Chapter 17 Amino Acids and Proteins

Copyright © 1804 Pearson Education Inc., publishing as Benjamin Cummings. 1

17.1 Functions of Proteins

17.2 Amino Acids

17.3 Amino Acids as Acids and Bases

Chapter 17 Amino Acids and Proteins


Functions of Proteins

Proteins perform many different functions.


Amino acids: Are the building blocks of proteins. Contain a carboxylic acid group and an amino

group on the alpha () carbon. Have different side groups R that give each

amino acid unique characteristics. R side chain

|H2N—C —COOH General structure of an

| -amino acid H

17.3 Amino Acids


Nonpolar Amino Acids Amino acids are classified as nonpolar when the

R groups are H, alkyl, or aromatic.


Polar Amino Acids Amino acids are classified as polar when the R

groups are alcohols, thiols, or amides.


Acidic and Basic Amino Acids Amino acids are classified as acidic when the R

group is a carboxylic acid. Amino acids are classified as basic when the R

group is an amine.


17.2 Chiral Objects

Chiral compounds have the same number of atoms arranged differently in space.

A chiral carbon atom is bonded to four different groups.

Your hands are chiral. Try to superimpose your thumbs, palms, back of hands, and little fingers.


Mirror Images The mirror images of chiral compounds cannot

be superimposed. When the H and I atoms are aligned, the Cl and

Br atoms are on opposite sides.


Achiral Structures are Superimposable

When the mirror image of an achiral structure is rotated, the structure can be aligned with the initial structure. Thus this mirror image is superimposable.


Some Everyday Chiral and Achiral Objects


Learning Check

Identify each as a chiral or achiral compound.


Solution

Identify each as a chiral or achiral compound.

Chiral Achiral Chiral

C

Cl

CH3H

CH2CH3

C

Cl

CH3H

H

A B

C

Cl

CH3H

Br

C


Both the –NH2 and the –COOH groups in an amino acid undergo ionization in water.

A zwitterion forms that has + and – charge. At the isoelectric point (pI), the + and – charges in

the zwitterion are equal. +

NH2—CH2—COOH H3N—CH2—COO–

Glycine Zwitterion of glycine

Zwitterions


In solutions more basic than the pI, the —NH3+

in the amino acid donates a proton.

+ OH–

H3N—CH2—COO– H2N—CH2—COO– Zwitterion Negative ion

at pI Higher pH

Amino Acids as Acids


In solution more acidic than the pI, the COO- in the amino acid accepts a proton.

+ H+ +

H3N—CH2—COO– H3N—CH2—COOH Zwitterion Positive ionat pI Low pH

Amino Acids as Bases


pH and Ionization Acidic amino acids such as aspartic acid have a

second carboxyl group that can donate and accept protons.

The pI for aspartic acid occurs at a pH of 2.8.


Electrophoresis

Electrophoresis separates amino acids according to their isoelectric points.

The positively charged amino acids move towards the negative electrode.

The negatively charged amino acids move toward the positive electrode.

An amino acid at its pI will not migrate in either direction.


Separation of Amino Acids When electrophoresis is completed, the amino

acids are identified as separate bands on the filter paper or thin layer plate.


CH3 CH3

+ | |H3N—CH—COOH H2N—CH2—COO–

(1) (2)

Which structure represents:

A. Alanine at a pH above its pI?

B. Alanine at a pH below its pI?

Learning Check


CH3 CH3

+ | |H3N—CH—COOH H2N—CH2—COO–

(1) (2)

Which structure represents:

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

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

Solution


The Peptide Bond

A peptide bond is an amide bond between the carboxyl group of one amino acid and the amino group of the next amino acid.

O CH3 O

+ || + | ||H3N—CH2—C—OH + H3N—CH—C—O–

O H CH3 O + || | | ||

H3N—CH2—C—N—CH—C—O– peptide bond


A Dipeptide

A peptide is named from the free amine (NH3+)

using -yl endings for the names of the amino acids. The last amino acid with the free carboxyl group

(COO-) uses its amino acid name.


Write the names and three-letter abbreviations of the amino acids in the tripeptides that could form from two glycine and one alanine.

Learning Check


Write the names and three-letter abbreviations of the amino acids in the tripeptides that could form from two glycine and one alanine.

Gly-Gly-Ala Glycylglycylalanine

Gly-Ala-Gly Glycylalanylglycine

Ala-Gly-Gly Alanylglycylglycine

Solution


Write the name of the following tetrapeptide using amino acid names and three-letter abbreviations.

Learning Check

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


Ala-Leu-Cys-Met

Alanylleucylcysteinylmethionine

Solution

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

Ala Leu Cys Met


A polypeptide containing 50 or more amino acids is called a protein.

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

Ala-Leu-Cys-Met

17.7 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


Primary Structures

The nonapeptides oxytocin and vasopressin have similar primary structures.

Only the amino acids at positions 3 and 8 differ.


InsulinInsulin: Was the first

protein to have its primary structure determined.

Of humans has a primary structure that is similar to the insulin of pigs and cows.


The secondary structures of proteins indicate the arrangement of the polypeptide chains in space.

The alpha helix is a three-dimensional arrangement of the polypeptide chain that gives a corkscrew shape like a coiled telephone cord.

17.9 Secondary Structure: Alpha Helix


Alpha Helix 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.


Secondary Structure: 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.


A triple helix: Consists of three alpha helix chains. 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.

Secondary Structure: Triple Helix


Essential amino acids: Are the ten amino

acids that are not synthesized by the body.

Must be obtained from the diet.

Essential Amino Acids


Essential amino acids are: Found in milk and eggs

(complete proteins). Not all found in grains

and vegetables (incomplete proteins). Obtained by combining

two or more vegetables that provide complementary proteins.

Essential Amino Acids


Indicate the type of structure as:

1) primary 2) alpha helix

3) beta pleated sheet 4) triple helix

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 aminoacids.

D. Three peptide chains woven like a rope.

Learning Check


Indicate the type of 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.C. 2 Corkscrew shape with H bonds between amino acids.D. 4 Three peptide chains woven like a rope.

Solution


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

different parts of the peptide chain. Is stabilized by

Hydrophobic and hydrophilic interactions Salt bridgesHydrogen bondsDisulfide bonds

17.10 Tertiary Structure


Tertiary Structure


Tertiary Structure

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


Globular Proteins

Globular proteins: Have compact,

spherical shapes. Carry out synthesis,

transport, and metabolism in the cells.

Such as myoglobin store and transport oxygen in muscle.

Myoglobin


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 amounts of beta-pleated sheet structures.


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

Learning check


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

Solution


17.11 Quaternary Structure The quaternary structure

contains two or more tertiary subunits.

Hemoglobin contains 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 Structural Levels



Identify the level of protein structure as:1) 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

Learning Check


Identify the level of protein structure

1. Primary 2. Secondary

3. Tertiary 4. Quaternary

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

Solution


Protein hydrolysis: Splits the peptide bonds to give smaller

peptides and amino acids. Occurs in the digestion of proteins. Occurs in cells when amino acids are needed

to synthesize new proteins and repair tissues.

17.12 Chemical Properties of Proteins


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.

+

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.

Denaturation


Denaturation Heat and organic compounds break apart H

bonds and disrupt hydrophobic interactions. Acids and bases break H bonds between polar

R groups and disrupt ionic bonds. Heavy metal ions react with S-S bonds to form

solids. Agitation such as whipping stretches chains

until bonds break.


Denaturation of protein occurs when:

An egg is cooked. The skin is wiped with

alcohol. Heat is used to cauterize

blood vessels. Instruments are sterilized in

autoclaves.

Applications of Denaturation


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

Learning Check


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

alanine

serine

valine

Solution


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?

Learning check


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

Solution


Chapter Summary 20 different amino acids occur in proteins. Amino acid side chains have acidic or basic

functional groups, or neutral groups that are either polar or nonpolar.

Each amino acid has a isoelectric point – the pH at which the numbers of positive and negative charges in a solution are equal.

A molecule can be identified as a chiral molecule if it contains a carbon atom that has four different groups.


Chapter Summary Contd. Chiral molecules do not have a mirror plane. Mirror

images of chiral molecules are non-superimposable. All amino acids except glycine have four different

groups bonded to the -carbon, as a result they are chiral.

Proteins are polymer of amino acids (polypeptides). Primary structure of a protein is the sequence in which

the amino acids are connected by peptide bonds.


Chapter Summary Contd. Primary structures are written with the amino

terminal amino acid on the left and carboxyl terminal amino acid on the right. Noncovalent interactions between side chain groups determine the overall three dimensional shape of a protein molecule.

Secondary structure of a protein is the regular repeating three dimensional structures held together by hydrogen bonding between the backbone atoms within a chain or in adjacent chain.


Chapter Summary Contd. Tertiary structure of a protein is the overall three

dimensional shape of a folded protein. Quaternary structure of a protein is the structure

that incorporates more than one peptide chain. The peptide bonds are broken by hydrolysis.

Denaturation is the loss of overall structure while retaining its primary structure.

Agents that can cause denaturation of protein includes heat, mechanical agitation, polar organic solvents, detergents, salts, etc.


End of Chapter 17

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Chapter 17 Amino Acids and Proteins