Interaction of Zn2+ with the bovine‐heart mitochondrial bc1 complex

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Abstract
A study is presented of the effect of Zn2+ on the enzymatic properties of the bovine‐heart cytochrome‐bc1 complex. Micromolar concentrations of Zn2+ reversibly inhibit the cytochrome‐c reductase activity of either the cholate‐solubilized or liposome‐reconstituted complex. Kinetic analysis of the redox reactions of the cytochromes indicate that Zn2+ affects the activity of the complex at the quinol oxidation site. The following have been determined: (a) Zn2+ inhibits the pre‐steady‐state reduction of cytochrome c1 by duroquinol either in the absence or in the presence of antimycin, (b) it does not inhibit the reduction of b cytochromes in the absence of antimycin or in the presence of myxothiazol, (c) it inhibits cytochrome‐b reduction in the presence of antimycin. Furthermore Zn2+ inhibits the antimycin‐promoted oxidant‐induced extrareduction of b cytochromes. Addition of Zn2+ to reduced bc1 complex causes a red shift in the absorption spectrum of cytochrome b566 and a substantial decrease in the signal intensity of the EPR spectrum of the Fe‐S protein. This is interpreted as an interaction of Zn2+ with the 2Fe–2S‐cluster region of the Fe‐S protein, thus giving rise to inhibition of the reductase activity and of the antimycin‐insensitive reduction route of b cytochromes. A Scatchard‐plot of 65Zn2+ binding to the native isolated complex gave a straight line from which a value of three binding sites and a single dissociation constant of 3 × 10−6 M can be calculated, which is practically equal to the concentration causing 50% inhibition of electron flow.

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