Operation of the modified Q cycle in
Complex III results in the reduction of
Cytochrome c, oxidation of
ubiquinol to
ubiquinone, and the transfer of four protons into the intermembrane space, per two-cycle process. Ubiquinol (QH2) binds to the Qo site of
complex III via
hydrogen bonding to His182 of the
Rieske iron-sulfur protein and Glu272 of
Cytochrome b. Ubiquinone (Q), in turn, binds the Qi site of complex III. Ubiquinol is divergently oxidized (gives up one electron each) to the
Rieske iron-sulfur '(FeS) protein' and to the
bL
heme. This oxidation reaction produces a transient semiquinone before complete oxidation to ubiquinone, which then leaves the Qo site of complex III. Having acquired one electron from ubiquinol, the 'FeS protein' is freed from its electron donor and is able to migrate to the Cytochrome c1 subunit. 'FeS protein' then donates its electron to Cytochrome c1, reducing its bound heme group. The electron is from there transferred to an oxidized molecule of
Cytochrome c externally bound to complex III, which then dissociates from the complex. In addition, the reoxidation of the 'FeS protein' releases the proton bound to His181 into the intermembrane space. The other electron, which was transferred to the
bL heme, is used to reduce the
bH heme, which in turn transfers the electron to the ubiquinone bound at the Qi site. The movement of this electron is energetically unfavourable, as the electron is moving towards the negatively charged side of the membrane. This is offset by a favourable change in EM from −100 mV in BL to +50mV in the BH heme. The attached ubiquinone is thus reduced to a
semiquinone radical. The proton taken up by Glu272 is subsequently transferred to a hydrogen-bonded water chain as Glu272 rotates 170° to hydrogen bond a water molecule, in turn hydrogen-bonded to a
propionate of the
bL heme. Because the last step leaves an unstable
semiquinone at the Qi site, the reaction is not yet fully completed. A second Q cycle is necessary, with the second electron transfer from cytochrome
bH reducing the semiquinone to ubiquinol. The ultimate products of the Q cycle are four protons entering the intermembrane space, two from the matrix and two from the reduction of two molecules of cytochrome c. The reduced cytochrome c is eventually reoxidized by
complex IV. The process is cyclic as the ubiquinol created at the Qi site can be reused by binding to the Qo site of complex III. == Notes ==