A Functional Dimer Model of Ubiquinol Cytochrome C Oxidoreductase

Abstract

Ubiquinol cytochrome c oxidoreductase (bc1 complex) serves as the electron junction in many respiratory systems. The enzyme possess both a quinol oxidase site (Qp-site) and a quinone reductase site (Qn-site) that operate together in a bifurcated electron transfer mechanism. While most electrons from complex I and II are funneled to cytochrome c, a small percent leak out of the respiratory system to produce the free radical, superoxide. In situ and other experimental systems, the enzyme exists as a dimer. But until recently, it was believed to operate as a functional monomer. Here we show that in order to explain the kinetic, superoxide, and antimycin stimulated data, a functional dimer model is required. A total of nine data sets consisting of kinetic and superoxide measurements from a range of experimental studies were used to constrain the models. The monomer and dimer models behave kinetically similar; however, the electron state distributions were vastly different. In the dimer catalytic mechanism, only one quinol oxidase site is active at any given time. Quinone reduction occurs randomly at either reductase site. But when antimycin A is bound to a single monomer, both Qp-sites are activated. With the superoxide production rate calibrated, model analysis reveals that the mammalian complex relative to the yeast and bacteria complexes either possess a less stable semiquinone at the Qp-site in or the estimated level of free radical scavenging is significantly underestimated.