Biology_Today_-_October_2016

AMINO ACIDS

Amino acids are organic acids (with carboxylic group –COOH) having amino group (–NH 2 ) generally attached
to α-carbon or carbon next to the carboxylic group. Carboxylic group provides an acidic property to the amino
acid while amino group gives it a basic reaction. The α-carbon also bears a variable hydrocarbon or alkyl group
R and hydrogen.

Neutral amino acids

They have one amino group and

one carboxylic group (mono-amino

monocarboxylic) with noncyclic

hydrocarbon chain, e.g., glycine,

alanine, valine.

COOH

H 2 NHC

CH 3

Alanine

COOH

H 2 NHC

H

Glycine

Acidic amino acids

COOH

H 2 NHC

CH 2. COOH

Aspartic acid

COOH

COOH

H 2 NHC

(CH 2 ) 2

Glutamate

These amino acids have

an extra carboxylic group

(mono-amino dicarboxylic),

e.g., glutamate (glutamic

acid), asparate (aspartic

acid).

Aromatic amino acids

They posses cyclic structure with a straight side chain bearing carboxylic

and amino group, e.g., phenylalanine, tryptophan (actually heterocyclic)

or tyrosine (having OH group).

CH 2 CHNH 2 COOH

Phenylalanine

Basic Amino acids

They have an

additional amino

group without

forming amides

(diamino mono-

carboxylic), e.g.,

arginine, lysine.

COOH

H 2 N

H 2 C

H

NH 2

C

(CH 2 ) 3

Lysine

COOH

H 2 NHC

H 2 COH

Serine

Alcoholic amino acids

These are

having alcoholic

or hydroxyl

group,e.g.,

serine,

threonine.

Heterocyclic amino acids

They have

nitrogen

in the ring

structure,

e.g., histidine,

proline. Proline

CHCOOH

H 2 CCH 2

H 2 C

H

N

Sulphur containing

amino acids

The amino

acids posses

sulphur, e.g.,

cysteine,

methionine.

COOH

HCNH 2

CH 2 SH

Cysteine

Types of Amino Acids (Depending on Structure and Reaction)

Amino acids have ionisable nature of NH 2 and COOH groups. Hence, in solution of different pH, the structure of amino acid changes.

H 3 NHCH 3 NHCH 2 N CH

RRR

COOH COO– COO–

(Cation) Zwitter ion (Anion)

+ +H+ + –H+

Table: Differences between Essential and Non-essential Amino Acids

Essential Amino Acids Non-essential Amino Acids

(i) They are those amino acids which the human body cannot

synthesise from raw materials.

They are amino acids which can be synthesised by human

body from raw materials.

(ii) They are 7 in number. They are 13 in number.

(iii) Essential amino acids are obtained from dietary proteins. They need not be present in the diet.

Peptide Formation

Peptide bond, (–NHCO–) is the bond between two amino acids, formed by elimination of water.

Functions of Amino Acids

(i) They are building blocks of proteins and enzymes.

(ii) Amino acid glycine provides nitrogen and carbon atoms for the synthesis of protoporphyrin
and heme (=haem).

(iii) Tryptophan produces plant hormone indole 3-acetic acid or IAA and vitamin nicotinamide.

(iv) Amino acid tyrosine gives rise to two hormones (thyroxine and adrenaline) and skin pigment
melanin.

COOH

H 2 NHC

R

Fig.: A part of peptide showing

peptide bonds linking amino acids

H 3 C

CH 2 OH

Peptide

bond

Peptide

bond

H

H

H

H

H

H

O

C

O

C

C

C

N

N

C

O

C