Chapter 19 Amino Acids and Proteins - OU Campus Logincms.cerritos.edu/uploads/ahoang/Powerpoint/Unit...

Chapter 19 Amino Acids and

Proteins

19.1 Proteins and Amino Acids

19.2 Amino Acids as Acids and Bases

1

Copyright © 2007 by Pearson Education, Inc.

Publishing as Benjamin Cummings

Functions of Proteins

� Proteins perform many different functions in the body.

TABLE 19.1

2

Amino Acids

Amino acids

� Are the building blocks of proteins.

� Contain a carboxylic acid group and an amino group

on the alpha (α) carbon.

� Are ionized in solution.

3

� Are ionized in solution.

� Each contain a different side group (R).

R R

│ + │H2N—C —COOH H3N—C —COO−

│ │H H

ionized form

Examples of Amino Acids

H

+ │H3N—C—COO−

│

H glycine

4

H glycine

CH3

+ │H3N—C—COO−

│H alanine

Types of Amino Acids

Amino acids are classified as

� Nonpolar (hydrophobic)

with hydrocarbon side

chains.

� Polar (hydrophilic) with

Nonpolar Polar

5

� Polar (hydrophilic) with

polar or ionic side chains.

� Acidic (hydrophilic) with

acidic side chains.

� Basic (hydrophilic) with

–NH2 side chains.

Acidic Basic



Nonpolar Amino Acids

A nonpolar amino acid has

� An R group that is H, an alkyl group, or aromatic.

6



Polar Amino Acids

A polar amino acid has

� An R group that is an alcohol, thiol, or amide.

7Copyright © 2007 by Pearson Education, Inc.


Acidic and Basic Amino Acids

An amino acid is

� Acidic with a carboxyl R group (COO−).

� Basic with an amino R group (NH3+).

Basic Amino Acids

8Copyright © 2007 by Pearson Education, Inc.


Learning Check

Identify each as (P) polar or (NP) nonpolar.

+

A. H3N–CH2–COO− (Glycine)

CH

9

CH3

|

CH–OH

+ │B. H3N–CH–COO− (Threonine)

Solution

Identify each as (P) polar or (NP) nonpolar.

+

A. H3N–CH2–COO− (Glycine) (NP) nonpolar

CH

10

CH3

|

CH–OH

+ │B. H3N–CH–COO− (Threonine) (P) polar

Fischer Projections of Amino Acids

Amino acids

� Are chiral except for glycine.

� Have Fischer projections that are stereoisomers.

� That are L are used in proteins.

11

L-alanine D-alanine L-cysteine D-cysteine

CH2SH

H2N H

COOH

CH2SH

H NH2

COOH

CH3

H NH2

COOH

CH3

H2N H

COOH

A zwitterion

• Has charged —NH3+ and COO- groups.

• Forms when both the —NH2 and the —COOH groups

in an amino acid ionize in water.

• Has equal + and − charges at the isoelectric point (pI).

Zwitterions and Isoelectric Points

12

• Has equal + and − charges at the isoelectric point (pI).

O O

║ + ║NH2—CH2—C—OH H3N—CH2—C—O–

Glycine Zwitterion of glycine

In solutions more basic than the pI,

� The —NH3+ in the amino acid donates a proton.

+ OH–

H N—CH —COO– H N—CH —COO–

Amino Acids as Acids

13

H3N—CH2—COO– H2N—CH2—COO–

Zwitterion Negative ionat pI pH > pICharge: 0 Charge: 1−

In solutions more acidic than the pI,

� The COO− in the amino acid accepts a proton.

+ H++

H N—CH —COO– H N—CH —COOH

Amino Acids as Bases

14

H3N—CH2—COO– H3N—CH2—COOH

Zwitterion Positive ionat pI pH< pICharge: 0 Charge: 1+

pH and Ionization

H+ OH−

+ +

H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO–

15

3 2 3 2 2 2

positive ion zwitterion negative ion

(at low pH) (at pI) (at high pH)

Electrophoresis: Separation of

Amino Acids

In electrophoresis, an electric current is used to separate

a mixture of amino acids, and

� The positively charged amino acids move toward the

negative electrode.

16

� The negatively charged amino acids move toward the

positive electrode.

� An amino acid at its pI does not migrate.

� The amino acids are identified as separate bands on the filter paper or thin-layer plate.

Electrophoresis

With an electric current, a mixture of lysine, aspartate,

and valine are separated.

17



CH3 CH3

+ | |H3N—CH—COOH H2N—CH—COO–

(1) (2)

Learning Check

18

Which structure represents:

A. Alanine at a pH above its pI?

B. Alanine at a pH below its pI?

CH3 CH3

+ | |H3N—CH—COOH H2N—CH—COO–

(1) (2)

Solution

19

Which structure represents:

A. Alanine at a pH above its pI? (2)

B. Alanine at a pH below its pI? (1)


Proteins

19.3

Formation of Peptides

20

Copyright © 2007 by Pearson Education, Inc


The Peptide Bond

A peptide bond

� Is an amide bond.

� Forms between the carboxyl group of one amino acid and the amino group of the next amino acid.

O CH3 O

21

O CH3 O+ || + | ||H3N—CH2—C—O– + H3N—CH—C—O–

O H CH3 O+ || | | ||

H3N—CH2—C—N—CH—C—O– + H2Opeptide bond

Formation of A Dipeptide

22

Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings

Learning Check

Write the dipeptide Ser-Thr.

OH CH3

| |

CH2 O HCOH O

+ | ║ + | ║

23

+ | ║ + | ║H3N─CH─C─O – + H3N─CH─C─O–

Ser Thr

Naming Dipeptides

A dipeptide is named with

� A -yl ending for the N-terminal amino acid.

� The full amino acid name of the free carboxyl group

(COO-) at the C-terminal end.

25


Write the three-letter abbreviations and names of the

tripeptides that could form from two glycine and one

alanine.

Learning Check

26

Write the names and three-letter abbreviations of the

tripeptides that could form from two glycine and one

alanine.

Glycylglycylalanine Gly-Gly-Ala

Solution

27

Glycylglycylalanine Gly-Gly-Ala

Glycylalanylglycine Gly-Ala-Gly

Alanylglycylglycine Ala-Gly-Gly

Learning Check

What are the possible tripeptides formed from one

each of leucine, glycine, and alanine?

28

Solution

Tripeptides possible from one each of leucine,

glycine, and alanine

Leu-Gly-Ala

Leu-Ala-Gly

Ala-Leu-Gly

29

Ala-Leu-Gly

Ala-Gly-Leu

Gly-Ala-Leu

Gly-Leu-Ala

Learning Check

Write the three-letter abbreviation and name for the

following tetrapeptide:

CH3

│CH3 S

30

CH3 S

│ │CH–CH3 SH CH2

│ │ │CH3 O H CH2 O H CH2O H CH2 O

+ │ ║ │ │ ║ │ │ ║ │ │ ║H3N–CH–C–N–CH–C–N–CH–C–N–CH–CO–

Solution

Ala-Leu-Cys-Met Alanylleucylcysteylmethionine

CH3

│CH3 S

│ │

31

│ │CH–CH3 SH CH2

│ │ │CH3 O H CH O H CH2O H CH2 O

+ │ ║ │ │ ║ │ │ ║ │ │ ║H3N–CH–C–N–CH–C–N–CH–C–N–CH–CO–

Ala Leu Cys Met


Proteins

19.4

Protein Structure: Primary and Secondary Levels

32



Primary Structure of Proteins

The primary structure of a protein is

� The particular sequence of amino acids.

� The backbone of a peptide chain or protein.

33Ala─Leu─Cys─Met

CH3

SH

CH2

CH3

S

CH2

CH2CH O

O-CCH

H

N

O

CCH

H

N

O

CCH

H

N

O

CCHH3N

CH3

CH3CH



Primary Structures

The nonapeptides oxytocin and vasopressin

� Have similar primary structures.

� Differ only in the amino acids at positions 3 and 8.

34


Primary Structure

of Insulin

Insulin

� Was the first protein to

have its primary structure

determined.

� Has a primary structure of

35

� Has a primary structure of

two polypeptide chains

linked by disulfide bonds.

� Has a chain A with 21

amino acids and a chain B

with 30 amino acids.


Secondary Structure – Alpha Helix

The secondary structures of proteins indicate the

three-dimensional spatial arrangements of the

polypeptide chains.

36

An alpha helix has

� A coiled shape held in place by hydrogen bonds

between the amide groups and the carbonyl

groups of the amino acids along the chain.

� Hydrogen bonds between the H of a –N-H group

and the O of C=O of the fourth amino acid down

the chain.

Secondary Structure – Alpha Helix

37Copyright © 2007 by Pearson Education, Inc


Secondary Structure – Beta

Pleated Sheet

A beta-pleated sheet is a secondary structure that

� Consists of polypeptide chains arranged side by

side.

� Has hydrogen bonds between chains.

38

� Has hydrogen bonds between chains.

� Has R groups above and below the sheet.

� Is typical of fibrous proteins such as silk.

Secondary Structure: β-Pleated

Sheet

39


Secondary Structure: Triple Helix

A triple helix

� Consists of three alpha helix chains woven together.

� Contains large amounts glycine, proline, hydroxy

40

� Contains large amounts glycine, proline, hydroxy proline, and hydroxylysine that contain –OH groups for hydrogen bonding.

� Is found in collagen, connective tissue, skin, tendons, and cartilage.


Indicate the type of protein structure as

1) primary 2) alpha helix

3) beta-pleated sheet 4) triple helix

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

Learning Check

41

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.

D. Three peptide chains woven like a rope.

Indicate the type of protein structure as:

1) primary 2) alpha helix

3) beta-pleated sheet 4) triple helix

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

B. 1 Sequence of amino acids in a polypeptide chain.

Solution

42

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.


Proteins

19.5

Protein Structure: Tertiary and Quaternary Levels

43



Essential amino acids

� Must be obtained from the diet.

� Are the ten amino acids not

Essential Amino Acids

TABLE 19.3

44

acids not synthesized by the body.

� Are in meat and

diary products.

� Are missing (one or more) in grains and vegetables.


The tertiary structure of a protein

� Gives a specific three dimensional shape to the

polypeptide chain.

� Involves interactions and cross links between

Tertiary Structure

45

different parts of the peptide chain.

� Is stabilized by

Hydrophobic and hydrophilic interactions. Salt bridges.

Hydrogen bonds.

Disulfide bonds.

Tertiary Structure

� The

interactions of

the R groups

give a protein

its specific

46

its specific

three-

dimensional

tertiary

structure.


Tertiary Structure

TABLE 19.5

47

Globular Proteins

Globular proteins

� Have compact, spherical shapes.

� Carry out synthesis, transport, and

Myoglobin

48

transport, and metabolism in the cells.

� Such as myoglobin store and transport oxygen in muscle.



Fibrous Proteins

Fibrous proteins

� Consist of long, fiber-like shapes.

� Such as alpha keratins make up hair, wool, skin, and nails.

� Such as feathers contain beta keratins with large

49

� Such as feathers contain beta keratins with large amounts of beta-pleated sheet structures.


Select the type of tertiary interaction

1) disulfide 2) ionic

3) H bonds 4) hydrophobic

Learning Check

50

A. Leucine and valine

B. Two cysteines

C. Aspartic acid and lysine

D. Serine and threonine

Select the type of tertiary interaction as:

1) disulfide 2) ionic

3) H bonds 4) hydrophobic

A. 4 Leucine and valine

Solution

51

B. 1 Two cysteines

C. 2 Aspartic acid and lysine

D. 3 Serine and threonine

Quaternary Structure

The quaternary structure

� Is the combination of two or

more tertiary units.

� Is stabilized by the same

interactions found in tertiary

hemoglobin

52

interactions found in tertiary

structures.

� Of hemoglobin consists of two

alpha chains and two beta

chains. The heme group in

each subunit picks up oxygen

for transport in the blood to the

tissues.



Summary of Protein Structure

TABLE 19.6

53


Summary of Protein Structures

54


Identify the level of protein structure as:

1) Primary 2) Secondary

3) Tertiary 4) Quaternary

A. Beta-pleated sheet

Learning Check

55

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

Identify the level of protein structure

1. Primary 2. Secondary

3. Tertiary 4. Quaternary

A. 2 Beta-pleated sheet.

Solution

56

A. 2 Beta-pleated sheet.

B. 1 Order of amino acids in a protein.

C. 4 A protein with two or more peptide chains.

D. 3 The shape of a globular protein.

E. 3 Disulfide bonds between R groups.

19.6

Protein Hydrolysis and Denaturation


Proteins

57



Protein hydrolysis

� Splits the peptide bonds to give smaller peptides

and amino acids.

� Occurs in the digestion of proteins.

Protein Hydrolysis

58

� Occurs in the digestion of proteins.

� Occurs in cells when amino acids are needed to

synthesize new proteins and repair tissues.

Hydrolysis of a Dipeptide

� In the lab, the hydrolysis of a peptide requires acid

or base, water and heat.

� In the body, enzymes catalyze the hydrolysis of

proteins.

59

proteins.

+

H3N CH COH

OCH3

+

H2O, H+

++

heat,

CH2

OH

H3N CH C

O

N

H

CH C

O

OH

CH3

CH2

OH

CH C

O

OHH3N

Denaturation involves

� The disruption of bonds in the secondary, tertiary

and quaternary protein structures.

� Heat and organic compounds that break apart H

Denaturation

60

� Heat and organic compounds that break apart H

bonds and disrupt hydrophobic interactions.

� Acids and bases that break H bonds between polar

R groups and disrupt ionic bonds.

� Heavy metal ions that react with S-S bonds to form

solids.

� Agitation such as whipping that stretches peptide

chains until bonds break.

Denaturation of protein occurs

when

� An egg is cooked.

� The skin is wiped with alcohol.

Applications of Denaturation

61

alcohol.

� Heat is used to cauterize blood vessels.

� Instruments are sterilized in autoclaves.



What are the products of the complete hydrolysis of

the peptide Ala-Ser-Val?

Learning Check

62

The products of the complete hydrolysis of the

peptide Ala-Ser-Val are

alanine

Solution

63

alanine

serine

valine

Tannic acid is used to form a scab on a burn. An

egg is hard boiled by placing it in boiling water.

What is similar about these two events?

Learning Check

64

Acid and heat cause the denaturation of protein.

They both break bonds in the secondary and tertiary

structures of proteins.

Solution

65

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