Complexes of the electron transport chain (ETC) establish a proton gradient acro

Question

Complexes of the electron transport chain (ETC) establish a proton gradient across the inner mitochondrial membrane at the same time as they carry electrons to the ultimate electron acceptor, O2. What are the two main ways in which protons are moved across the membrane? Explain how these mechanisms work, and how they compare to one another. Which mechanism is used by each of the main complexes of the ETC (I, III, IV)? If this is unknown for one or more of the complexes, provide evidence supporting either of the two mechanisms.

Explanation / Answer

An electron transport chain (ETC) is a series of compounds that transfer electrons from electron donors toelectron acceptors via redox reactions, and couples this electron transfer with the transfer of protons (H+ions) across a membrane. This creates an electrochemical proton gradient that drives ATP synthesis, or the generation of chemical energy in the form of adenosine triphosphate (ATP). The final acceptor of electrons in the electron transport chain is molecular oxygen

The two ways by which protopns are transferred:

1. The underlying force driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free energy is the energy available ("free") to do work. Any reaction that decreases the overall Gibbs free energy of a system is thermodynamically spontaneous.

2. a transmembrane proton electrochemical gradient as a result of the redox reactions.If protons flow back through the membrane, they enable mechanical work, such as rotating bacterial flagella. ATP synthase, an enzyme highly conserved among all domains of life, converts this mechanical work into chemical energy by producing ATP,which powers most cellular reactions and help intransport of protons.

In Complex I , NADH:ubiquinone oxidoreductase or more simply NADH dehydrogenase EC 1.6.5.3, two electrons are removed from NADH and transferred to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH2), freely diffuses within the membrane, and Complex I translocates four protons (H+) across the membrane, thus producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide

n Complex II (succinate dehydrogenase; EC 1.3.5.1) additional electrons are delivered into the quinone pool (Q) originating from succinate and transferred (via FAD) to Q. Complex II consists of four protein

subunits: SDHA, SDHB, SDHC, and SDHD. Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD). Complex 2 is a parallel electron transport pathway to complex 1, but unlike complex 1, no protons are transported to the intermembrane space in this pathway. Therefore, the pathway through complex 2

In Complex III, the Q-cycle contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH2 at the QO site and sequentially transferred to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Qi site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol (2H+2e-) oxidations at the Qo site to form one quinol (2H+2e-) at the Qi site. (in total six protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules).contributes less energy to the overall electron transport chain process.

,For example In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is

NADH ? Complex I ? Q ? Complex III ? cytochrome c ? Complex IV ? O2

where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers. The electron acceptor is molecular oxygen

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