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An Overview of the Q Cycle

For some of you who are reading this, you may have never heard of the Q cycle before, but you probably know something about cellular respiration.


How are they related?


Well, the Q cycle plays an indispensable role in aerobic respiration. Let’s have a quick look at what this is!

A little bit about aerobic respiration first...

Adenosine triphosphate, also known as ATP, is an extremely important molecule that acts as the energy currency of the cell. The hydrolysis of ATP into adenosine diphosphate (ADP) and a phosphate group results in release of a huge amount of stored energy. This, in turn, powers many cellular reactions.


Most ATP is generated through aerobic respiration in the mitochondria of a cell. To dive deeper into the process, aerobic respiration involves several stages, including glycolysis, citric acid cycle (also known as TCA cycle or Krebs cycle) and oxidative phosphorylation, which take place in the cytoplasm, the mitochondrial matrix and the inner mitochondrial membrane, respectively.


Both glycolysis and the citric acid cycle produce a small amount of ATP by substrate level phosphorylation, while the vast majority of ATP is produced during oxidative phosphorylation using proteins of the electron transport chain.

Moving forward, in oxidative phosphorylation, the electrons produced in previous stages are transferred to oxygen in order to generate water through a series of large protein complexes that are embedded in the inner mitochondrial membrane. This series of protein complexes is known as electron transport chain (ETC). In general, electron flow through the ETC is a highly exergonic process (i.e. energy is released during the process) that powers the transport of protons from the matrix to the intermembrane space, generating a proton gradient across the inner mitochondrial membrane. Protons will then flow back into the matrix through ATP synthase, thereby powering the synthesis of ATP from ADP and an inorganic phosphate group (Pi).


What is the Q cycle?

The Q cycle describes a series of reactions where electrons are transferred from ubiquinol (QH2) all the way up to another electron carrier called cytochrome c as part of the electron transport that occurs in oxidative phosphorylation.


The Q cycle takes place in Complex III of the electron transport chain (ETC) in the inner mitochondrial membrane. This process is coupled to transmembrane proton transport, thereby contributing to the biochemical generation of a proton gradient across the inner mitochondrial membrane. As a result, this proton gradient can be used to drive ATP synthesis by the ATP synthase.

Check out Figure 1 if you need help with visualising the process!


Why do cells need the Q cycle?

The Q cycle plays an indispensable role in the ETC.


QH2 is a 2e- carrier that passes two electrons all the way to cytochrome c via Complex III, whereas cytochrome c is a 1e- carrier (meaning that it can only accept one electron). Hence, the Q cycle allows cells to switch from a 2e- carrier to a 1e- carrier.


If the Q cycle fails to function properly, only one of the two electrons on QH2 can be passed to cytochrome c and further down to oxygen. This would lead to a huge reduction in ATP production since only half the electrons produced in the previous stage could be passed to its final acceptor - oxygen.


Given that cells need ATP to support many cellular processes, including active transport and muscle movement - without sufficient energy, the cells would start to die and eventually, so would the organism itself.

Figure 1: Oxidative phosphorylation and the electron transport chain. Electrons carried by NADH and FADH2 generated in glycolysis and citric acid cycle flow through proteins of the electron transport chain. This drives the pumping of protons across the inner membrane to generate proton motive force, which, in turn, powers the synthesis of ATP through ATP synthase. Figure taken from Shutterstock.

Q: an electron carrier and redox centre

Q stands for coenzyme Q, which is a hydrophobic (meaning "water-hating") quinone derivative. It can diffuse within the inner mitochondrial membrane, acting as mobile electron carriers that transfer electrons between complexes of the electron transport chain.



The fully oxidized state (Q) is known as ubiquinone with two keto groups. The addition of one electron and one proton gives rise to the semiquinone form (QH•). The proton on the semiquinone may dissociate to generate a semiquinone radical anion (QH•-).


Meanwhile, the fully reduced form of coenzyme Q is called ubiquinol (QH2), which is generated by the addition of a second electron and proton to semiquinone. Once QH2 is produced, electron transfer on coenzyme Q is coupled with proton binding and release. This in turn contributes to the proton gradient across the inner mitochondrial membrane, which can drive ATP synthesis through oxidative phosphorylation.

Figure 2: Oxidation states of coenzyme Q. Ubiquinone(Q) is reduced to semiquinone (QH•) by accepting one electron and one proton. Semiquinone radical anion (QH•-) could be formed by dissociation of a proton from semiquinone. On the other hand, the reduction of semiquinone to ubiquinol (QH2) requires another electron and proton.

Complex III

Complex III is also known as Q-cytochrome c oxidoreductase, cytochrome c reductase, or cytochrome bc1 complex.


This protein is responsible for accepting electrons from ubiquinol and passing it to cytochrome c. It is a homodimer (meaning a molecule constituting a pair of identical protein monomers) with each monomer consisting of 11 polypeptide chains. It contains three important subunits that allow electron transport, namely:

  1. Cytochrome c1 that contains a single haem group.

  2. Cytochrome b that consists of two different haem groups: bL (L for low affinity) and bH (H for high affinity)

  3. The iron-sulphur protein with a 2Fe-2S centre, known as the Rieske Centre

A cytochrome is a redox-active (i.e. active in reduction-oxidation reactions) protein that contains a haem prosthetic group as cofactor (i.e. a non-protein compound that acts as a catalyst; e.g. metal ions or organic compounds).


In the centre of the haem group is an iron atom bonded to four equatorial nitrogen atoms in a porphyrin ring (i.e. a large ring structure that is made out of four pyrroles). In addition to these equatorial covalent bonds, the iron is also axially coordinated to the side chains of two amino acids, allowing the iron to adopt octahedral coordination.


The iron can shift between a reduced ferrous state (Fe2+) and an oxidized ferric state (Fe3+) during electron transfer. Likewise, the iron in Rieske Centre is also capable of accepting and donating electrons.

Cytochrome c

Cytochrome c is another small, soluble cytochrome protein that moves along the intermembrane space side of the inner mitochondrial membrane. It has a binding site on Complex III where it accepts electrons from ubiquinol. The reduced form of cytochrome c can then diffuse away to the binding site on complex IV, where it donates the electron and becomes oxidized again.

Figure 3: Structures of cytochrome c and the haem group. Cytochrome c is a redox-active protein containing a haem c group. The haem group has a porphyrin ring with a central iron atom as the redox centre, which can shift its oxidation states between +2 and +3. Not all haem groups are exactly the same, the R groups can differ different haem molecules. Tienerated using figures from Shutterstock.


What takes place during the Q cycle?

The overall reaction in the Q cycle can be summarized as the following equation where one ubiquinol molecule is oxidized to ubiquinone, with its two electrons passed on to two molecules of oxidized cytochrome c to form two reduced cytochrome c molecules. Meanwhile, two protons (H+) are taken from the matrix and four protons are released into intermembrane space of mitochondria.

QH2 + 2 Cyt c (oxidized) + 2H+matrix → Q + 2 Cyt c (reduced) + 4 protons

Let’s zoom in on this a little bit and review the process of the Q cycle in more detail!

The Q cycle consists of two half cycles.

Both Complex I and Complex II of electron transport chain pass their electrons to Q, generating its reduced form QH2. Two QH2 molecules travel onto Complex III and bind to it consecutively, each releasing two electrons and two protons. There are two quinone binding sites on Complex III, namely Qin and Qout. Qout is near the intermembrane space side of the membrane, while Qin is near the matrix side of the membrane.

In the first half-cycle, QH2 binds to the Qout site, releasing its two protons to the intermembrane space of mitochondria. Apart from the two protons, there are also two electrons released by QH2. These two electrons follow different pathways. One of the electrons is passed to the 2Fe-2S group on the Rieske Centre, and then to the haem group of cytochrome c1 before reaching the oxidized cytochrome c molecule, converting it to its reduced form. The reduced cytochrome c can then detach from complex III and diffuses away to complex IV.

The other electron cannot be accepted by that cytochrome c molecule. Therefore, an additional Q molecule attaches to the Qin site of Complex III to recycle the second electron. The second electron passes through haem bL and haem bH groups of cytochrome b, and finally onto a ubiquinone molecule attached on Qin site. The Q is reduced to a semiquinone radical anion (Q•-) by accepting the second electron of QH2.

The overall equation of the first half cycle is the following:

QH2 + Cyt c (oxidized) → Q•- + Cyt c (reduced) + 2 protons

In the second half-cycle, another QH2 binds to the Qout site. Similarly, the two protons are pumped into the intermembrane space of mitochondria and the two electrons follow the same pathways as before. The electron following the upper pathway shown in Figure 4 ends up in another oxidized cytochrome c molecule, and, again, reduces cytochrome c. The other electron passes through two haem groups on cytochrome b to the partially reduced semiquinone radical anion (Q•-), making a fully reduced quinone radical anion. This quinone radical anion immediately takes up two protons from the matrix to form QH2. This QH2 regenerated on Qin site can detach, and then reattach onto complex III’s Qout site to produce more reduced cytochrome c.

The overall equation of the second half cycle is the following:

QH2 + Q•- + Cyt c (oxidized) + 2H+matrix → Q + QH2 + Cyt c (reduced) + 2H+cytoplasm

In essence, the cytochrome b component is a recycling device that allows both electrons on QH2 to be used effectively. Even though two QH2 molecules participate in the reaction, one is regenerated through cytochrome b. This is the “cyclic” component of the Q cycle.

Overall, one net QH2 molecule passes its two electrons to two molecules of cytochrome c in a single Q cycle.

Hence, the Q cycle can be expressed in the following overall reaction:

QH2 + 2 Cyt c (oxidized) + 2H+matrix → Q + 2 Cyt c (reduced) + 4 protons


Figure 4: A schematic diagram of the Q cycle. The Q cycle takes place in Complex III, which is embedded in the inner mitochondrial membrane. In the first half cycle, the two electrons of QH2 are transferred, one to an oxidized form of cytochrome c and the second to another Q molecule on Qin site to form reduced form of cytochrome c and a semiquinone (QH·), respectively. Meanwhile, two protons are released into the intermembrane space. In the second half cycle, a second QH2 releases its two electrons to Complex III. One of them goes to reduce cytochrome c again, whilst the other goes to reduce QH• to the semiquinone radical anion (Q·-) in order to pick up two protons from the matrix. As a result, a fully reduced ubiquinol (QH2) is formed. The electron transfer in the second half cycle results in the uptake of two protons from the matrix.

Why should we care about the Q cycle?

The Q cycle is crucial to make use of both electrons on QH2 effectively by switching from the two-electron carrier QH2 to the one-electron carrier cytochrome c. Without the Q cycle, one of the two electrons on QH2 could not be transferred through the rest of the ETC, and thus not sufficient proton gradient would be generated to power ATP synthesis. Lack of power supply could lead to severe consequences on cellular metabolism, and eventually result in cell death.

In most cases, the Q cycle refers to the series of reactions that take place with coenzyme Q on complex III of the respiratory chain. However, this does not mean that the Q cycle can only occur in mitochondria.


There is also important application of the Q cycle in other biological systems. For instance, in the thylakoid membranes of photosynthetic organisms, a similar reaction is done with plastoquinone by cytochrome b6f complex. In fact, the current Q cycle model in cytochrome b6f complex derives from the Q cycle mechanism in complex III.

All in all, it is remarkable that the Q-cycle mechanism is highly conserved across different organisms, indicating common evolutionary origin of Q-cycle mechanism as well as protein complexes involved. This also suggests energy conservation mechanism is crucial for a wide range of organisms!

Author: Helen Luojia Zhang, BSc Biochemistry

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