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Chapter 16 Amino Acids, Proteins, and Enzymes

16.1 Functions of Proteins16.2 Amino Acids

16.3 Amino Acids as Acids and Bases

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Functions of Proteins

Proteins perform many different functions in the body.

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Amino Acids

Amino acids • are the building blocks of proteins.• contain a carboxylic acid group and an amino group on

the alpha () carbon.• are ionized in solution.• each contain a different side group (R). R side chain R

│ + │H2N—C —COOH H3N—C —COO−

│ │ H H ionized form

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Examples of Amino Acids

H + │H3N—C—COO−

│ H glycine

CH3

+ │H3N—C—COO−

│ H alanine

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Types of Amino Acids

Amino acids are classified as• nonpolar (hydrophobic)

with hydrocarbon side chains.

• polar (hydrophilic) with polar or ionic side chains.

• acidic (hydrophilic) with acidic side chains.

• basic (hydrophilic) with –NH2 side chains.

Nonpolar Polar

AcidicBasic

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Nonpolar Amino Acids

An amino acid is nonpolar when the R group is H, alkyl, or aromatic.

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Polar Amino Acids

An amino acid is polar when the R group is an alcohol, thiol, or amide.

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Acidic and Basic Amino Acids

An amino acid is • acidic when the R group is a carboxylic acid.• basic when the R group is an amine.

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Learning Check

Identify each as (1) polar or (2) nonpolar. +

A. H3N–CH2–COO− (Glycine)

CH3 | CH–OH + │

B. H3N–CH–COO − (Threonine)

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Solution

Identify each as (1) polar or (2) nonpolar. +

A. H3N–CH2–COO− (Glycine) (2) nonpolar

CH3

| CH–OH + │

B. H3N–CH–COO − (Threonine) (1) polar

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Fischer Projections of Amino Acids

Amino acids • are chiral except glycine.• have Fischer projections that are stereoisomers.• that are L are the only amino acids used in proteins.

L-Alanine D-Alanine L-Cysteine D-Cysteine

CH2SH

H2N H

COOH

CH2SH

H NH2 COOH

CH3

H NH2

COOH

CH3

H2N H

COOH

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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).

O O ║ + ║

NH2—CH2—C—OH H3N—CH2—C—O–

glycine zwitterion of glycine

Zwitterions

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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 pH > pI

Charge: 0 Charge: 1-

Amino Acids as Acids

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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 pH< pI

Charge: 0 Charge: 1+

Amino Acids as Bases

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pH and ionization

H+ OH–

+ +H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO– positive ion zwitterion negative ionlow pH pI high pH

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Separation of Amino Acids

When an electric current is used to separate a mixture ofamino acids• 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 does not migrate.• the amino acids are identified as separate bands on

the filter paper or thin layer plate.

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Separation of Amino Acids

With an electric current, a mixture of lysine, aspartate,and valine are separated.

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CH3 CH3

+ | |H3N—CH—COOH H2N—CH—COO–

(1) (2)Which structure represents:

A. Alanine at a pH above its pI?

B. Alanine at a pH below its pI?

Learning Check

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CH3 CH3

+ | |H3N—CH—COOH H2N—CH—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