Transamination in Amino Acid Metabolism

Organisms vary widely in their ability to synthesize amino acids. Dietary amino acid requirements for organisms vary from all to none. Mammals require about half of the amino acids in their diet for growth and maintenance of normal nitrogen balance (see here). Amino acids that must be provided in the diet to meet an animal's metabolic needs are called essential amino acids (Table 20.1). Those that need not be provided because they can be biosynthesized in adequate amounts are called nonessential amino acids.

Transamination is the process by which an amino group, usually from glutamate, is transferred to an -keto acid, with formation of the corresponding amino acid plus -ketoglutarate. Thus, transamination provides a route for redistribution of amino acid nitrogen. Transamination reactions are catalyzed by transaminases (aminotransferases) (see here).

Aminotransferases utilize a coenzyme - pyridoxal phosphate - which is derived from vitamin B 6. The functional part of pyridoxal phosphate (see here) is an aldehyde functional group attached to a pyridine ring. Catalysis involves a Schiff base intermediate (see here).

Transamination reactions have equilibrium constants close to one. Therefore, the direction of a transamination reaction proceeds in large part as a function of the intracellular concentrations of the reactants. This means that transamination can be used not only for amino acid synthesis, but also for degradation of amino acids that accumulate in excess of need. The reaction here shows how transamination is used for degradation of amino acids.

Most aminotransferases use glutamate/-ketoglutarate as one of the two amino/keto acid pairs involved. Aminotransferases involving aspartate/oxaloacetate and alanine/pyruvate are also quite abundant. Two important enzymes in the clinical diagnosis of human disease are serum glutamate-oxaloacetate transaminase (SGOT) and serum glutamate-pyruvate transaminase (SGPT):

> SGOT: Oxaloacetate + Glutamate <-> Aspartate + -Ketoglutarate
> SGPT: Glutamate + Pyruvate -------> -Ketoglutarate + Alanine

These enzymes, abundant in heart and liver, are released as part of the cell injury that occurs in myocardial infarction, infectious hepatitis, or other damage to either organ. Assays of these enzyme activities in blood serum can be used both in diagnosis and in monitoring the progress of a patient during treatment.

See also: Citric Acid Cycle Intermediates in Amino Acid Metabolism (from Chapter 21), Amino Acids, , Utilization of Ammonia, Amino Acid Degradation, Ammonia Transport in the Body, Coenzymes in Nitrogen Metabolism