chemistry

Q cycle

Q cycle is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Q cycle rather than just read about it. In short: The Q cycle (named for quinol) describes a series of sequential oxidation and reduction of the lipophilic electron carrier Coenzyme Q (CoQ) between the ubiquinol and ubiquinone forms. These reactions can result in the net movement of protons across a lipid bilayer (in the case of the mitochondria, the inner mitochondrial membrane).

Key takeaways

Q cycle belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Q cycle to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Q cycle from memory before moving on to harder problems.

Reference excerpt

The Q cycle (named for quinol) describes a series of sequential oxidation and reduction of the lipophilic electron carrier Coenzyme Q (CoQ) between the ubiquinol and ubiquinone forms. These reactions can result in the net movement of protons across a lipid bilayer (in the case of the mitochondria, the inner mitochondrial membrane). The Q cycle was first proposed by Peter D. Mitchell, though a modified version of Mitchell's original scheme is now accepted as the mechanism by which Complex III moves protons (i.e. how complex III contributes to the biochemical generation of the proton or pH, gradient, which is used for the biochemical generation of ATP). The first reaction of Q cycle is the 2-electron oxidation of ubiquinol by two oxidants, c1 (Fe3+) and ubiquinone:

CoQH2 + cytochrome c1 (Fe3+) + CoQ' → CoQ + CoQ'−• + cytochrome c1 (Fe2+) + 2 H+ (intermembrane) The second reaction of the cycle involves the 2-electron oxidation of a second ubiquinol by two oxidants, a fresh c1 (Fe3+) and the CoQ'−• produced in the first step:

CoQH2 + cytochrome c1 (Fe3+) + CoQ'−• + 2 H+ (matrix)→ CoQ + CoQ'H−2 + cytochrome c1 (Fe2+) + 2 H+ (intermembrane) These net reactions are mediated by electron-transfer mediators including a Rieske 2Fe-2S cluster (shunt to c1) and cb (shunt to CoQ' and later to CoQ'−•) In chloroplasts, a similar reaction is done with plastoquinone by cytochrome b6f complex.

Process 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

References Trumpower, B.L. (2002) Biochim. Biophys. Acta 1555, 166-173 Hunte, C., Palsdottir, H. and Trumpower, B.L. (2003) FEBS Letters 545, 39-46 Trumpower, B.L. (1990) J. Biol. Chem., 11409-11412

Illustrations

Worked examples

Example 1 — a first encounter with Q cycle

Start with the simplest possible case. Write down what Q cycle claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Q cycle before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Q cycle ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Q cycle

In research: Q cycle appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Q cycle in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Q cycle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biochemical reactions, Cellular respiration, Metabolism, so understanding it makes those chapters shorter.
In everyday life: Look for Q cycle outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Q cycle in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Q cycle means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Q cycle out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Q cycle in simple terms?

Why does Q cycle matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Q cycle?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Q cycle.