Functional flexibility of electron flow between quinol oxidation Qo site of cytochrome bc1 and cytochrome c revealed by combinatory effects of mutations in cytochrome b,iron-sulfur protein and cytochrome c1

Transfer of electron from quinol to cytochrome c is an integral part of catalytic cycle of cytochrome bc₁. It is a multi-step reaction involving: i) electron transfer from quinol bound at the catalytic Q_o site to the Rieske iron-sulfur (2Fe-2S]) cluster, ii) large-scale movement of a domain containing 2Fe-2S] cluster (ISP-HD) towards cytochrome c₁, iii) reduction of cytochrome c₁ by reduced 2Fe-2S] cluster, iv) reduction of cytochrome c by cytochrome c₁.In this work, to examine this multi-step reaction we introduced various types of barriers for electron transfer within the chain of 2Fe-2S] cluster, cytochrome c₁ and cytochrome c. The barriers included: impediment in the motion of ISP-HD, uphill electron transfer from 2Fe-2S] cluster to heme c₁ of cytochrome c₁, and impediment in the catalytic quinol oxidation. The barriers were introduced separately or in various combinations and their effects on enzymatic activity of cytochrome bc₁ were compared. This analysis revealed significant degree of functional flexibility allowing the cofactor chains to accommodate certain structural and/or redox potential changes without losing overall electron and proton transfers capabilities. In some cases inhibitory effects compensated one another to improve/restore the function. The results support an equilibrium model in which a random oscillation of ISP-HD between the Q_o site and cytochrome c₁ helps maintaining redox equilibrium between all cofactors of the chain. We propose a new concept in which independence of the dynamics of the Q_o site substrate and the motion of ISP-HD is one of the elements supporting this equilibrium and also is a potential factor limiting the overall catalytic rate.