42 



BACTERIAL ENZYMES 



oxidase, the net result being the oxidation of the dehydro- 

 genase substrate. 



Enzymes which transfer hydrogen direct from their sub- 

 strate to oxygen are called "oxidases"; the next simplest 

 system is the direct cytochrome system described above. 

 The majority of dehydrogenases are unable to transfer 

 hydrogen from their substrate to cytochrome without the 

 intermediation of a further carrier in the form of a coenzyme. 

 Dehydrogenases of this third type can be looked upon as 

 enzymes with prosthetic groups so loosely attached that the 

 latter can become detached from the protein and act as 

 hydrogen carriers between one enzyme system and the next. 

 In this case the action of the dehydrogenase is to transfer 

 hydrogen from the substrate to the coenzyme. The formulae 

 of coenzymes I and II have been given on p. 31. These 

 substances act as hydrogen acceptors by reduction of one 

 of the double bonds in the pyridine ring of nicotinamide. 

 The reduced coenzyme is not autoxidisable but acts as the 

 specific substrate for a coenzyme dehydrogenase (" diaphorase " 

 for coenzyme I, " cytochrome reductase " for coenzyme II) 

 which transfers the hydrogen to cytochrome. In this case 

 the reaction chain is (substrate types S3 and S4, Table III) : 



AH2 + Coenzyme 



C0E.H2+ Cytochrome 



Cytochrome 



Reduced Cyt. + — >■ Cytochrome+H^G 



oxidase 



and the hydrogen passes through two intermediate carriers 



before linking with oxygen. 



