Question

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

Short answer

The Q cycle has two steps that produce two cytochromes molecules, release 4H+ into the intermembrane space, and renew ubiquinone.

Step-by-step solution

Define electron transport chain and its composition

The electron transport chain (ETC) is made up of five protein complexes, four of which help transfer electrons from NADH and FADH2 to molecular oxygen and one of which aids in ATP generation.The Q cycle, which occurs at complex III, is a component of the electron transport chain, which is found in mitochondria's inner membrane.

By transferring the electron of NADH and pumping four hydrogens across the membrane, Complex I forms the hydron ion gradient, whereas Complex II obtains the electron of FADH2. The coenzyme Q commonly known as ubiquinone (Q), accepts electrons from both complexes (I and II) and delivers them to complex III.

The two rounds of the Q cycle

Complex III - Cytochrome b, the Rieske centre (which contains the 2Fe - 2S center), and a cytochrome c protein make up the third complex. The electron is carried by a heme group in these cytochromes. During electron transport in complex III, the heme group cycles between Fe²⁺ and Fe³⁺.

The last acceptor of the electron, Cytochrome C which is a water-soluble protein that can easily diffuse in the intermembrane gap, can only accept one electron at a time, which is why this step occurs in two steps.

Complex III releases cytochrome c, which delivers an electron to complex IV along with four protons into the mitochondrial membrane's intermembrane gap.

• 1^st stage (Round 1) - Ubiquinol (CoQH2) and ubiquinone (CoQ) bind to two different locations on complex III. Each electron is transferred to two separate routes by CoQH2. One electron is transported to Fe - S and then to cytochromeC through cytochrome c1, while the other is transferred to cytochrome b and then to CoQ bound, and 2H+ ions are released into the intermembrane space during this process. From absorbing one of the electrons from the ubiquinone, the CoQ bound at the second site enters an intermediary CoQH - radical state.
• 2^ndstage (Round 2) - The cycle repeats again in this stage, with another CoQH2 binding to complex III and transferring two electrons, as in the first stage, and two more H + ions discharged into the intermembrane. One electron goes through c and another through b, assisting in the reduction of to CoQH - to CoQH2 before it dissociates from complex III and may be recycled.

The electron transport chain's complex III is seen in the diagram below. The inner mitochondrial membrane is depicted in grey. The ubiquinone form of CoQ10 is Q, while the ubiquinol (dihydroxyquinone) form is QH2.

Thus, the Q cycle has two steps that produce two cytochrome C molecules, release 4H+ into the intermembrane space, and renew ubiquinone.