1

Amino Acids, Proteins, and Enzymes

Primary and Secondary Structure

Tertiary and Quaternary Structure

Protein Hydrolysis and Denaturation

2

Primary Structure of Proteins

The particular sequence of amino acids that is the backbone of a peptide chain or protein

H3N CH

CH3

C

O

N

H

CH C

O

N

H

CH C

O

N

H

CH C O-

OCH

CH CH3

CH3

CH2

SH

CH2

CH2

S

CH3

+

Ala-Leu-Cys-Met

3

Secondary Structure – Alpha Helix

• Three-dimensional arrangement of amino acids with the polypeptide chain in a corkscrew shape

• Held by H bonds between the H of –N-H group and the –O of C=O of the fourth amino acid along the chain

• Looks like a coiled “telephone cord”

4

Secondary Structure – Beta Pleated Sheet

• Polypeptide chains are arranged side by side

• Hydrogen bonds form between chains

• R groups of extend above and below the sheet

• Typical of fibrous proteins such as silk

5

Secondary Structure – Triple Helix

• Three polypeptide chains woven together

• Glycine, proline, hydroxy proline and hydroxylysine

• H bonding between –OH groups gives a strong structure

• Typical of collagen, connective tissue, skin, tendons, and cartilage

6

Learning Check P1

Indicate the type of structure as

(1) primary (2) alpha helix

(3) beta pleated sheet (4) triple helix

A. Polypeptide chain 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

D. Three peptide chains woven like a rope

7

Solution P1

Indicate the type of structure as

(1) primary (2) alpha helix

(3) beta pleated sheet (4) triple helix

A. 3 Polypeptide chain held side by side by H bonds

B. 1 Sequence of amino acids in a polypeptide chain

C. 2 Corkscrew shape with H bonds between amino acids

D. 4 Three peptide chains woven like a rope

8

Tertiary Structure

• Specific overall shape of a protein• Cross links between R groups of amino

acids in chain

disulfide –S–S– +

ionic –COO– H3N–

H bonds C=O HO–

hydrophobic –CH3 H3C–

9

Learning Check P2

Select the type of tertiary interaction as

(1) disulfide (2) ionic

(3) H bonds (4) hydrophobic

A. Leucine and valine

B. Two cysteines

C. Aspartic acid and lysine

D. Serine and threonine

10

Solution P2

Select the type of tertiary interaction as

(1) disulfide (2) ionic

(3) H bonds (4) hydrophobic

A. 4 Leucine and valine

B. 1 Two cysteines

C. 2 Aspartic acid and lysine

D. 3 Serine and threonine

11

Globular and Fibrous Proteins

Globular proteins Fibrous proteins

“spherical” shape long, thin fibers

Insulin Hair

Hemoglobin Wool

Enzymes Skin

Antibodies Nails

12

Quaternary Structure

• Proteins with two or more chains• Example is hemoglobin

Carries oxygen in blood

Four polypeptide chains

Each chain has a heme group to

bind oxygen

13

Learning Check P3

Identify the level of protein structure1. Primary 2. Secondary3. Tertiary 4. Quaternary

A. Beta pleated sheetB. Order of amino acids in a proteinC. A protein with two or more peptide chainsD. The shape of a globular proteinE. Disulfide bonds between R groups

14

Solution P3

Identify the level of protein structure1. Primary 2. Secondary3. Tertiary 4. Quaternary

A. 2 Beta pleated sheetB. 1 Order of amino acids in a proteinC. 4 A protein with two or more peptide chainsD. 3 The shape of a globular proteinE. 3 Disulfide bonds between R groups

15

Protein Hydrolysis

• Break down of peptide bonds • Requires acid or base, water and

heat• Gives smaller peptides and

amino acids • Similar to digestion of proteins

using enzymes• Occurs in cells to provide amino

acids to synthesize other proteins and tissues

16

Hydrolysis of a Dipeptide

H3N CH

CH3

C

O

N

H

CH C

OCH2

OH

OH

+

H3N CH

CH3

COH

O

+ CH C

OCH2

OH

OHH3N

H2O, H+

++

heat

17

Denaturation

Disruption of secondary, tertiary and quaternary protein structure byheat/organics

Break apart H bonds and disrupt hydrophobic attractions acids/ bases

Break H bonds between polar R groups andionic bonds

heavy metal ions React with S-S bonds to form solids

agitation Stretches chains until bonds break

18

Applications of Denaturation

• Hard boiling an egg• Wiping the skin with alcohol swab for

injection• Cooking food to destroy E. coli.• Heat used to cauterize blood vessels• Autoclave sterilizes instruments• Milk is heated to make yogurt

19

Learning Check P4

What are the products of the complete hydrolysis of Ala-Ser-Val?

20

Solution P4

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

alanine

serine

valine

21

Learning Check P5

Tannic acid is used to form a scab on a burn. An egg becomes hard boiled when placed in hot water. What is similar about these two events?

22

Solution P5

Acid and heat cause a denaturation of protein. They both break bonds in the secondary and tertiary structure of protein.