Protonmotive activity of the cytochrome bc1 complex in chromatophores of Rhodobacter capsulatus in the presence of myxothiazol and antimycin A

Photosynthetic electron transport and anaerobic metabolism in purple non-sulfur phototrophic bacteria

Purple non-sulfur phototrophic bacteria, exemplified by Rhodobacter capsulatus and Rhodobacter sphaeroides, exhibit a remarkable versatility in their anaerobic metabolism. In these bacteria the photosynthetic apparatus, enzymes involved in CO2 fixation and pathways of anaerobic respiration are all induced upon a reduction in oxygen tension. Recently, there have been significant advances in the understanding of molecular properties of the photosynthetic apparatus and the control of the expression of genes involved in photosynthesis and CO2 fixation. In addition, anaerobic respiratory pathways have been characterised and their interaction with photosynthetic electron transport has been described. This review will survey these advances and will discuss the ways in which photosynthetic electron transport and oxidation-reduction processes are integrated during photoautotrophic and photoheterotrophic growth.

The bc1 complexes of Rhodobacter sphaeroides and Rhodobacter capsulatus

Photosynthetic bacteria offer excellent experimental opportunities to explore both the structure and function of the ubiquinol-cytochrome c oxidoreductase (bc1 complex). In both Rhodobacter sphaeroides and Rhodobacter capsulatus, the bc1 complex functions in both the aerobic respiratory chain and as an essential component of the photosynthetic electron transport chain. Because the bc1 complex in these organisms can be functionally coupled to the photosynthetic reaction center, flash photolysis can be used to study electron flow through the enzyme and to examine the effects of various amino acid substitutions. During the past several years, numerous mutations have been generated in the cytochrome b subunit, in the Rieske iron-sulfur subunit, and in the cytochrome c1 subunit. Both site-directed and random mutagenesis procedures have been utilized. Studies of these mutations have identified amino acid residues that are metal ligands, as well as those residues that are at or near either the quinol oxidase (Qo) site or the quinol reductase (Qi) site. The postulate that these two Q-sites are located on opposite sides of the membrane is supported by these studies. Current research is directed at exploring the details of the catalytic mechanism, the nature of the subunit interactions, and the assembly of this enzyme.

A rapid burst preceding the steady-state rate of H(+)-transhydrogenase during illumination of chromatophores of Rhodobacter capsulatus. Implications for the mechanism of interaction between protonmotive force and enzyme

At the onset of illumination of chromatophores there was a burst (t1/2 approx. 5 ms) in the rate of the H(+)-transhydrogenase reaction before establishment of the steady-state rate. The burst was suppressed at high pH with a pKa of approx. 8.5. The burst and the steady-state rate were inhibited by either (i) a combination of myxothiazol and carbonylcyanide-p-trifluoromethoxyphenylhydrazone, or (ii) NAD+, or (iii) dicyclohexylcarbodiimide. The results support a model in which substrate binding to H(+)-transhydrogenase is relatively fast. A subsequent slow step is accelerated by the protonmotive force and a third step, possibly product release, is rate-limiting in steady-state turnover during illumination.

Electron transport pathways to nitrous oxide in Rhodobacter species

1. Electron transport components involved in nitrous oxide reduction in several strains of Rhodobacter capsulatus and in the denitrifying strain of Rhodobacter sphaeroides (f. sp. denitrificans) have been investigated. Detailed titrations with antimycin A and myxothiazol, inhibitors of the cytochrome bc1 complex, show that part of the electron flow to nitrous oxide passes through this complex. The sensitivity to myxothiazol varies between strains and growth conditions of R. capsulatus; the higher rates of nitrous oxide reduction correlate with the higher sensitivities. Partial inhibition of the nitrous oxide reductase enzyme with azide decreased the sensitivity to myxothiazol of the strains that had the highest nitrous oxide reductase activity. 2. Inhibition of nitrous oxide reduction in cells of R. capsulatus by myxothiazol could be restored under dark conditions by addition of N,N,N',N'-tetramethyl-p-phenylene diamine. The highest activities observed after addition of this electron carrier were found in the strains that had the highest sensitivity to myxothiazol, consistent with the premise that this inhibitor is more effective at the higher flux rates to nitrous oxide. 3. Addition of nitrous oxide to cells of R. capsulatus strain N22DNAR+ under darkness caused oxidation of both b- and c-type cytochromes. The oxidation of b cytochromes was less pronounced in the presence of myxothiazol, consistent with a role for the cytochrome bc1 complex in the electron pathway to nitrous oxide. Ferricyanide, in the absence of myxothiazol, caused a similar extent of oxidation of b cytochromes, but a greater oxidation of c-type, suggesting that there was a pool of c-type cytochrome that was not oxidisable by nitrous oxide. The time course showed that both the b- and c-type cytochromes were oxidised within a few seconds of the addition of nitrous oxide. During the following seconds there was a partial re-reduction of the cytochromes such that after approximately 1 min a lower steady-state of oxidation was attained and this persisted until the nitrous oxide was exhausted. 4. A mutant, MTCBC1, of R. capsulatus that specifically lacked a functional cytochrome bc1 complex reduced nitrous oxide, albeit at 30% of the rate shown by the parent strain MT1131. A reduced minus nitrous-oxide-oxidised difference spectrum for MTCBC1 in the absence of myxothiazol was similar to the corresponding difference spectrum observed for strain N22DNAR+ in the presence of myxothiazol. It is suggested that these difference spectra identify the cytochrome components, including a b-type, involved in a pathway that is alternative to, and independent of, the cytochrome bc1 complex.(ABSTRACT TRUNCATED AT 400 WORDS)

The coupling between protonmotive force and the NAD(P)+ transhydrogenase in chromatophores from photosynthetic bacteria

1. The activity of NAD(P)+ transhydrogenase in chromatophores of Rhodobacter capsulatus relaxed from a high rate during illumination to a lower rate after darkening with a half-time of approximately 100 ms. 2. The dissipative ionic current flowing across the chromatophore membrane was increased in the presence of transhydrogenase substrates. This is attributed to proton current through the transhydrogenase enzyme. Subject to the assumption that transhydrogenase does not conduct in the absence of nucleotide substrates, the ratio of protons translocated across the membrane per hydride ion transferred was 0.4 +/- 0.5. Within the error and uncertainities in the calibration procedure, this ratio may be consistent with a stoichiometry of one but higher values seem unlikely. The ratio of hydride ion transferred in the transhydrogenase to electrons transferred through the cyclic electron transport system was approximately 0.2. 3. The Kappm values for the transhydrogenase substrates were determined for chromatophores in illuminated and darkened suspensions over a range of pH. These values are discussed in relation to the equivalent parameters reported for mitochondria transhydrogenase [Rydstrom, J. (1977) Biochim. Biophys. Acta 255, 9641-9646] and were used to calculate the concentrations of substrates which effectively saturate the enzyme. 4. At substrate concentrations which were in excess of 8 X Kappm the dependence of transhydrogenase rate on the value of the membrane potential (zero pH gradient) was determined at pH 6.3, 6.9, 7.6 and 9.0. The relation was similar at pH 6.9 and 7.6. At alkaline pH the apparent threshold in the relation became more prominent as it was shifted to slightly higher values of membrane potential. At acid pH a shift in the opposite direction diminished the apparent threshold and saturation at high membrane potential became more dominant. We use these data in an attempt to discriminate between two models of energy transduction: (a) the driving force exerted by the membrane potential is mediated by a pH gradient formed through the operation of a proton well in the transhydrogenase; (b) the membrane potential increases a rate constant for charge translocation through transhydrogenase by decreasing the effective height of the Eyring barrier for charge transfer across the membrane through the enzyme. The second model leads to a more simple description than the first of the pH dependence of transhydrogenase rate on membrane potential.4+ transhydrogenase activity in chromatopho