214 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Other, The most active and the most widely distributed transaminase is 

 the glutamic-oxaloacetic enzyme : 



L-glutamic acid + oxaloacetic acid ^ 



a-ketoglutaric acid + aspartic acid 



The following reaction is also very common : 



amino acid + a-ketoglutaric acid ^ 



a-ketonic acid + glutamic acid 



The participation of oxaloacetic acid in transaminations appears to be 

 limited to the glutamic-oxaloacetic system. Another common system is the 

 glutamic-pyruvic one : 



L-glutamic acid + pyruvic acid ^ 



a-ketoglutaric acid + L-alanine 



It has long been thought from our knowledge of these two systems that 

 one of the members of the pair of substrates for a transaminase must be a 

 dicarboxylic acid. Since then, leucine-pyruvate, phenylalanine-pyruvate 

 and ornithine-pyruvate transaminations have been demonstrated. However 

 it is not possible to exclude the presence of a trace of glutamate, thus : 



pyruvic acid + glutamic acid ^ alanine + a-ketoglutatic acid 

 amino acid + a-ketoglutaric acid ^ a-ketonic acid + glutamic acid 



Numerous transaminases exist. Their specificity appears to be narrow in 

 some cases and very much wider in others. 



Each transaminase consists of a specific apoenzyme and a coenzyme 

 which is pyridoxal phosphate. 



It was long believed that glutamine and asparagine did not take part in 

 transamination reactions except after hydrolysis. In fact enzymatic systems 

 have been demonstrated which catalyse transminations from glutamine and 

 asparagine to many ketonic acids. Glutamine is an even better donor than 

 glutamate but the specificity of glutamine transaminase for the a-ketonic 

 acid is low. 



(/) Trans deaminations 



One of the mechanisms which has been proposed to account for the 

 oxidative deamination of amino acids invokes a transamination followed by 

 a deamination. This mechanism appears to be capable of explaining the 

 rapid and reversible deaminations whose character is not in accordance 

 with the properties and action of the L-amino acid oxidases. A transami- 

 nation to a-ketoglutaric acid would remove the amino group from an 



