Role of the Isoprenyl Tail of Ubiquinone in Reaction with Respiratory Enzymes: Studies with Bovine Heart Mitochondrial Complex I and Escherichia coli bo -Type Ubiquinol Oxidase † (original) (raw)
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Bioscience, biotechnology, and biochemistry, 2016
We previously produced the unique ubiquinone QT ("decoupling" quinone), the catalytic reduction of which in NADH-quinone oxidoreduction with bovine heart mitochondrial NADH-ubiquinone oxidoreductase (complex I) is completely decoupled from proton translocation across the membrane domain. This feature is markedly distinct from those of typical short-chain quinones such as ubiquinone-1. To further characterize the features of the QT reaction with complex I, we herein synthesized three QT analogs, QT2-QT4, and characterized their electron transfer reactions. We found that all aspects of electron transfer (e.g. electron-accepting activity and membrane potential formation) vary significantly among these analogs. The features of QT2 as decoupling quinone were slightly superior to those of original QT. Based on these results, we conclude that the bound positions of QTs within the quinone binding cavity susceptibly change depending on their side-chain structures, and the positions...
Journal of bioenergetics and biomembranes, 2001
Studies of the structure-activity relationships of ubiquinones and specific inhibitors are helpful to probe the structural and functional features of the ubiquinone reduction site of bovine heart mitochondrial complex I. Bulky exogenous short-chain ubiquinones serve as sufficient electron acceptors from the physiological ubiquinone reduction site of bovine complex I. This feature is in marked contrast to other respiratory enzymes such as mitochondrial complexes II and III. For various complex I inhibitors, including the most potent inhibitors, acetogenins, the essential structural factors that markedly affect the inhibitory potency are not necessarily obvious. Thus, the loose recognition by the enzyme of substrate and inhibitor structures may reflect the large cavity like structure of the ubiquinone (or inhibitor) binding domain in the enzyme. On the other hand, several phenomena are difficult to explain by a simple one-catalytic site model for ubiquinone.
Journal of Biological Chemistry
NADH-quinone oxidoreductase (complex I) couples electron transfer from NADH to quinone with proton translocation across the membrane. Quinone reduction is a key step for energy transmission from the site of quinone reduction to the remotely located proton-pumping machinery of the enzyme. Although structural biology studies have proposed the existence of a long and narrow quinone-access channel, the physiological relevance of this channel remains debatable. We investigated here whether complex I in bovine heart submitochondrial particles (SMPs) can catalytically reduce a series of oversized ubiquinones (OS-UQs), which are highly unlikely to transit the narrow channel because their side chain includes a bulky “block” that is ~13 Å across. We found that some OS-UQs function as efficient electron acceptors from complex I, accepting electrons with an efficiency comparable to ubiquinone-2. The catalytic reduction and proton translocation coupled with this reduction were completely inhibit...
Hybrid ubiquinone: novel inhibitor of mitochondrial complex I
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2002
We synthesized novel ubiquinone analogs by hybridizing the natural ubiquinone ring (2,3-dimethoxy-5-methyl-1,4-benzoquinone) and hydrophobic phenoxybenzamide unit, and named them hybrid ubiquinones (HUs). The HUs worked as electron transfer substrates with bovine heart mitochondrial succinate-ubiquinone oxidoreductase (complex II) and ubiquinol-cytochrome c oxidoreductase (complex III), but not with NADH-ubiquinone oxidoreductase (complex I). With complex I, they acted as inhibitors in a noncompetitive manner against exogenous short-chain ubiquinones irrespective of the presence of the natural ubiquinone ring. Elongation of the distance between the ubiquinone ring and the phenoxybenzamide unit did not recover the electron accepting activity. The structure/activity study showed that high structural specificity of the phenoxybenzamide moiety is required to act as a potent inhibitor of complex I. These findings indicate that binding of the HUs to complex I is mainly decided by some specific interaction of the phenoxybenzamide moiety with the enzyme. It is of interest that an analogous bulky and hydrophobic substructure can be commonly found in recently registered synthetic pesticides the action site of which is mitochondrial complex I.
European journal of biochemistry / FEBS, 1996
To characterize the structural features of the ubiquinone reduction site of glucose dehydrogenase (GlcDH) in Escherichia coli, we performed structure/activity studies of a systematic set of synthetic ubiquinone analogues and specific inhibitors (synthetic capsaicins) of this site. Considering the proposed similarity of the quinone binding domain motif between GlcDH and one subunit of mitochondrial complex I [Friedrich, T., Strohdeicher, M., Hofhaus, G., Preis, D., Sahm, H. & Weiss, H. (1990) FEBS Lett. 265, 37-40], we compared the structure/activity profiles of the substrates and inhibitors for GlcDH with those for bovine heart mitochondrial complex i. With respect to GlcDH, replacement of one or both methoxy groups in the 2 and 3 positions of benzoquinone ring by ethoxy group(s) resulted in a drastic decrease in the electron accepting activity. The presence of a 5-methyl group and the conformational property of the 6-alkyl side chain did not significantly contribute to the activity...
Three Molecules of Ubiquinone Bind Specifically to Mitochondrial Cytochrome bc1 Complex
Journal of Biological Chemistry, 2001
Bifurcated electron flow to high potential "Rieske" iron-sulfur cluster and low potential heme b L is crucial for respiratory energy conservation by the cytochrome bc 1 complex. The chemistry of ubiquinol oxidation has to ensure the thermodynamically unfavorable electron transfer to heme b L. To resolve a central controversy about the number of ubiquinol molecules involved in this reaction, we used high resolution magic-angle-spinning nuclear magnetic resonance experiments to show that two out of three n-decyl-ubiquinones bind at the ubiquinol oxidation center of the complex. This substantiates a proposed mechanism in which a charge transfer between a ubiquinol/ubiquinone pair explains the bifurcation of electron flow.
Archives of Biochemistry and Biophysics, 2000
Determination of the number of ubiquinone-and inhibitor-binding sites in the mitochondrial complex I (NADH:ubiquinone oxidoreductase) is a controversial question with a direct implication for elaborating a suitable model to explain the bioenergetic mechanism of this complicated enzyme. We have used combinations of both selective inhibitors and common ubiquinone-like substrates to demonstrate the multiplicity of the reaction centers in the complex I in contrast with competition studies that have suggested the existence of a unique binding site for ubiquinone. Our results provide new evidence for the existence of at least two freely exchangeable ubiquinone-binding sites with different specificity for substrates, as well as for a different kinetic interaction of inhibitors with the enzyme.
A New Ubiquinone Metabolite and Its Activity at the Mitochondrial bc 1 Complex
Chemical Research in Toxicology, 2007
Ubichromanol, a reductive cyclization product of ubiquinone, acts as radical scavenging antioxidant and is similarly effective as R-tocopherol. However, nothing is known so far on the two-electron oxidation product of this antioxidant and its bioactivity. This study demonstrates that ubichromanol yields a ubiquinone-like compound with a hydroxyl-substituted side chain (UQOH) on oxidation. HPLC/MS and HPLC/ECD measurements revealed its natural presence in bovine liver mitochondria. The bioactivity of this formerly unknown compound as substrate for mitochondrial complex III was tested by measurements of the quinol:cytochrome c oxidoreductase activity in bovine submitochondrial particles and isolated mitochondrial bc 1 complex. Consistently in both model systems, reduced UQOH exhibited substrate efficiencies below that of native ubiquinone but a significantly higher efficiency than R-tocopheryl quinone. Model calculations revealed that on binding of reduced UQOH to the bc 1 complex the polar hydroxyl group was located close to hydrophobic amino acid residues. This fact could in part explain the lower efficiency of reduced UQOH in comparison to ubiquinone as a substrate for the mitochondrial bc 1 complex. Therefore, the hydroxylation of the aliphatic or isoprenoid side chains of bioquinones, which is typical for quinoid oxidation products of chromanols, such as R-tocopherol and ubichromanol, disturbs substrate binding at the mitochondrial electron-transfer complexes, which usually interact with ubiquinone.
Tightly-bound ubiquinone in the Escherichia coli respiratory Complex I
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2012
a b s t r a c t NADH:ubiquinone oxidoreductase (Complex I), the electron input enzyme in the respiratory chain of mitochondria and many bacteria, couples electron transport to proton translocation across the membrane. Complex I is a primary proton pump; although its proton translocation mechanism is yet to be known, it is considered radically different from any other mechanism known for redox-driven proton pumps: no redox centers have been found in its membrane domain where the proton translocation takes place. Here we studied the properties and the catalytic role of the enzyme-bound ubiquinone in the solubilized, purified Complex I from Escherichia coli. The ubiquinone content in the enzyme preparations was 1.3 ± 0.1 per bound FMN residue. Rapid mixing of Complex I with NADH, traced optically, demonstrated that both reduction and re-oxidation kinetics of ubiquinone coincide with the respective kinetics of the majority of Fe-S clusters, indicating kinetic competence of the detected ubiquinone. Optical spectroelectrochemical redox titration of Complex I followed at 270-280 nm, where the redox changes of ubiquinone contribute, did not reveal any transition within the redox potential range typical for the membrane pool, or loosely bound ubiquinone (ca. + 50-+ 100 mV vs. NHE, pH 6.8). The transition is likely to take place at much lower potentials (E m ≤−200 mV). Such perturbed redox properties of ubiquinone indicate that it is tightly bound to the enzyme's hydrophobic core. The possibility of two ubiquinone-binding sites in Complex I is discussed.