590 E. C. Slater and J. P. Colpa-Boonstra 



COOH COOH 



I 1 



HC— SE CH 



I ^ II + ESH (25) 



CHa CH 



I I 



COOH COOH 



Sum COOH COOH 



I I 



CHa CH 



I + 2c+++ ?=i II + 2c++ + 2H+ 



CHa CH 



I I 



COOH COOH 



In the absence of reaction (18), the complex between fumarate and the 

 enzyme — SH dissociates according to reaction (25). 



It is, indeed, possible that this mechanism operates in the absence of 

 cyanide and provides an alternative pathway for the oxidation of succinate 

 not involving the reduction of the iron atom of cytochrome b. 



The effects of antimycin can also be adequately incorporated into these 

 mechanisms, but that will be discussed elsewhere (Colpa-Boonstra, 1959). 



The reaction mechanisms described for DPNH oxidation can be 

 summarized (but rather inadequately) in the conventional shortened forms 

 as follows : 



Phqsphorylating 



K (X) 



DPNH '-^^ fp -> Ii -^^ 26+++-< ^2c -- 



Non-phosphorylating 



6+++ 



DPNH^^fp-^ :2c -> 



It should be noted that the two pathways proceeding from ferrocytochrome 

 h or flavoprotein to ferricytochrome c are not alternative, but describe two 

 compulsorily-linked reactions proceeding at the same rate. 



