<P> In Complex II (succinate dehydrogenase or succinate - CoQ reductase; EC 1.3. 5.1) additional electrons are delivered into the quinone pool (Q) originating from succinate and transferred (via flavin adenine dinucleotide (FAD)) to Q. Complex II consists of four protein subunits: succinate dehydrogenase, (SDHA); succinate dehydrogenase (ubiquinone) iron - sulfur subunit, mitochondrial, (SDHB); succinate dehydrogenase complex subunit C, (SDHC) and succinate dehydrogenase complex, subunit D, (SDHD). Other electron donors (e.g., fatty acids and glycerol 3 - phosphate) also direct electrons into Q (via FAD). Complex 2 is a parallel electron transport pathway to complex 1, but unlike complex 1, no protons are transported to the intermembrane space in this pathway . Therefore, the pathway through complex 2 contributes less energy to the overall electron transport chain process . </P> <P> In Complex III (cytochrome bc complex or CoQH - cytochrome c reductase; EC 1.10. 2.2), the Q - cycle contributes to the proton gradient by an asymmetric absorption / release of protons . Two electrons are removed from QH at the Q site and sequentially transferred to two molecules of cytochrome c, a water - soluble electron carrier located within the intermembrane space . The two other electrons sequentially pass across the protein to the Q site where the quinone part of ubiquinone is reduced to quinol . A proton gradient is formed by one quinol (2H + 2e -) oxidations at the Q site to form one quinone (2H + 2e -) at the Q site . (in total four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules). </P> <P> When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation . </P> <P> In Complex IV (cytochrome c oxidase; EC 1.9. 3.1), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome c and transferred to molecular oxygen (O), producing two molecules of water . At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient . The activity of cytochrome c oxidase is inhibited by cyanide . </P>

Where are the proteins of the electron transport chain located