Rich 1984 Biochim Biophys Acta

Rich PR (1984) Electron and proton transfers through quinones and cytochrome bc complexes. Biochim Biophys Acta 768:53-79.

» PMID: 6322844

Rich Peter R (1984) Biochim Biophys Acta

Abstract: It is the aim of this article to discuss the details of electron and proton transfers through quinones and cytochrome bc complexes. Emphasis will be placed on the molecular organisation and mobility of components, on the chemistry of the individual redox steps, and on the relation of these factors to overall protonmotive ability.

• Bioblast editor: Gnaiger E

Labels:

Enzyme: Complex III 

Q, MitoFit 2021 CoQ 

Selected quotes

• "Solid state" and "liquid state" as extremes of molecular organisation: The biological electron transfer chains consist

of a number of large multiprotein complexes which generally span the lipid bilayer membranes in which they are situated. They are often connected to each other electronically by smaller components, which may be located in the membrane or in the aqueous phase. A specific example of the interaction of succinate dehydrogenase and NADH dehydrogenase with the bc1 complex in mitochondria will be used for discussion. It is known that quinone plays a role as the electronic connector. Two extreme types of molecular organisation may be envisaged, which may loosely be termed 'liquid state' and 'solid state'. .. In a liquid-state system, one or all components are freely diffusing and hence randomly arranged in the membrane as a two-dimensional solution. Such a notion was originally applied to the quinone pool of mitochondria by Green [21] and by Kröger and Klingenberg [22]. Interactions occur by collisional processes and long-lived intermediates are not readily observed, since electron transfer is rapid after an appropriate complex has been formed [23]. The system will be rate-limited by diffusional processes and an increase in reactant concentrations will produce an increased rate. In an ideal solution, a quinol produced by a given dehydrogenase is potentially able to interact with a large number of cytochrome bc1 complexes within its expected lifetime. This number of complexes, N, will be determined by the cycling time (between quinol and quinone) of the quinol, by diffusion constants, by association/dissociation rate constants, and by the distances between components. (N.B., In the following, N represents the number of acceptors within range of a quinol of lifetime t. The actual number of acceptors which lie in the area swept out by a single given quinol will be rather lower than N.)

Some numerical values

• 2 10 6 cm2/g protein for the surface area of the inner membrane [25]
• cytochrome bc 1 content of around 0.2 nmol per mg protein [26]
• area per bc1 dimer of around 200 000 Anstrom2 (2000 nm2) [27]
• diffusion coefficient, D, for ubiquinone of 10-8 cm2 s-1 by analogy with phospholipid

• State 3 {OXPHOS state}

• throughput of the cytochrome bc1 complex can be as high as 200 electrons/s per monomer in state 3 mitochondria (Einstein-Smoluchowski equation)
• ratio of active quinone/bc1 monomer of at least 5 [22]
• turnover time per Q species of around 50 ms (assuming two electrons are throughput to cytochrome c from each QH 2 ~ Q

cycle)

• distance travelled x for the quinol form (assuming that it is quinol for only one-half of the 50 ms cycle) of around 2240 Angstrom (224 nm)
• approx. 80 cytochrome bc1 dimers are potentially reachable by a given quinol during its lifetime in state 3

• State 4 {LEAK state

• assuming a bc1 throughput of around 20 electrons s per monomer
• then x becomes 7000 Angstrom
• N rises to around 800

Cited by

• Komlódi T, Cardoso LHD, Doerrier C, Moore AL, Rich PR, Gnaiger E (2021) Coupling and pathway control of coenzyme Q redox state and respiration in isolated mitochondria. Bioenerg Commun 2021.3. https://doi.org/10.26124/bec:2021-0003