groups in different areas of the peptide chain Is stabilized by:

Hydrophobic and hydrophilic interactions Salt bridges (electrostatic interactions) Hydrogen bonds Disulfide bridges

19.8 Tertiary Structure

Tertiary Structure

Disulfide Bonds:

(electrostatic interactions)

Tertiary Structure

The interactions of the R groups give a protein its specific three-dimensional tertiary structure

Select the type of tertiary interaction as:

1) disulfide 2) salt bridge 3) H-bonds 4) hydrophobic

A. Leucine and valine

B. Cysteine and cysteine

C. Aspartate and lysine

D. Serine and threonine

Learning Check

19.9 Quaternary Structure

The quaternary structure contains two or more tertiary subunits (protein chains)

Held together by same interactions as tertiary structure

Hemoglobin contains four chains

The heme group in each subunit picks up oxygen in the blood for transport to the tissues

Summary of Structural Levels

Identify the level of protein structure:

A. Beta-pleated sheet

B. Order of amino acids in a protein

C. A protein with two or more peptide chains

D. The shape of a globular protein

E. Disulfide bonds between R groups

Learning Check

Learning Check

In myoglobin, about one-half of the 153 amino acids have nonpolar

side chains.

A. Where would you expect those amino acids to be located in the tertiary

structure?

B. Where would you expect the polar side chains to be?

C. Why is myoglobin more soluble in water than silk or wool?

Learning Check

State whether the following statements apply to primary, secondary, tertiary, or quaternary protein structure:

A. Side groups interact to form disulfide bonds or salt bridges.

B. Peptide bonds join amino acids in a polypeptide chain.

C. Hydrogen bonding between carbonyl oxygen atoms and nitrogen atoms of amide groups causes a polypeptide to coil.

D. Hydrophobic side chains seeking a nonpolar environment move toward the inside of the folded protein.

E. Protein chains of collagen form a triple helix.

F. A protein contains four tertiary subunits.

19.10 Reactions of Proteins

Hydrolysis Denaturation

Amide hydrolysis (review section 16.8):

Protein hydrolysis:

Splits the peptide bonds to give smaller peptides and amino acids

Catalyzed by enzymes

Occurs in the digestion of proteins Occurs in cells when amino acids are needed to synthesize new proteins

and repair tissues

Hydrolysis of Amides

R C OH

O

+ N RR C N

O

+ H2OR

H H

H

acid

or base

Disruption of bonds in the native secondary, tertiary and quaternary protein structures

Covalent amide bonds (primary structure) are not affected

Loss of biological activity with loss of structure

Denaturation

Cooking food containing protein

Wiping the skin with alcohol

(denaturation of bacterial proteins)

Hair permanents

Applications of Denaturation

The products of the complete hydrolysis of the peptide Ala-Ser-Val are:

Learning Check

Learning Check

What structural level of a protein is affected by denaturation?

End of Chapter 19!