Extraneous Agents and Cell Metabolism 99 



and succinate, and GTP and ADP subsequently react to form 

 GDP and ATP : 



succinyl CoA + GDP + P -> succinate + GTP + CoA 

 GTP + ADP -^ ATP + GDP 



Other a-ketonic acids probably react analogously to a-keto- 

 glutarate, at least as far as the reactions with TPP, lipoic 

 acid and coenzyme A are concerned. In the case of pyruvate 

 and possibly other cases, the acyl coenzyme A arising in the 



most substrates a-ketonic acids 



I 



a- lipoic acid 



DPN 



flavoprotein [succinate 



(?or vitamin K}"* (acyl coenzyme A 



iron porphyrin 



Fig. 1. Pathway of hydrogen transport for 

 different types of substrates. 



primary stages is assumed to react with AMP to form acetyl 

 AMP, and subsequently acetate and ATP (Berg, 1955). 



Apart from a-ketonic acids there are a few other substrates 

 which do not react according to the common rule (reaction 1). 

 They are succinate, and fatty acids attached to coenzyme A, 

 possibly also some amino acids. In these cases DPN is not 

 involved in hydrogen transport. Instead there is a direct 

 transfer of hydrogen atoms to a flavoprotein, and thence to 

 iron porphyrins. These exceptions arise from the thermo- 

 dynamic properties of the substrates. 



Fig. 1 summarizes the stages in hydrogen and electron 



