AMINO ACID METABOLISM

BY MUHAMMAD HASEEB TARIQ

PROTEINS

Proteins are formed of repeating units of Amino acids.

Half of the body protein (collagen) is present in supportive tissue.

Other half is present Intracellularly.

GENERAL CLASSIFICATION

Amino Acids are classified in 3 categories:• Essential (Can’t be synthesized in body)• Non-Essential (Can be synthesized in body)• Semi-Essential

Total amino acids are 20.Essential amino acids are 10.

AMINO ACID POOL

Body contains 100g of protein present freely inside the body. This is amino acid pool.

Amino acids of pool are in dynamic state.These participate in different metabolic reactions

occurring in the body.CONTENTS:50% of the pool is formed of :• Glutamate • Glutamine.10% of pool is formed of Essential amino acids.

SOURCES:• Protein turnover• Ingestion

PROTEIN TURNOVER:The amount of protein degraded and

synthesized constantly in the body is called as protein turnover (300-400g).

Factors: • Ubiquitin tags a protein and increase its

degradation.• Amino acid sequence of Proline , Glutamine ,

Serine and Threonine increases the degradation.

Above factors are responsible for affecting Protein turnover.

INGESTION:• 30-50g protein is daily lost.• RDA for proteins is 1g/kg body weight/day.• Excess amino acids are not stored in body.

GENRAL ASPECTS OF AMINO ACID METABOLISM

Amino acids undergo 2 steps along metabolism. These are:

• Transamination• Deamination

TRANSAMINATIONDefinition:The transfer of an amino group (-NH2 ) from an

amino acid to a keto acid is known as Transamination.

Features:• The reaction is catalysed by transaminases.• All reactions require PLP derivative of B6.

• No free NH3 is liberated, just transfer occurs.

• Excess amino acids are diverted to generate energy.

• Nitrogen is finally concentrated in Glutamate.• Only Glutamate undergoes oxidative

deamination to liberate ammonia.• Serum transaminases can be used for

diagnostic and prognostic purposes.

MECHANISM OF TRANSAMINATIONPING PONG BI BI MODEL:

• Amino group is transferred to Pyridoxal phosphate converting it to Pyridoxamine phosphate.

• Amino group is transferred to a keto acid converting Pyridoxamine phosphate to Pyridoxal phosphate.

• During the reactions there is formation of Schiff base.

DEAMINATION

Definition:The removal of amino group as NH3 is called as

Deamination.

Mostly it occurs in:• Liver• Kidneys

Deamination is of 2 types:• Oxidative deamination (coupled with

oxidation)• Non-oxidative deamination

OXIDATIVE DEAMINATIONGlutamate acts as a collection centre for amino

groups.Deamination requires enzyme Glutamate

Dehydrogenase (GDH).It is inhibited by ATP, GTP and allosterically

activated by ADP, GDP.Amino acid oxidases can also cause deamination.GDH:Zn containing enzyme.6 identical subunits. Molecular wt. 56000 each.

EFFECT OF PROTEIN RICH MEAL:After ingestion of protein rich meal glutamate

level is increased, which leads to increased levels of NH3 in body.

AMINO ACID OXIDASES:These are flavo proteins, possessing FMN and

FAD.Activity of L-amino acid oxidases is low than D-

amino acid oxidases.Hydrogen peroxide is produced in these

reactions which is then acted upon by Catalase.

NON-OXIDATIVE DEAMINATIONFollowing enzymes cause it :• Dehydratase (acts on Serine, Threonine ,

Homoserine)• Desulfhydrases (acts on Cystein ,

Homocystein)• Histidase (acts on histidine)

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