TRANSAMINATION

• The transfer of an amino group from an amino acid to a keto acid is known as transamination.
• It is catalysed by a group of enzymes called transaminases  (recently aminotransferases)

Occurs in two stages

1. Transfer of the amino group to the coenzyme pyridoxal phosphate (PLP)  (bound to the coenzyme) to form pyridoxamine phosphate.
2. The amino group of pyridoxamine phosphate is then transferred to a keto acid to produce a new amino acid and the enzyme with PLP is regenerated.

• All the transaminases require pyridoxal phosphate (PLP), a derivative of vitamin B6.
• Important for the redistribution of amino groups and production of non-essential amino acids. It involves both catabolism (degradation) and anabolism (synthesis) of amino acids.
• Transamination diverts the excess amino acids towards energy generation.
• The amino acids undergo transamination to finally concentrate nitrogen in glutamate. Glutamate is the only amino acid that undergoes oxidative deamination to a significant extent to liberate free NH3 for urea synthesis.
• All amino acids except lysine, threonine, proline and hydroxyproline participate in transamination.

• The effects produced due to elevation in blood NH3 levels
•  Elevation in blood NH3 level may be genetic or acquired.
• Gentic is impairment in urea synthesis due to a defect in any one of the five enzymes .
• The acquired hyperammonemia may be due to hepatitis, alcoholism etc. where the urea synthesis becomes defective, hence NH3 accumulates.
• Accumulation of NH3 shifts the equilibrium to the right with more glutamate formation, hence more utilization of D-ketoglutarate.
• D- Ketoglutarate is a key intermediate in TCA cycle and its depleted levels impair the TCA cycle.
• The net result is that production of energy (ATP) by the brain is reduced. The toxic effects of NH3 on brain are, therefore, due to impairment in ATP formation.

•  When the plasma level of ammonia is highly elevated, intravenous administration of sodium benzoate and phenyllactate is done.
• These compounds can respectively condense with glycine and glutamate to form water soluble products that can be easily excreted.