Amino Acids,Peptides,

and Proteins

Introduction to Amino Acids There are about 26 amino acids, many others are

also known from a variety of sources. Amino acids are the building blocks used to

make proteins and peptides. The different amino acids have a variety of

structural parts which result in different polarities and solubilities.

Each amino acid has at least one amine and one acid functional group

The amino group may be at α or β or γ positions relative to the carboxyl group

H2N C

H

C

R side chain

O

OH carboxyl groupamino group

Amino acids as zwitterions

Due to internal transfer of a proton from the -COOH group to the -NH2 group the amino acid has both a negative charge at the carboxylate group and a positive charge at the ammonium group . This salt-like ion is called a zwitterion.

Amino acids are crystallin solids with relatively high melting points due to this salt-like nature

In solutions the charge of the amino acid changes

H2N C

H

C

R

O

OH H2N C

H

C

R

O

O

H

Zwitterion

H2N C

H

CR

O

O

H

ZwitterionpH = 7

H2N C

H

CR

O

OH

H

pH = 0

H2N C

H

CR

O

O

pH = 11

Classification of Amino Acids

There are basically four different classes of amino acids determined by different side chains:

1. Non polar /Neutral R = H, CH3, alkyl groups, aromatic

2. Polar / Neutral R contain polar groups –CH2OH, –CH2SH, –CH2CO–

NH2

3. Polar/Acidic R contain –COOH group 4. Polar/ BasicR contain –NH2 group

1) Non polar /Neutral Amino Acids

Glycine Alanine

Valine LeucineIsoleucine

Phenylalanine

Proline

H3N CH C

CH2

O

O

CH CH3

CH3

H3N CH C

CH

O

O

CH3

CH3

H3N CH C

CH

O

O

CH3

CH2

CH3

H3N CH C

H

O

O

H3N CH C

CH3

O

OH3N CH C

CH2

O

O

H2N

C O

O

serineThreonine

Cysteine

Methionine

2) Polar /Neutral Amino Acids

Tyrosine

H3N CH C

CH2

O

O

OH

H3N CH C

CH

O

O

OH

CH3

H3N CH C

CH2

O

O

SH

H3N CH C

CH2

O

O

CH2

S

CH3

H3N CH C

CH2

O

O

OH

 Asparagine Glutamine

H3N CH C

CH2

O

O

HN

Tryptophan

3) Polar /Basic Amino AcidsH3N CH C

CH2

O

O

CH2

CH2

CH2

NH2

Histidine

LysineH3N CH C

CH2

O

O

CH2

CH2

NH

C

NH2

NH

Arginine

4) Polar / Acidic Amino Acids

Aspartatic acidGlutamic acid

H3N CH C

CH2

O

O

C

O-

O

H3N CH C

CH2

O

O

CH2

C

OH

O

Configuration of Amino Acids

Most of the naturally occurring amino acids have L- configuration while naturally occurring carbohydrates have the D configuration

Proteins and peptidesProteins are the basis for the major

structural components of animal and human tissue.

Proteins are natural polymer molecules consisting of amino acid units linked together by amide or peptide bonds

The number of amino acids in proteins may range from two to several thousand.

RHC

NH3

C O

O

C

OHN

HC

R'

C

OHN

HC

R''

C

OHC

R

HN

Amide bonds

Formation of a Peptide

Amide bond is formed between the –COOH of an amino acid and the –NH2 of the next amino acid

Polymerization A condensation reaction between the carboxyl

of one amino acid and the amino group of another forming a an amide linkage known as peptide bond.

This polymerization is used to form peptides, proteins and enzymes

Peptides• Oligopeptides − condensation of 2 – 10 AA

units • Polypeptides − condensation of 11 – 100 AA

units Proteins − more than 100 AA units

Reactions of Amino acids

The Strecker synthesis

Synthesis of Amino acids

End of the chapterThank you