1

20.1 Functions of Proteins

20.2 Amino Acids

20.3 Amino Acids as Acids and Bases

Chapter 20 Amino Acids and Proteins

2

Functions of Proteins Proteins perform many different functions.

3

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

Amino Acids

4

Nonpolar Amino Acids Amino acids are classified as nonpolar when the

R groups are H, alkyl, or aromatic.

5

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

groups are alcohols, thiols, or amides.

6

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.

7

Fischer Projections of Amino Acids

All amino acids except glycine are chiral. Amino acids have stereoisomers. In biological systems, only L amino acids are used

in proteins.

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

COOH

HH2N

CH3

COOH

NH2H

CH3

COOH

NH2 H

CH2SH

COOH

HH2N

CH2SH

8

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

9

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

10

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

11

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.

12

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.

13

Separation of Amino Acids When electrophoresis is completed, the amino

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