ENZYMES 153 



Transaminases 



The reactions catalyzed require a coenzyme derived from the Bg 

 vitamins shown on page 340. The general process may be written 

 thus: 



-OOC(CH2)2CHCOO- + RC— COO- t=^ 



Alanine, aspartic acid, and perhaps other amino acids are formed 

 primarily according to this reaction, which thus becomes important 

 for the subsequent synthesis of proteins. 



Transmethylases 



A series of enzymatic reactions transfers methyl groups from one 

 compound to another, leading to the formation of choline, methionine, 

 betaine, creatine, serine, and a number of related compounds, all of 

 metabolic importance. 



CH3 



1 

 CH3S(CH2)2CHGOO- (CH3)3NCH2COO- NH2— C— N— CH2GOO- 



I 11 



NH3+ +NH2 



methionine betaine creatine 



There are still many uncertainties in the overall sequence of reactions, 

 but much progress is being made. 



Transacylases 



Transacylases catalyze transfers of acyl groups, for example, in the 

 synthesis of acetylcholine by means of an acyl derivative of coenzyme 

 A (CoA). 



+ choline 



Acetyl CoA + HOCH2CH2N(CH3)3 - 



acetylase 



CH3COOCH2CH2N(CH3)3 + CoA 



Coenzyme A participates in several reactions in the metabolism of 

 carbohydrates and lipides. Its structure is as follows: 



