474 5. QUINONES 



enzymic substrate oxidation (Kikuchi and Barron, 1959; Koide, 1962). 

 Hydroquinones, either added or formed from quinones, may donate elec- 

 trons into the sequence, most probably at the cytochrome c level. Wil- 

 liams (1963) has reported a very interesting kinetic study of this reaction, 

 which proceeds in two steps: 



QH., + cyto-Co^ ±1? QH • + cyto-c,ed + H+ 

 QH • + cyto-Cox ^ Q + cyto-Cred + H+ 



the second proceeding more rapidly than the first. The accelerating effect 

 of quinone on the reaction is attributed to the formation of the semiqui- 

 none, Q + QH2 ±> QH • . Ionic strength effects and entropy changes in- 

 dicate that an electron transfer between oppositely charged ions occurs, 

 and it was suggested that the formation of the active complex might in- 

 volve interactions of the aromatic ring of the hydroquinone with hydro- 

 phobic regions of the cytochrome c. T. Nakamura (1960 a, b, 1961) has 

 also obtained evidence for the initial formation of the semiquinone in the 

 oxidation of p-benzohydroquinone by a laccase preparation, and for the 

 importance of the semiquinone in determining the over-all rate of oxidation. 



(D) Establish alternate or bypass pathways for electron flow. This is, of 

 course, closely related to the previous mechanism, but specifically it is 

 the introduction of an alternate pathway: 



Q 



which may have properties quite different from those of the original path- 

 way. A simple example is the acceleration of the reduction of cytochrome c 

 with NADH as a substrate by menadione in an enzyme from lymphosar- 

 coma, and to a lesser extent by p-benzoquinone and 1,4-naphthoquinone 

 (Strength, 1956). On the other hand, 1,2-naphthoquinone augments the 

 oxidation of NADH but does not reduce cytochrome c. The functioning 

 of p-benzoquinone as a mediator between NADH and cytochrome c has 

 also been noted in electron transport systems from Fusarium (Kikuchi and 

 Barron, 1959). More complex situations of this type have been observed 

 and are particularly important in oxidative phosphorylations, to be dis- 

 cussed later (page 477). 



(E) Block specific site in electron transport sequence. Certain antimalarial 

 3-alkyl-l,4-naphthoquinones, such as SN-5949, are known to be potent 

 inhibitors of electron transport through a region also sensitive to antimycin 

 A. Although this type of inhibition will be discussed elsewhere, it is well 

 to remember that some of the simpler naphthoquinones also might act to 



