PROSTHETIC GROUPS 33 



This forms the prosthetic group of enzymes known as 

 flavoproteins and again acts as hydrogen carrier by 

 alternate reduction and oxidation of the double bond in 

 the isoalloxazine ring. 



Riboflavin-phosphate 



Consists of riboflavin with a single phosphate radicle; 

 occurs as the prosthetic group of a flavoprotein enzyme known 

 as cytochrome reductase (see below). 



Thiamindiphosphate, Aneurindiphosphate, or Cocarboxylase 



N==C — NHi CH3 



I I CI c==c-CH2-CH20— P-O-P-OH 



H3C— C C CH2 N^ I OH OH 



II II ^c-1 



N CH H 



Thiamindiphosphate was first identified as the prosthetic 

 group of yeast carboxylase, the enzyme which decarboxylates 

 pyruvic acid to acetaldehyde. It is found in many tissues 

 and bacteria which do not possess carboxylase and plays a 

 part in many of the reactions involving pyruvic acid (see 

 Chap. VII), such as oxidative decarboxylation. It also acts 

 in some as yet undefined way in the oxidation of certain 

 fatty acids. 



Pyridoxal phosphate, Pyridoxamine phosphate 



CHO 9 ^^2^^2 ? 



^ I 



\.J OH 



tCH^O-P-OH H0/\CH^0-P-0H 



3 N 



(a) Pyridoxal phosphate. (6) Pyridoxamine phosphate. 



Pyridoxal phosphate has the probable structure shown above, 

 although the exact position of the phosphate group is not yet 

 certain. It acts as the prosthetic group of the amino-acid 

 decarboxylases (see Chap. IX) and of bacterial transaminase. 



CHRM. A. B. 



