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Understanding the cytochrome bc complexes by what they don't do. The Q-cycle at 30

2005; Elsevier BV; Volume: 11; Issue: 1 Linguagem: Inglês

10.1016/j.tplants.2005.11.007

1878-4372

Jonathan L. Cape, Michael K. Bowman, David Kramer,

Electron Spin Resonance Studies

The cytochrome (cyt) bc1, b6f and related complexes are central components of the respiratory and photosynthetic electron transport chains. These complexes carry out an extraordinary sequence of electron and proton transfer reactions that conserve redox energy in the form of a trans-membrane proton motive force for use in synthesizing ATP and other processes. Thirty years ago, Peter Mitchell proposed a general turnover mechanism for these complexes, which he called the Q-cycle. Since that time, many opposing schemes have challenged the Q-cycle but, with the accumulation of large amounts of biochemical, kinetic, thermodynamic and high-resolution structural data, the Q-cycle has triumphed as the accepted model, although some of the intermediate steps are poorly understood and still controversial. One of the major research questions concerning the cyt bc1 and b6f complexes is how these enzymes suppress deleterious and dissipative side reactions. In particular, most Q-cycle models involve reactive semiquinone radical intermediates that can reduce O2 to superoxide and lead to cellular oxidative stress. Current models to explain the avoidance of side reactions involve unprecedented or unusual enzyme mechanisms, the testing of which will involve new theoretical and experimental approaches. The cytochrome (cyt) bc1, b6f and related complexes are central components of the respiratory and photosynthetic electron transport chains. These complexes carry out an extraordinary sequence of electron and proton transfer reactions that conserve redox energy in the form of a trans-membrane proton motive force for use in synthesizing ATP and other processes. Thirty years ago, Peter Mitchell proposed a general turnover mechanism for these complexes, which he called the Q-cycle. Since that time, many opposing schemes have challenged the Q-cycle but, with the accumulation of large amounts of biochemical, kinetic, thermodynamic and high-resolution structural data, the Q-cycle has triumphed as the accepted model, although some of the intermediate steps are poorly understood and still controversial. One of the major research questions concerning the cyt bc1 and b6f complexes is how these enzymes suppress deleterious and dissipative side reactions. In particular, most Q-cycle models involve reactive semiquinone radical intermediates that can reduce O2 to superoxide and lead to cellular oxidative stress. Current models to explain the avoidance of side reactions involve unprecedented or unusual enzyme mechanisms, the testing of which will involve new theoretical and experimental approaches. general scheme originally proposed by Peter Mitchell for the proton translocation mechanism of the cyt bc complexes. This hypothesis proposes that some of the electrons derived from QH2 oxidation in the cyt bc complex are recycled back into the Q-pool via a Q-reductase site on the enzyme, thereby increasing the proton translocation stoichiometry relative to a simple linear electron shuttle mechanism (see Box 2 for details). This type of reaction is called bifurcated electron transfer and its details are the subject of contentious debate. the three accessible redox states of quinone species – quinone (Q), semiquinone (SQ) and quinol (QH2) – each differing by one electron. Both the QH2 and SQ species are capable of assuming different protonation states at physiological pH. It is not clear which of these species participate in the Q-cycle. We use the vague term SQ, which does not discriminate between protonation states. a putative SQ species that is formed in the Qo site of the cyt bc complex from QH2 oxidation by the Rieske 2Fe2S cluster. Although certain models of the Q-cycle invoke the existence of this intermediate species, its thermodynamic stability is still open to debate. However, other models deny the existence of this intermediate altogether during normal Q-cycle turnover, favoring a concerted oxidation of QH2 to Q in the Qo site. the proton motive force that results from the vectorial transfer of protons across the energy conserving membrane by various electron transport proteins This force is partitioned into both osmotic (ΔpH) and electric field (ΔΨ) components that are energetically interchangeable for most purposes [4]. The proton motive force both activates and drives the synthesis of ATP at the CF1CFo-ATP synthase. the quinol oxidase (Qo) and quinone reductase (Qi) sites housed in the cyt b subunit of the bc complex. The Qo site catalyzes the transfer of electrons from QH2 to the Rieske 2Fe2S cluster and cyt bL, and the Qi site catalyzes the reduction of quinone by cyt bH. In the b6f complex, the Qi site contains an additional heme cofactor, tentatively named heme ci or x, whose function is presently unknown. the negatively and positively charged sides of the energetic membrane. QH2 oxidation and proton release from the Qo site occurs on the p-side of the membrane, and quinone reduction with associated proton uptake occurs on the n-side of the membrane. the cyt bc complex contains two separate redox chains. The high-potential chain transfers electrons from the QH2 at the Qo site to a soluble protein electron carrier via the Rieske 2Fe2S cluster and cyt c1 (in mitochondria) or cyt f (in the chloroplast). The low-potential chain links the Qo site to the Qi site via two b-type hemes, cyt bL and cyt bH, named for their respective redox potentials.