PROTEIN CHEMISTRY-1AMINO ACIDS

Dr. Vijay Marakala, MBBS, MD.

BIOCHEMISTRY

IMS, MSU.

OUTLINE

Introduction

General structure of amino acid

Classification of amino acids

Importance of amino acids

Peptide bond

Biologically important peptides

LEARNING OBJECTIVES

Know the general structure of an amino acid and the structures of the 20 different R groups.

Understand how peptide bonds link amino acid residues in a polypeptide

Biomedical importance of amino acids

PROTEIN CHEMISTRY-1AMINO ACIDSIntroduction

Proteins are the most abundant molecules in living systems

Every life process depends on this class of molecules. E.g:- digestion, absorption and metabolism etc

All proteins are linear polymers of AMINO ACIDS that are linked together by covalent PEPTIDE BONDS

AMINO ACIDS

More than 300 different amino acids have been described in nature

Only as constituents of mammalian proteins.

These 20 amino acids are called STANDARD AMINO ACIDS

General structure of amino acid

C–OHH–N C

HH

R

R-group (Variable)

-carbon

side chain

O

carboxyl

All amino acids differ from each other with respect to their side chain group

amino

General structure of amino acid At physiologic pH (approximately pH 7.4)

At physiologic pH, the carboxyl group is dissociated, forming the negatively charged carboxylate ion (–COO-), and the amino group is protonated (–NH3

+).

General structure of amino acid

C

α-CARBON ATOM

β-CARBON ATOM

General structure of amino acid

Generally amino acids present in proteins are

L-α-amino acids

Classification of amino acids

I. Based on chemical structure of side chain

II. Based on nutritional requirement

III. Based on polarity of the sidechains

IV. Based on metabolic products of amino acids

Classification of amino acids

1. Aliphatic amino acids

2. Aromatic amino acids

3. Hydroxyl amino acids

4. Sulphur containing amino acids

5. Charged amino acids

6. Amide group containing amino acids

7. Imino acids

Classification of amino acidsALIPHATIC AMINO ACIDS

Unbranched amino acids Branched chain amino acids

Classification of amino acids

AROMATIC AMINO ACIDS

Classification of amino acids

HYDROXY AMINO ACIDS

Classification of amino acids

SULPHUR CONTAINING AMINO ACIDS

Classification of amino acids

CHARGED -ACIDIC AMINO ACIDS

Classification ofamino acids

CHARGED -BASIC AMINO ACIDS

Classification of amino acidsAMIDE GROUP [-CONH2] CONTAINING AMINO ACIDS

Classification of amino acids

IMINO ACIDS

Classification of amino acids

BASED ON NUTRITIONAL REQUIREMENT

Essential amino acids

Semi-essential

amino acids

Non-essential

amino acids

Classification of amino acids

BASED ON NUTRITIONAL REQUIREMENT

Essential amino acids are required for health but not synthesized in the body and therefore

have to be supplied through the diet.

Isoleucine, Leucine, Valine

Threonine,

Lysine, Methionine,

Phenylalanine, Tryptophan

PVT TIM HALL

Classification of amino acidsBASED ON NUTRITIONAL REQUIREMENT

Growing children require them in food.

But they are not essential for the adult

Histidine and arginine

Classification of amino acidsBASED ON NUTRITIONAL REQUIREMENT

These amino acids are required for health and are synthesized in the body by metabolic pathways.

Glycine, Alanine, Cysteine, Serine,

Tyrosine, Aspartic acid, Glutamic acid,

Asparagine, Glutamine and Proline.

Classification of amino acidsBASED ON POLARITY OF THE SIDECHAINS

HYDROPHILIC OR POLAR AMINO ACIDS

Acidic amino acids

Basic amino acids

Glycine, Serine, Threonine, Cysteine, Tyrosine

Glutamine and Asparagine

HYDROPHOBIC OR NON-POLAR AMINO ACIDS

Alanine, Valine, Leucine, Isoleucine

Phenylalanine, Tryptophan

Methionine

Proline

III. Based on polarity of the sidechains

Classification of amino acidsBASED ON METABOLIC PRODUCTS OF AMINO ACIDS

Leucine

Lysine, Isoleucine, Phenylalanine,

Tyrosine, Tryptophan

All other amino acids

IV. Based on metabolic products of amino acids

21st and 22nd

amino acids

BIOMEDICAL IMPORTANCE OF AMINO ACIDS

1. Formation of proteins and peptides

2. Amino acids are precursors of important biomolecules such as adrenaline, melanin, neurotransmitters, melatonin etc

3. Enzyme activity : -SH group of cysteine

4. Transport and storage form of ammonia

5. As a buffer

6. Detoxification reaction

7. Formation of biologically important compound

PEPTIDE BOND

PEPTIDE BONDS

AA 1 AA 2

AA 1 AA 2 AA 3

AA 1 AA 2 AA 3 AA 4

DIPEPTIDE

TRIPEPTIDE

TETRAPEPTIDE

Amino acid residues

PEPTIDE BOND

The linkage formed between amino acids is an amide bond called a peptide bond

This linkage formed by simple condensation of the α-carboxyl group of one amino acid with the α-amino group of another.

PEPTIDE BOND FORMATION

Biologically important peptides

Insulin – 51 amino acids

Glucagon -29 amino acids

ACTH – 39 amino acids

TRH – 3 amino acids

Oxytocin – 9 amino acids

Antidiuretic hormone – 9 amino acids

Glutathione – 3 amino acids

Next class Protein structure