PROSTHETIC GROUPS 31 



group and the protein varies in strength so that some prosthetic 

 groups are firmly fixed to the protein whilst others are in such 

 a loose combination that they may wander from one protein 

 molecule to another. In this second case the non-protein 

 moiety is called a " coenzyme." There is still some doubt 

 whether there is any difference between prosthetic groups and 

 coenzymes other than in the strength of the link with the 

 protein, or whether there is a fundamental difference in that 

 the prosthetic group is an integral part of the enzyme 

 structure while the coenzyme acts as a separate carrier of 

 hydrogen ions, etc., from one enzyme to another. This 

 controversial point is outside the scope of the present dis- 

 cussion so we shall discuss all prosthetic groups and 

 coenzymes under one heading. 



PROSTHETIC GROUPS 

 The prosthetic groups (and coenzymes) which have been 

 identified up to the present are:' 

 Ademne-nicotinamide-dinucleotide (Coenzyme I) 



N= C— NHp 



I I 



HC C— N^ O O CONH2 



II II >H II II .^ ' ^ 

 N — C — N^ RIBOSE— P-o-P-0— RIBOSE— N, 



OH O" 



This acts as the coenzyme for certain dehydrogenases and 

 these enzymes display a specificity towards coenzyme I as 

 the hydrogen acceptor in the same way as they display 

 specificity towards their substrate as hydrogen donator. 

 The molecule appears to act as a carrier of hydrogen by 

 alternate reduction and oxidation of the nicotinamide group. 

 By accepting hydrogen from one dehydrogenase system and 

 transferring it to another dehydrogenase system working 

 in reverse, the coenzyme acts as a hydrogen carrier between 

 what are called " coenzyme-linked-dehydrogenase systems " 

 (see Chap. VII). 



