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Chapter 20: Problem 14

Describe in your own words the path of electrons through the \(\mathrm{Q}\) cycle of Complex III.

Short Answer

Expert verified
The Q cycle of Complex III in the electron transport chain involves the movement of electrons from QH2, through several proteins, to cytochrome c and ubiquinone (Q). The cycle facilitates the movement of protons across the membrane, contributing to the proton gradient needed for ATP synthesis.

Step by step solution

01

Initiation of the Q Cycle

The Q cycle begins when a molecule of ubiquinol (QH2) binds to the Qo site (also known as Qp) of Complex III. This QH2 has gained two protons and two electrons from a previously occurring chemical reaction in Complex I or II of the electron transport chain. Upon binding, the QH2 is oxidized, releasing two electrons which follow separate paths.
02

Path of the First Electron

The first electron goes through a series of proteins including an Fe-S cluster and cytochrome c1. The electron is passed on and ultimately reduces a molecule of cytochrome c, a soluble protein in the intermembrane space. At the same time, this oxidation of cytochrome c1 causes the release of a proton (H+) from the matrix.
03

Path of the Second Electron

The second electron reduces an ubiquinone (Q) molecule that is located at the Qi site (also known as Qn), forming a semiquinone (SQ) intermediate. It's important to note that this occurs in a nearby position, within the same Complex III.
04

Completion of the Q Cycle

The cycle is completed when a second QH2 molecule binds to the Qo site, allowing another pair of electrons to be removed. The process described above is repeated - one electron reduces another molecule of cytochrome c, whereas the second reduces the semiquinone (SQ) intermediate at the Qi site to ubiquinol (QH2), resulting in the uptake of two protons from the mitochondrial matrix. The reduced ubiquinol (QH2) remains bound to the Qi site, ready for the next Q cycle, and a total of four protons have been moved to the intermembrane space.

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Most popular questions from this chapter

A wealthy investor has come to you for advice. She has been approached by a biochemist who seeks financial backing for a company that would market dinitrophenol and dicumarol as weight-loss medications. The biochemist has explained to her that these agents are uncouplers and that they would dissipate metabolic energy as heat. The investor wants to know if you think she should invest in the biochemist's company. How do you respond?

Consider the oxidation of succinate by molecular oxygen as carried out via the electron-transport pathway \\[ \text { Succinate }+\frac{1}{2} \mathrm{O}_{2} \longrightarrow \text { fumarate }+\mathrm{H}_{2} \mathrm{O} \\] a. What is the standard free energy change \(\left(\Delta G^{\circ}\right)\) for this reaction if \\[ \mathscr{E}_{\mathrm{o}}^{\prime}(\mathrm{Fum} / \mathrm{Succ})=+0.031 \mathrm{V} \text { and } \mathscr{E}_{\mathrm{o}}^{\prime}\left(\frac{1}{2} \mathrm{O}_{2} / \mathrm{H}_{2} \mathrm{O}\right)=+0.816 \mathrm{V} \\] b. What is the equilibrium constant \(\left(K_{\mathrm{eq}}\right)\) for this reaction? c. Assume that (1) the actual free energy release accompanying succinate oxidation by the electron-transport pathway is equal to the amount released under standard conditions (as calculated in part a \(),(2)\) this energy can be converted into the synthesis of ATP with an efficiency \(=0.7\) (that is, \(70 \%\) of the energy released upon succinate oxidation is captured in ATP synthesis), and (3) the oxidation of 1 succinate leads to the phosphorylation of 2 equivalents of ATP. Under these conditions, what is the maximum ratio of [ATP]/ [ADP] attainable by oxidative phosphorylation when \(\left[\mathrm{P}_{\mathrm{i}}\right]=1 \mathrm{m} M ?\) (Assume \(\Delta G^{\circ \prime}\) for ATP synthesis \(=+30.5 \mathrm{kJ} / \mathrm{mol} .\) )

Assume that the free energy change \((\Delta G)\) associated with the movement of 1 mole of protons from the outside to the inside of a bacterial cell is \(-23 \mathrm{kJ} / \mathrm{mol}\) and \(3 \mathrm{H}^{+}\) must cross the bacterial plasma membrane per ATP formed by the bacterial \(\mathrm{F}_{1} \mathrm{F}_{0}-\mathrm{ATP}\) synthase. ATP synthesis thus takes place by the coupled process: $$3 \mathrm{H}_{\mathrm{out}}^{+}+\mathrm{ADP}+\mathrm{P}_{\mathrm{i}} \rightleftharpoons 3 \mathrm{H}_{\mathrm{in}}^{+}+\mathrm{ATP}+\mathrm{H}_{2} \mathrm{O}$$ a. If the overall free energy change \(\left(\Delta G_{\text {overall }}\right)\) associated with ATP synthesis in these cells by the coupled process is \(-21 \mathrm{kJ} / \mathrm{mol}\), what is the equilibrium constant \(\left(K_{\mathrm{eq}}\right)\) for the process? b. What is \(\Delta G_{\text {synthesis }},\) the free energy change for ATP synthesis, in these bacteria under these conditions? c. The standard free energy change for ATP hydrolysis ( \(\Delta G^{\text {o' }}\) hydrolysis) is \(-30.5 \mathrm{kJ} /\) mol. If \(\left[\mathrm{P}_{\mathrm{i}}\right]=2 \mathrm{m} M\) in these bacterial cells, what is the \([\mathrm{ATP}] /[\mathrm{ADP}]\) ratio in these cells?

Considering that all other dehydrogenases of glycolysis and the TCA cycle use NADH as the electron donor, why does succinate dehydrogenase, a component of the TCA cycle and the electron transfer chain, use FAD as the electron acceptor from succinate, rather than \(\mathrm{NAD}^{+}\) ? Note that there are two justifications for the choice of FAD here-one based on energetics and one based on the mechanism of electron transfer for FAD versus \(\mathrm{NAD}^{+}\).

In the course of events triggering apoptosis, a fatty acid chain of cardiolipin undergoes peroxidation to release the associated cytochrome \(c .\) Lipid peroxidation occurs at a double bond. Suggest a mechanism for the reaction of hydrogen peroxide with an unsaturation in a lipid chain, and identify a likely product of the reaction.
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