4l6 FURTHER EVOLUTION 



Rueff.^* This ' active acetate ' is a thioester of acetic acid 

 with coenzyme A : 



,0\ N, 







\ 



-P P — CHj — CH CH N C CH 



^« \h_/oh < 



I 



0PQ3H, I 



OCH2. C (CHj)^. CHOH. CO. NHCHo. CHo. CO. NHCH.,. CH^S. CO. CH3 



The thioester bond of coenzyme A is associated with a large 

 supply of energy and when it is broken by hydrolysis 8,200 

 cal/mole are liberated. ^^ Thus acetyl coenzyme A may be 

 described as a ' macroergic ' compound. It must arise during 

 metabolism at the expense of energy derived from the phos- 

 phate bonds of ATP or from a simultaneous oxidation 

 according to the equation^® 



CH3CO.C00H + coenzyme A + ^Oa^ 



acetyl coenzyme A + CO2 + H2O 



However, this oxidation does not require the presence of free 

 oxygen, it can also occur anaerobically with the transfer of 

 hydrogen to other organic substances through the mediation 

 of diphosphopyridine nucleotide. When it has been activated 

 in this way by coenzyme A, the acetyl residue can enter into 

 the most diverse condensation reactions leading to the forma- 

 tion of new carbon-carbon bonds and the lengthening of the 

 carbon chain. 



It later appeared that coenzyme A can activate, not merely 

 acetic acid, but also other organic acids of both the aliphatic 

 and aromatic series which also form thioesters with coenzyme 

 A, e.g. succinyl coenzyme A" and ?50valeryl coenzyme A.^® 



The tremendous biological importance of coenzyme A rests 

 on the fact that only through its mediation can small organic 

 molecules combine together by carbon-carbon bonds to form 

 complicated organic substances. That is to say, this is the 

 only way in which one of the most important processes in 

 the synthesis of the carbon skeletons of the components of 

 protoplasm can take place. It is quite clear that a process 



