Control of proteoliposomal cytochrome c oxidase: the overall reaction.
The control of cytochrome c oxidase incorporated into proteoliposomes has been investigated as a function of membrane potential (delta psi) and pH gradient (delta pH). The oxidase generates a pH gradient (alkaline inside) and a membrane potential (negative inside) when respiring on external cytochrome c. Low levels of valinomycin collapse delta psi and increase delta pH; the respiration rate decreases. High levels of valinomycin, however, decrease delta pH as valinomycin can also act as a protonophore. Nigericin (in the absence of valinomycin) increases delta psi and collapses delta pH; the respiration rate increases. On a millivolt equivalent basis delta pH is a more effective inhibitor of activity than is delta psi. In the absence of any ionophores the cytochrome oxidase proteoliposomes enter a steady state, in which there are both delta pH and delta psi components of control. Present and previous data suggest that the respiration rate responds in a linear way ("ohmically") to increasing delta pH but in a nonlinear way to delta psi ("non-ohmically"). High levels of both delta psi and delta pH do not completely inhibit turnover (maximal respiratory control values lie between 6 and 10). The controlled steady state involves the electrophoretic entry and electroneutral exit of K+ from the vesicles. A model is presented in which the enzyme responds to both delta pH and delta psi components of the proton-motive force, but is more sensitive to delta pH than to delta psi at an equivalent delta mu H+. The steady state of the proteoliposome system can be represented for any set of permeabilities and enzyme activity levels using the computer simulation programme Stella.
Nicholls
P
P
Department of Biological Sciences, Brock University, St. Catharines, Ont., Canada.
Cooper
C E
CE
Wrigglesworth
J M
JM
D000818:Animals; D003574:Cytochrome c Group / Q000378:metabolism; D003576:Electron Transport Complex IV / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008081:Liposomes; D008564:Membrane Potentials; D008956:Models, Chemical; D010100:Oxygen / Q000378:metabolism; D010539:Permeability; D011188:Potassium / Q000378:metabolism; D011522:Protons
D003574:Cytochrome c Group; D008081:Liposomes; D011522:Protons; D003576:Electron Transport Complex IV; D011188:Potassium; D010100:Oxygen