The sum of flux control coefficients in the electron-transport chain of mitochondria.

The sum of the flux control coefficients for group-transfer reactions such as electron transport has been proposed to be two when the coefficients are calculated from experiments in which the concentrations of the electron carriers are changed (CE) but one when they are calculated from changes in the rates of the electron-transfer processes (Cv). We tested this proposal using electron transport in uncoupled beef heart, potato tuber and rat liver mitochondria. First, with ascorbate plus N,N,N',N"-tetramethyl-p-phenylenediamine as substrate, the CE flux control coefficients of ascorbate, N,N,N',N"-tetramethyl-p-phenylenediamine, mitochondria and oxygen over electron-transport rate were measured by direct titration of the concentrations of these electron carriers. CE values were close to zero, one, one and zero, respectively, giving a sum of CE flux control coefficients of approximately two. At higher concentrations of N,N,N',N'-tetramethyl-p-phenylenediamine, its CE control decreased and the sum decreased towards one as predicted. Secondly, the Cv control coefficients of groups of electron-transfer processes with succinate or ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine as substrate were measured. This was achieved by measuring the effects of KCN (or malonate or N,N,N',N'-tetramethyl-p-phenylenediamine) on system flux when intermediates were allowed to relax and on local flux when intermediates were held constant. The Cv flux control coefficients were calculated as the ratio of the effects on system flux and on local flux. The sum of the Cv flux control coefficients was approximately one. Whether a sum of one or a sum of two was obtained depended entirely on the definition of control coefficients that was used, since either sum was obtained from the same set of data depending on the method of calculation. Both definitions are valid, but they give different information. It is important to be aware of which definition is being used when analysing control coefficients in electron-transport chains and other group-transfer systems.