Complex III: CoQ-Cyt.C Reductase Functions as * Proton pump, and * Catalyses transfer of electrons SECTION TWO This system catalyses transfer of electrons from CoQ.H2 to Cyt-c Cyt-b and Cyt-c1. The electrons from CoQ.H2 is first accepted by Cyt-b566 and then transferred to Cyt-b562, which reduces Co.Q.H2 CoQ Fe+++ accepts electron and is oxidised to Fe++. The system also acts as a proton pump. It is believed that 4 (four) protons are pumped across the mitochondrial membrane during the oxidation. The energy change permits ATP formation (Site II). Role of Q cycle and Rieske FeS in complex III Electrons are passed from Q.H2 to cytochrome C complex III-Q cytochrome C oxidoreductase: Q. H2 + 2 Cyt C + 2H+ - Q + 2 CytC + 4H+ Oxidised matrix reduced membrane space The process is believed to involve cytochromes C1, b1 and bHH and a Rieske FeS. Rieske FeS is an unusual FeS in which one of the Fe atoms is linked to two histidine -SH groups rather than two cysteine -SH groups and is called as the Q cycle. Q may exist in three forms: * Oxidised quinone, * Reduced quinol, or * Semiquinone. The semiquinone is formed transiently during the cycle, one turn of which results in the oxidation of 2Q.H2 to Q, releasing 4H+ into the intermembrane space, and the reduction of one Q to QH2, causing 2H+ to be taken up from the matrix. Note It is to be noted that while Q carries two electrons, the cytochromes carry only one, thus the oxidation of one Q.H2 is coupled to the reduction of two molecules of cytochrome C the Q. cycle. Other options are not paicipating in ETC.Ref: MN Chatterjea Textbook of Medical Biochemistry, 8th edition, page no: 142