Question

What does the Q cycle accomplish in each of its two rounds?

Short answer

The Q cycle reduces two molecules of cytochrome C and releases four protons.

Step-by-step solution

Q cycle

The Q cycle, which is carried out by Complex III, is what causes H⁺to move from the matrix to the intermembrane space. This movement is sparked by the movement of electrons from cytochrome b to cytochrome c.

Steps of the Q cycle

Reactions 1 through 7 are needed for the first cycle, while Reactions 1 through 6 and 8 are needed for the second cycle, which is technically two cycles in total. The Q cycle's reactions are as follows:

(1) On the matrix side of the membrane, Complex I provides coenzyme QH₂.

(2) QH₂ diffuses to the membrane's cytosolic side and binds to the cytochrome b subunit of Complex III at the Q_o site.

(3) QH₂ produces Q semiquinone and 2H⁺ when it lowers the Rieske iron-sulfur protein (ISP). The ISP continues by lowering heme c1.

(4) Coenzyme Q is created when Q lowers heme b_L.

(5) Q diffuses to the matrix side and attaches to the Q_i location on cytochrome b (only in Cycle 1).

(6) Heme b_H is decreased by heme b_L.

(7) Heme b_H transforms Q into Q. (Only Cycle 1)

(8) Heme b_H converts the Q bound in the Q_i site to QH₂. (Only Cycle 2)

Thus, the net reaction is the movement of four protons from the matrix into the intermembrane space and the transfer of two electrons from QH₂ to cytochrome c1.