Rapid redox equilibrium between the mitochondrial Q pool and cytochrome b during triphasic reduction of cytochrome b by succinate

Heat stress-induced effects of photosystem I: an overview of structural and functional responses

Temperature is one of the main factors controlling the formation, development, and functional performance of the photosynthetic apparatus in all photoautotrophs (green plants, algae, and cyanobacteria) on Earth. The projected climate change scenarios predict increases in air temperature across Earth's biomes ranging from moderate (3-4 °C) to extreme (6-8 °C) by the year 2100 (IPCC in Climate change 2007: The physical science basis: summery for policymakers, IPCC WG1 Fourth Assessment Report 2007; Climate change 2014: Mitigation of Climate Change, IPCC WG3 Fifth Assessment Report 2014). In some areas, especially of the Northern hemisphere, even more extreme warm seasonal temperatures may occur, which possibly will cause significant negative effects on the development, growth, and yield of important agricultural crops. It is well documented that high temperatures can cause direct damages of the photosynthetic apparatus and photosystem II (PSII) is generally considered to be the primary target of heat-induced inactivation of photosynthesis. However, since photosystem I (PSI) is considered to determine the global amount of enthalpy in living systems (Nelson in Biochim Biophys Acta 1807:856-863, 2011; Photosynth Res 116:145-151, 2013), the effects of elevated temperatures on PSI might be of vital importance for regulating the photosynthetic response of all photoautotrophs in the changing environment. In this review, we summarize the experimental data that demonstrate the critical impact of heat-induced alterations on the structure, composition, and functional performance of PSI and their significant implications on photosynthesis under future climate change scenarios.

Ubiquinone at center N is responsible for triphasic reduction of cytochrome b in the cytochrome bc(1) complex

We have examined the pre-steady state reduction kinetics of the Saccharomyces cerevisiae cytochrome bc(1) complex by menaquinol in the presence and absence of endogenous ubiquinone to elucidate the mechanism of triphasic cytochrome b reduction. With cytochrome bc(1) complex from wild type yeast, cytochrome b reduction was triphasic, consisting of a rapid partial reduction phase, an apparent partial reoxidation phase, and a slow rereduction phase. Absorbance spectra taken by rapid scanning spectroscopy at 1-ms intervals before, during, and after the apparent reoxidation phase showed that this was caused by a bona fide reoxidation of cytochrome b and not by any negative spectral contribution from cytochrome c(1). With cytochrome bc(1) complex from a yeast mutant that cannot synthesize ubiquinone, cytochrome b reduction by either menaquinol or ubiquinol was rapid and monophasic. Addition of ubiquinone restored triphasic cytochrome b reduction, and the duration of the reoxidation phase increased as the ubiquinone concentration increased. When reduction of the cytochrome bc(1) complex through center P was blocked, cytochrome b reduction through center N was biphasic and was slowed by the addition of exogenous ubiquinone. These results show that ubiquinone residing at center N in the oxidized cytochrome bc(1) complex is responsible for the triphasic reduction of cytochrome b.

Laser flash photolysis studies of electron transfer in complex III from yeast mitochondria

The kinetics of reduction of cytochrome b and cytochrome c1 of yeast Complex III by 5-deazariboflavin semiquinone, generated by laser flash photolysis under anaerobic conditions, have been investigated. The reduction of cytochrome b occurs in two phases with first-order rate constants of 1300 and 670 s-1, whereas the reduction of cytochrome c1 appears as a unique exponential phase with an intermediate value of 800 s-1. Under these experimental conditions, about 50% of cytochrome b is reduced in comparison with cytochrome c1. After photoreduction, the re-oxidation of the cytochromes by internal re-equilibrium occurs in both cases, following pseudo-first-order kinetics at a rate constant of 43 s-1 for cytochrome b and 39 s-1 for cytochrome c1. These results, which agree with the data from the rapid mixing technique (A.-L. Tsai, J.S. Olson, G. Palmer, J. Biol. Chem. 262 (1987) 8677-8684), have implications for the mechanistic understanding of inner Complex III electron transfer. One of the goals of the investigation reported here is to provide direct evidence for the hypothesis of a proton-motive Q cycle for the mechanism of electron transfer in Complex III. Moreover, these results demonstrate the usefulness of laser flash photolysis in studying the redox kinetic properties of mitochondrial Complex III.

The kinetic mechanism of ubiquinol: cytochrome c reductase at steady state

The steady-state kinetics of ubiquinol: cytochrome c reductase (cytochrome bc1 complex) is analyzed in this work. The graphical pattern of the titrations is clearly indicative of a ping-pong mechanism, but the two products ubiquinone and reduced cytochrome c behave competitively with their substrate and noncompetitively with the other substrate. Hence, the mechanism of the reductase is of a ping-pong two-site type. A minimal reaction scheme for the enzymatic mechanism is proposed and approximate values of its rate constants are deduced on the assumption that each substrate is in rapid equilibrium at its catalytic site. This has been substantiated by presteady-state measurements of the reduction and oxidation of cytochrome b by a short-chain homolog of ubiquinol. Values of the rate constants of the reaction scheme have been deduced from the steady-state titrations for a series of 2,3-dimethoxy-5-methyl quinols having different hydrophobic substituents in position 6 of the ring. The results provide a quantitative estimation of the specificity of the quinol catalytic site in the transmembrane portion of the bc1 complex. In particular, a reasonable correlation is found between the rate of the second-order reaction of quinols with the enzyme and their solubility in lipids.

Reduction of the Q-pool by duroquinol via the two quinone-binding sites of the QH2: cytochrome c oxidoreductase. A model for the equilibrium between cytochrome b-562 and the Q-pool

The steady-state reduction of exogenous ubiquinone-2 by duroquinol as catalysed by the ubiquinol: cytochrome c oxidoreductase was studied in bovine heart mitoplasts. The reduction of ubiquinone-2 by duroquinol proceeds both in the absence of inhibitors of the enzyme, in the presence of outside inhibitors, e.g., myxothiazol, and in the presence of inside inhibitors, e.g., antimycin, but not in the presence of both inside and outside inhibitors. It is concluded that both the Qin-binding domain and the Qout-binding domain may independently catalyse this reaction. The rate of the reduction of ubiquinone-2 by duroquinol via the Qin-binding domain is dependent on the type of outside inhibitor used. The maximal rate obtained for the reduction of ubiquinone-2 by DQH2 via the Qout-binding domain, measured in the presence of antimycin, is similar to that catalysed by the Qin-binding domain of the non-inhibited enzyme and depends on the redox state of the high-potential electron carriers of the respiratory chain. The reduction of ubiquinone-2 by DQH2 via the Qin-binding domain can be described by a mechanism in which duroquinol reduces the enzyme, upon which the reduced enzyme is rapidly oxidized by ubiquinone-2 yielding ubiquinol-2. By determination of the initial rate under various conditions and simulation of the time course of reduction of ubiquinone-2 using the integrated form of the steady-state rate equation the values of the various kinetic constants were calculated. During the course of reduction of ubiquinone-2 by duroquinol in the presence of outside inhibitors only cytochrome b-562 becomes reduced. At all stages during the reaction, cytochrome b-562 is in equilibrium with the redox potential of the ubiquinone-2/ubiquinol-2 couple but not with that of the duroquinone/duroquinol couple. At low pH values, cytochrome b-562 is reduced in a single phase; at high pH separate reduction phases are observed. In the absence of inhibitors three reduction phases of cytochrome b-562 are discernible at low pH values and two at high pH values. In the presence of antimyin cytochrome b becomes reduced in two phases. Cytochrome b-562 is reduced in the first phase and cytochrome b-566 in the second phase after substantial reduction of ubiquinone-2 to ubiquinol-2 has occurred. In ubiquinone-10 depleted preparations, titration of cytochrome b-562, in the presence of myxothiazol, with the duroquinone/duroquinol redox couple yields a value of napp = 2, both at low and high pH.(ABSTRACT TRUNCATED AT 400 WORDS)

Triphasic reduction of bH and the absence of equilibration at the i-site of bc1 complex

The concept of a stabilized semiquinone radical forming the basis of a two-electron gate has long been familiar in the context of the quinone-reducing site in photosynthetic systems and has already been suggested to play a role at the i-site of the bc1-type complexes. It is here pointed out that this concept is sufficient to explain the so-called triphasic reduction kinetics of cytochrome bH, in which cytochrome bH goes partially reduced, is reoxidized and then goes fully reduced. The rate constants for binding and unbinding of quinone, quinol and semiquinone at the i-site are discussed, and a kinetic model featuring slow release of the i-site semiquinone is shown to display many features of the kinetics of electron transfer at the i-site.

The interaction of quinone analogues with wild-type and ubiquinone-deficient yeast mitochondria

The interaction of the exogenous quinones, duroquinone (DQ) and the decyl analogue of ubiquinone (DB) with the mitochondrial respiratory chain was studied in both wild-type and a ubiquinone-deficient mutant of yeast. DQ can be reduced directly by NADH dehydrogenase, but cannot be reduced by succinate dehydrogenase in the absence of endogenous ubiquinone. The succinate-driven reduction of DQ can be stimulated by DB in a reaction inhibited 50% by antimycin and 70-80% by the combined use of antimycin and myxothiazol, suggesting that electron transfer occurs via the cytochrome b-c1 complex. Both DQ and DB can effectively mediate the reduction of cytochrome b by the primary dehydrogenases through center o, but their ability to mediate the reduction of cytochrome b through center i is negligible. Two reaction sites for ubiquinol seem to be present at center o: one is independent of endogenous Q6 with a high reaction rate and a high Km; the other is affected by endogenous Q6 and has a low reaction rate and a low Km. By contrast, only one ubiquinol reaction site was observed at center i, where DB appears to compete with endogenous Q6. DB can oxidize most of the pre-reduced cytochrome b, while DQ can oxidize only 50%. On the basis of these data, the possible binding patterns of DB on different Q-reaction sites and the requirement for ubiquinone in the continuous oxidation of DQH are discussed.

Characterisation of binding of the methoxyacrylate inhibitors to mitochondrial cytochrome c reductase

The three E-beta-methoxyacrylate (MOA) inhibitors oudemansin A, strobilurin A and MOA stilbene [3-methoxy-2(2-styrylphenyl)propenic acid-methylester], which differ by more than one order of magnitude in their binding affinity to the mitochondrial ubihydroquinone:cytochrome c oxidoreductase (bc1 complex), bind to a site that is not identical to the binding site for ubihydroquinone, the substrate of the outer ubiquinone reaction site (Qo centre). Although the ubihydroquinone molecule is still bound in the presence of the MOA inhibitors, its electrons cannot be transferred to the iron-sulfur centre. A shift of the relative position of the ubihydroquinone molecule in the reaction centre due to a conformational distortion of cytochrome b induced by the binding of the MOA inhibitor seems to be the reason for the blocked electron transfer. Further analysis shows that ubihydroquinone affects the Kd values of all three MOA inhibitors tested: the values are raised by a constant factor of two, although the inhibitors bind with quite different affinity. The iron-sulfur protein is not involved in the binding of the MOA inhibitors. These results have direct implications for the proper use of MOA inhibitors in experiments designed to analyse the structure/mechanism relationship in cytochrome c reductase. In particular, point mutations recently described in MOA-inhibitor-resistant mutants can no longer be taken to affect necessarily the ubihydroquinone binding site.

Pre-steady-state reduction kinetics of QH2:cytochrome c oxidoreductase and the Q-pool: evidence for a special quinone not in rapid equilibrium with the Q-pool

The pre-steady-state kinetics of the reduction of the prosthetic groups of QH2:cytochrome c oxidoreductase in bovine heart submitochondrial particles were studied in relation to the kinetics of the Q-10 reduction, using duroquinol as substrate. The prosthetic groups, including semiquinone, were measured with EPR and low-temperature-diffuse reflectance spectroscopy, the samples being prepared with the rapid-freeze quench technique. For the determination of the redox state of ubiquinone in the pre-steady state the rapid chemical quench technique was used as an extension of the rapid-freeze quench technique, and Q-10 and QH2-10 were measured with reversed-phase HPLC after extraction with petroleum ether. Ubiquinone was reduced biphasically, 8% of total Q-10 (equal to 1 mol Q-10/mol cytochrome c1), being reduced within 5 ms, and the rest, the Q-pool, at a much lower rate. The initial rapid reduction of this special Q-10 was accompanied by rapid formation of Qi and rapid reduction of a large part of the cytochrome b-562. Both semiquinone formation and reduction of b-562 showed transient kinetics due to a contribution of the reaction pathway via centre o when the iron-sulphur cluster and cytochrome c1 were oxidised. The majority of the special quinol was located at centre i, probably bound, but also at centre o some bound quinol was formed. This was visible when antimycin was present, the antimycin-insensitive bound quinol being totally sensitive to myxothiazol. Myxothiazol alone accelerated the reduction of the Q-pool via centre i, but also the equilibration of cytochrome b-562 with the Q-pool. Antimycin drastically lowered the rate of reduction of the Q-pool and additionally seemed to block the rapid electron transfer from part of the Rieske iron-sulphur cluster to cytochrome c1. It is concluded that, during the pre-steady-state, cytochrome b-562 is not in equilibrium with the Q-pool and that the rate of equilibration is probably determined by the rate of dissociation of the special bound quinol from centre i.

The effect of ring substituents on the mechanism of interaction of exogenous quinones with the mitochondrial respiratory chain

In uncoupled pig-heart mitochondria the rate of the reduction of duroquinone by succinate in the presence of cyanide is inhibited by about 50% by antimycin. This inhibition approaches completion when myxothiazol is also added or British anti-Lewisite-treated (BAL-treated) mitochondria are used. If mitochondria are replaced by isolated succinate:cytochrome c oxidoreductase, the inhibition by antimycin alone is complete. The reduction of a plastoquinone homologue with an isoprenoid side chain (plastoquinone-2) is strongly inhibited by antimycin with either mitochondria or succinate:cytochrome c reductase. The reduction by succinate of plastoquinone analogues with an n-alkyl side chain in the presence of mitochondria is inhibited neither by antimycin nor by myxothiazol, but is sensitive to the combined use of these two inhibitors. On the other hand, the reduction of the ubiquinone homologues Q2, Q4, Q6 and Q10 and an analogue, 2,3-dimethoxyl-5-n-decyl-6-methyl-1,4-benzoquinone, is not sensitive to any inhibitor of QH2:cytochrome c reductase tested or their combined use, either in normal or BAL-treated mitochondria or in isolated succinate:cytochrome c reductase. It is concluded that quinones with a ubiquinone ring can be reduced directly by succinate:Q reductase, whereas those with a plastoquinone ring can not. Reduction of the latter compounds requires participation of either center i or center o (Mitchell, P. (1975) FEBS Lett. 56, 1-6) or both, of QH2:cytochrome c oxidoreductase. It is proposed that a saturated side chain promotes, while an isoprenoid side chain prevents reduction of these compounds at center o.