MITOCHONDRIAL STRUCTURR AND FUNCTION 97 



and formnhi II correspondingly, 

 lla 



K /(b).\ 



y<f /(a)-. where c* =c, or c. 



The most recent information suggests an arrangement quite different from that 

 of / and //. As shown in jormuhi III below, coenzyme Q is localized in the suc- 

 cinic chain while a new component which we shall call dithiolipoic dehydrogenase 

 (Dt) is part of the DPNH chain: 



/// 



(b)2<( > ci-c-a 



^fi) d/ 



The position of b is still indeterminate, and it is not excluded that it may be the 

 link between fg and Q. The dotted lines indicate that additional components may 

 be implicated. 



The oxidation-reduction components of the electron transfer chain may be 

 classified into two categories: /) the fixed components, which are usually rela- 

 tively high-molecular weight proteins from 100,000 upwards; and 2) the mobile 

 components, which are relatively small molecules of molecular weight 500-13,000. 

 The fixed components include the two flavoproteins, cytochromes b, c-^ and a, 

 whereas the mobile components include cytochrome c, coenzyme Q and a-tocoph- 

 erol. The available evidence suggests that the isolated fixed components cannot 

 react with one another. For example, the isolated succinic or DPNH dehydro- 

 genases cannot react with isolated cytochrome c^ with any measurable speed. 

 Though information is still far from adequate, the indications are that the fixed 

 components react with one another through small molecules which shuttle elec- 

 trons from one to the other, and that these small molecules operate within lipo- 

 proteins which bridge together pairs of fixed components. Thus there are at least 

 four sites in the succinic and DPNH chains where mobile components and lipo- 

 proteins could be anticipated and there could be additional sites where electrons 

 would flow from one chain to the other, assuming separate succinic and DPNH 

 chains and the intercommunication of these two chains. 



This picture of the interaction of the fixed oxidation-reduction components 

 through shuttling small molecules housed in bridging lipoproteins is at present an 

 unabashed extrapolation from limited experimental evidence. But the auguries are 

 good that this picture represents the true state of affairs and I shall bring to your 

 attention some of the observations which give us this sense of confidence, so that 

 you can judge for yourselves. 



