Chemistry of amino acids

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Chemistry of Amino Acids
R.C. Gupta Professor and Head Dept. of
Biochemistry National Institute of Medical
Sciences Jaipur, India
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EM B - R C G
EMB-RCG
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Amino acids
Carboxylic acids having one or more amino groups
E M B - R C G
Carbon atoms are numbered 1, 2, 3 etc starting
from the carboxyl group
Also named a, b, g etc starting from
carbon atom next to the carboxyl group
EMB-RCG
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4    3 2 1
R CH2 CH2 CH2 COOH      g
b a
E M B - R C G
EMB-RCG
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E M B - R C G
EMB-RCG
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D- and L- Isomers
Amino group is present on right hand side of
a-carbon in D-amino acids, and on left hand side
in L-amino acids
E M B - R C G
R
R
R


H C NH2
H2N C H


COOH
COOH
D-Amino acid L-Amino acid
EMB-RCG
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E M B - R C G
EMB-RCG
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Polar and non-polar amino acids
E M B - R C G
EMB-RCG
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E M B - R C G
EMB-RCG
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EMB-RCG
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Polar amino acids
Non-polar amino acids
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4. Amino acids with side chains having acidic
groups or their amides
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5. Amino acids with side chains having basic
groups
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6. Amino acids with side chains containing
aromatic rings
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7. Imino acids
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Two cysteine residues in a protein may be linked
through their SH groups to form a cystine residue
S

Cystine
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Some enzymes contain an unusual amino acid,
selenocysteine, at their catalytic site

Selenocysteine
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Essential amino acids
E M B - R C G
EMB-RCG
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E M B - R C G
EMB-RCG
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E M B - R C G
EMB-RCG
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E M B - R C G
EMB-RCG
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Non-standard amino acids
E M B - R C G
EMB-RCG
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Some non-a-amino acids are also present
in human beings e.g. b-alanine, taurine,
b-aminoisobutyric acid, g-aminobutyric
acid etc
CH2 CH2 COOH
CH2 CH2 SO3H


NH2
NH2
b-Alanine
Taurine
CH2 CH COOH
CH2CH2CH2COOH



NH2
NH2
CH3
g-Aminobutyric acid
b-Aminoisobutyric acid
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b-Alanine is a constituent of pantothenic acid
and of acyl carrier protein and coenzyme A
Taurine is a neurotransmitter, and is a
constituent of bile acids and bile salts
E M B - R C G
b-Aminoisobutyric acid is a metabolite of some
pyrimidines
Aminobutyric acid (GABA) is a neurotransmitter
EMB-RCG
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Physical properties
E M B - R C G
EMB-RCG
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Amphoteric nature
All amino acids have at least one carboxyl and
one amino group
Both of these are ionizable
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E M B - R C G
EMB-RCG
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E M B - R C G
EMB-RCG
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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E M B - R C G
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Formation of peptide bonds
One of the most important reactions of amino
acids
E M B - R C G
The bond by which amino acids are linked with
each other in peptides and proteins
Formed between carboxyl group of one amino acid
and amino group of another amino acid
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The actual formation of peptide bonds in living
cells is complex
The reaction occurs in stages, and requires the
presence of enzymes, coenzymes and other factors
Peptide bond I
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E M B - R C G
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Reactions of carboxyl and amino groups
The carboxyl and amino groups of
amino acids can undergo their usual reactions
such as
E M B - R C G
Acylation
Esterification
Formation of salts
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Reactions of sulphydryl groups
The sulphydryl group of cysteine can undergo
reversible oxidation and reduction
E M B - R C G
Disulphide bonds can be formed between the
sulphydryl groups of two cysteine residues
The sulphydryl groups of cysteine residues are
essential for the biological activity of many
proteins
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Reactions for determination of amino acid
sequence of proteins
E M B - R C G
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E M B - R C G
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E M B - R C G
EMB-RCG
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Sangers reaction
E M B - R C G
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Edmans reaction
E M B - R C G
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S
R
O
S
R
O






NC H2NCHC
NHCNHCHC
N-Terminal amino acid residue
Phenyliso- thiocyanate
Phenylthiohydantoic acid residue
S
S


C
C
H2O
N NH
NH NH
Acid, nitromethane
OH
C
CH
C
CH




O
R
O
R
Phenylthiohydantoin
Phenylthiohydantoic acid
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Reactions for identification of specific amino
acids
E M B - R C G
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Ninhydrin reaction
E M B - R C G
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O

O
O?NH
4




OH
N
OH




O
O
O
Coloured complex
Ninhydrin
RCHCOOH

NH2
a-Amino acid
RCHO CO2 NH3
O
O


OH
OH
H
OH
2 NH3


O
O
Reduced ninhydrin
Ninhydrin
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Xanthoproteic reaction
E M B - R C G
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Millon-Nasse reaction
E M B - R C G
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Aldehyde reaction
E M B - R C G
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Hopkins-Cole reaction
E M B - R C G
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Sakaguchis reaction
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Lead sulphide reaction
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E M B - R C G
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Thank you