Abstract:In an attempt to establish the relative importance of diffusional and chemical control in the reactivity of the two substrates, ubiquinol and cytochrome c, we have undertaken an extensive characterization of the steady-state kinetics of ubiquinol-cytochrome c reductase (EC 1.10.2.2) when present in open submitochondrial particles from bovine heart. The kinetic pattern follows a Ping Pong mechanism; contrary to the situation found with the isolated enzyme [Speck and Margoliash (1984) J. Biol. Chem. 259, 1064-10… Show more
“…In addition, proteinâbinding during electron transfer may require unfolding, contributing to the high activation energy and low collision efficiency observed for electron transfer (e.g. [13,14]).…”
The function of the coenzyme Q (CoQ) pool in the inner mitochondrial membrane is reviewed in view of recent findings suggesting a supramolecular organization of the mitochondrial respiratory complexes. In spite of the structural evidence for preferential aggregations of the inner membrane components, most kinetic evidence is in favor of a dispersed organization based on random collisions of the small connecting redox components, in particular CoQ, with the individual complexes. The shape of the CoQ molecule in the pool, suggested to be a folded one, is in agreement with its very rapid lateral diffusion mobility in the membrane midplane. Since the structural evidence in favor of specific supercomplexes is rather strong, it cannot be excluded that electron transfer may follow either pool behavior or preferential channeling depending on the physiological conditions. Another function ascribed to the CoQ pool is the antioxidant action of the reduced CoQ molecules; although it cannot be excluded that protein-bound ubisemiquinones may be a source of oxygen radicals, particularly at the level of complex III, the available evidence suggests that the mitochondrial pool only behaves as an antioxidant under physiological conditions. Ă 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.
“…In addition, proteinâbinding during electron transfer may require unfolding, contributing to the high activation energy and low collision efficiency observed for electron transfer (e.g. [13,14]).…”
The function of the coenzyme Q (CoQ) pool in the inner mitochondrial membrane is reviewed in view of recent findings suggesting a supramolecular organization of the mitochondrial respiratory complexes. In spite of the structural evidence for preferential aggregations of the inner membrane components, most kinetic evidence is in favor of a dispersed organization based on random collisions of the small connecting redox components, in particular CoQ, with the individual complexes. The shape of the CoQ molecule in the pool, suggested to be a folded one, is in agreement with its very rapid lateral diffusion mobility in the membrane midplane. Since the structural evidence in favor of specific supercomplexes is rather strong, it cannot be excluded that electron transfer may follow either pool behavior or preferential channeling depending on the physiological conditions. Another function ascribed to the CoQ pool is the antioxidant action of the reduced CoQ molecules; although it cannot be excluded that protein-bound ubisemiquinones may be a source of oxygen radicals, particularly at the level of complex III, the available evidence suggests that the mitochondrial pool only behaves as an antioxidant under physiological conditions. Ă 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.
“…SMP were prepared from bovine heart mitochondria as described elsewhere [19]. CoQ-depleted SMP were prepared by pentane extraction of lyophilized particles according to Szarkowska [20], and reconstitution with CoQ 10 was achieved by adding the quinone in pentane to the dried sample as described by Norling et al [21] The CoQ content of diÂĄerent types of particles was determined by high performance liquid chromatography (HPLC) as described elsewhere [22].…”
The mitochondrial respiratory chain is a powerful source of reactive oxygen species, considered as the pathogenic agent of many diseases and of aging. We have investigated the role of Complex I in superoxide radical production in bovine heart submitochondrial particles and found, by combined use of specific inhibitors of Complex I and by Coenzyme Q (CoQ) extraction from the particles, that the one-electron donor in the Complex to oxygen is a redox center located prior to the binding sites of three different types of CoQ antagonists, to be identified with a Fe^S cluster, most probably N2 on the basis of several known properties of this cluster. Short chain CoQ analogs enhance superoxide formation, presumably by mediating electron transfer from N2 to oxygen. The clinically used CoQ analog, idebenone, is particularly effective in promoting superoxide formation. Ă 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.
“…For mitochondrial systems, recent work has involved assay of the isolated complex under steady-state conditions. In the earlier work Fato et al (257) assayed the rate as a function of [substrate] with either cyt c or quinol limiting, but the pH was not varied. More recently, Brandt & Okun (228) have measured the steady-state activation energy as a function of pH, and observed a strong dependence, at least in the alkaline range.…”
Section: Activation Barriers In Quinol Oxidationmentioning
Key Words oxidoreductase, respiratory chain, electron transfer, crystallography, membrane protein s Abstract The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc 1 complexes and the chloroplast and cyanobacterial b 6 f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b 6 f complex.
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