72 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



(h) Coenzyme A (Co A) 



This essential compound is universally distributed. It is formed by the 

 joining of adenosine-3, 5-diphosphate, pantothenic acid-4'-phosphate and 

 thioethanolamine (cysteamine). 



Cysteamine 



KC 



CHj-SH 



Pantothenic 



acid-4'- 

 phosphate 



OH 



Adenosine-3, 5-diphosphate 



The hydrogen atom of the -SH group at the cysteamine end of coenzyme 

 A may be substituted by an acetyl group to give "active acetate" or acetyl- 

 Co A. The metabolic role of "active acetate" is a primary one, for it acts as 

 a universal donor of acetyl groups. It contains an acylmercaptan bond, 

 a carboxyl group and a sulphhydryl group being condensed together, with 

 loss of water, in an anhydride linkage. This is a so-called "energy-rich" 

 bond, its hydrolysis setting free about 16,000 calories per mole. Despite this 

 strongly exergonic hydrolysis, the acylmercaptan bond of C0A-S-CO-CH3 

 is very stable in aqueous solution at physiological pH's. It is only in the 

 presence of specific enzyme catalysts that the bond is hydrolysed. 



(/) Cyanocohalamin {Vitamin B^^ 



Cyanocobalamin is widely distributed in living organisms ; it is found in 

 bacteria, in algae and in animal tissues, but it does not appear to be present 

 in the green leaves of plants. For man, it is an important vitamin, being 

 one of the "extrinsic factors" of haemopoiesis. It was crystallized in 1948; 

 the crystals are dark red, melt at 320° and their solution has well pronounced 

 absorption bands at 278, 361 and 550 m/x. It contains cobalt and phos- 

 phorous and the molecular weight is around 1,500. On acid hydrolysis, 

 cyanocobalamin yields 5,6-dimethylbenzimidazole, ribofuranose, phos- 

 phoric acid, l-amino-2-propanol and a cobalt complex in which the metal 



