18:4/ Enzymes: Kinetics of Oxidations 



347 



transport against a diffusion gradient, and to make muscular contraction 

 possible. 



The enzymes of the cytochrome chain consist of three known types. 

 These are the pyridine nucleotides (PN) which contain the coenzyme 

 DPN or TPN, the flavin-adenine-dinucleotides which contain the pros- 

 thetic group FAD, and the cytochromes (cyt), all of which contain a 

 heme group. The structure of a heme group is shown in Figure 1 and 

 that of DPN and FAD in Figure 6. In intact mitochondria, from 

 vertebrates or yeast, these enzymes react as follows 



+ 2H 



ATP ATP 



1:2 



PN + V-^FADH 2 \ /cytb"^cytc;' 



^PNH + H + /\ FAD ]l\[cytb"J \cytc? 



+ 2H + 

 ADP + P ADP + P 



( 



ATP 



'cytc"fr -tfcyta" 



(cytc'V \cyta" 



ADP+P 



'cyt a 5 



H 2 



12"2 



(cyta'a/ \0 2 + 4H + 

 4: 1 



The number of primes after each cytochrome indicates the valence 



Niacinamide 



H— N-H 



N^ 



N 



>N' 



H 



S H, 



Adenine 



? ? 



O-P— O— P— o 

 II II H/V+ 



H 2 C OH HO CHo 



H h: ~ if_|4 



OH OH 



Ribose 



fK / 



H H 



H H H 

 H O O O H 



I I I I I 

 H— C— C— C-C— C- 



H H H H 



DPN 



O O H 



t t I 



O— P— O-P-O-C-H. 



H— N— H 



N 



OH OH 



H~ 



^N 



H 



^irNl 



\H H 



H 



OH OH 



H^ \ 



Flavin 



Ribose 



Adenine 



(6,7-dimethyl-9-d-ribityl-isoalloxain) 



FAD 



Figure 6. Structure of DPN and FAD. 



