<prompt> The enzyme marker of electron transpo system isCytochrome reductaseFumarasePyruvateMalate dehydrogenase  <response>: 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 