The structure of Wolinella succinogenes quinol: fumarate reductase and its relevance to the superfamily of succinate: quinone oxidoreductases

The di-heme family of respiratory complex II enzymes

The di-heme family of succinate:quinone oxidoreductases is of particular interest, because its members support electron transfer across the biological membranes in which they are embedded. In the case of the di-heme-containing succinate:menaquinone reductase (SQR) from Gram-positive bacteria and other menaquinone-containing bacteria, this results in an electrogenic reaction. This is physiologically relevant in that it allows the transmembrane electrochemical proton potential Δp to drive the endergonic oxidation of succinate by menaquinone. In the case of the reverse reaction, menaquinol oxidation by fumarate, catalysed by the di-heme-containing quinol:fumarate reductase (QFR), evidence has been obtained that this electrogenic electron transfer reaction is compensated by proton transfer via a both novel and essential transmembrane proton transfer pathway ("E-pathway"). Although the reduction of fumarate by menaquinol is exergonic, it is obviously not exergonic enough to support the generation of a Δp. This compensatory "E-pathway" appears to be required by all di-heme-containing QFR enzymes and results in the overall reaction being electroneutral. In addition to giving a brief overview of progress in the characterization of other members of this diverse family, this contribution summarizes key evidence and progress in identifying constituents of the "E-pathway" within the framework of the crystal structure of the QFR from the anaerobic epsilon-proteobacterium Wolinella succinogenes at 1.78Å resolution. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Expression, purification and molecular modeling of another HdrC from Acidithiobacillus ferrooxidans which binds only one [4Fe-4S] cluster

The heterodisulfide reductase complex HdrABC from Acidithiobacillus ferrooxidans was predicted to have novel features that work in reverse to catalyse the sulfane sulfur of GSnH species (n > 1) into sulfite in sulfur oxidation. There are two different highly upregulated genes potentially encoding the HdrC subunit in A. ferrooxidans grown in sulfur-containing medium. An HdrC containing iron-sulfur cluster from A. ferrooxidans corresponding to one of the genes had been expressed and biophysically characterized. Comparatively, here we report the cloning, expression, and characterization of another HdrC from A. ferrooxidans. This HdrC was expressed in inclusion bodies in all conditions tested. This purified HdrC displayed brown color and contained the [4Fe-4S] cluster confirmed by the UV-scanning and EPR spectra. This HdrC owned two identical motifs (Cx(2)Cx(2)Cx(3)C) including total of eight cysteine residues for [4Fe-4S] cluster binding. To our surprise, the site-directed mutagenesis results of these eight residues revealed that respective removal of the sulfhydryl group of Cys73, Cys76, Cys79, and Cys37 resulted in the cluster loss, but those of Cys27, Cys30, Cys33, and Cys83 had no influence, which demonstrated that this HdrC bound only one cluster, and it might be responsible for causing the HdrABC in A. ferrooxidans working in reverse. Molecular modeling results also supported the above results and showed that this cluster was ligated by Cys73, Cys76, and Cys79 in one motif and Cys37, however, in another motif.

Limited reversibility of transmembrane proton transfer assisting transmembrane electron transfer in a dihaem-containing succinate:quinone oxidoreductase

Membrane protein complexes can support both the generation and utilisation of a transmembrane electrochemical proton potential (Deltap), either by supporting transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane or by supporting transmembrane proton transfer. The first mechanism has been unequivocally demonstrated to be operational for Deltap-dependent catalysis of succinate oxidation by quinone in the case of the dihaem-containing succinate:menaquinone reductase (SQR) from the Gram-positive bacterium Bacillus licheniformis. This is physiologically relevant in that it allows the transmembrane potential Deltap to drive the endergonic oxidation of succinate by menaquinone by the dihaem-containing SQR of Gram-positive bacteria. In the case of a related but different respiratory membrane protein complex, the dihaem-containing quinol:fumarate reductase (QFR) of the epsilon-proteobacterium Wolinella succinogenes, evidence has been obtained that both mechanisms are combined, so as to facilitate transmembrane electron transfer by proton transfer via a both novel and essential compensatory transmembrane proton transfer pathway ("E-pathway"). Although the reduction of fumarate by menaquinol is exergonic, it is obviously not exergonic enough to support the generation of a Deltap. This compensatory "E-pathway" appears to be required by all dihaem-containing QFR enzymes and results in the overall reaction being electroneutral. However, here we show that the reverse reaction, the oxidation of succinate by quinone, as catalysed by W. succinogenes QFR, is not electroneutral. The implications for transmembrane proton transfer via the E-pathway are discussed.

A threonine on the active site loop controls transition state formation in Escherichia coli respiratory complex II

In Escherichia coli, the complex II superfamily members succinate:ubiquinone oxidoreductase (SQR) and quinol:fumarate reductase (QFR) participate in aerobic and anaerobic respiration, respectively. Complex II enzymes catalyze succinate and fumarate interconversion at the interface of two domains of the soluble flavoprotein subunit, the FAD binding domain and the capping domain. An 11-amino acid loop in the capping domain (Thr-A234 to Thr-A244 in quinol:fumarate reductase) begins at the interdomain hinge and covers the active site. Amino acids of this loop interact with both the substrate and a proton shuttle, potentially coordinating substrate binding and the proton shuttle protonation state. To assess the loop's role in catalysis, two threonine residues were mutated to alanine: QFR Thr-A244 (act-T; Thr-A254 in SQR), which hydrogen-bonds to the substrate at the active site, and QFR Thr-A234 (hinge-T; Thr-A244 in SQR), which is located at the hinge and hydrogen-bonds the proton shuttle. Both mutations impair catalysis and decrease substrate binding. The crystal structure of the hinge-T mutation reveals a reorientation between the FAD-binding and capping domains that accompanies proton shuttle alteration. Taken together, hydrogen bonding from act-T to substrate may coordinate with interdomain motions to twist the double bond of fumarate and introduce the strain important for attaining the transition state.

Homology modeling of dissimilatory APS reductases (AprBA) of sulfur-oxidizing and sulfate-reducing prokaryotes

Background

The dissimilatory adenosine-5′-phosphosulfate (APS) reductase (cofactors flavin adenine dinucleotide, FAD, and two [4Fe-4S] centers) catalyzes the transformation of APS to sulfite and AMP in sulfate-reducing prokaryotes (SRP); in sulfur-oxidizing bacteria (SOB) it has been suggested to operate in the reverse direction. Recently, the three-dimensional structure of the Archaeoglobus fulgidus enzyme has been determined in different catalytically relevant states providing insights into its reaction cycle.

Methodology/Principal Findings

Full-length AprBA sequences from 20 phylogenetically distinct SRP and SOB species were used for homology modeling. In general, the average accuracy of the calculated models was sufficiently good to allow a structural and functional comparison between the beta- and alpha-subunit structures (78.8–99.3% and 89.5–96.8% of the AprB and AprA main chain atoms, respectively, had root mean square deviations below 1 Å with respect to the template structures). Besides their overall conformity, the SRP- and SOB-derived models revealed the existence of individual adaptations at the electron-transferring AprB protein surface presumably resulting from docking to different electron donor/acceptor proteins. These structural alterations correlated with the protein phylogeny (three major phylogenetic lineages: (1) SRP including LGT-affected Archaeoglobi and SOB of Apr lineage II, (2) crenarchaeal SRP Caldivirga and Pyrobaculum, and (3) SOB of the distinct Apr lineage I) and the presence of potential APS reductase-interacting redox complexes. The almost identical protein matrices surrounding both [4Fe-4S] clusters, the FAD cofactor, the active site channel and center within the AprB/A models of SRP and SOB point to a highly similar catalytic process of APS reduction/sulfite oxidation independent of the metabolism type the APS reductase is involved in and the species it has been originated from.

Conclusions

Based on the comparative models, there are no significant structural differences between dissimilatory APS reductases from SRP and SOB; this might be indicative for a similar catalytic process of APS reduction/sulfite oxidation.

Recent progress on obtaining theoretical and experimental support for the “E-pathway hypothesis” of coupled transmembrane electron and proton transfer in dihaem-containing quinol:fumarate reductase

Reconciliation of apparently contradictory experimental results obtained on the quinol: fumarate reductase (QFR), a dihaem-containing respiratory membrane protein complex from Wolinella succinogenes, was previously obtained by the proposal of the so-called E-pathway hypothesis. According to this hypothesis, transmembrane electron transfer via the haem groups is strictly coupled to co-transfer of protons via a transiently established, novel pathway, proposed to contain the side chain of residue Glu-C180 and the distal haem ring-C propionate as the most prominent components. This hypothesis has recently been supported by both theoretical and experimental results. Multiconformation continuum electrostatics calculations predict Glu-C180 to undergo a combination of proton uptake and conformational change upon haem reduction. Strong experimental support for the proposed role of Glu-C180 in the context of the "E-pathway hypothesis" is provided by the effects of replacing Glu-C180 with Gln or Ile by site-directed mutagenesis, the consequences of these mutations for the viability of the resulting mutants, together with the structural and functional characterisation of the corresponding variant enzymes, and the comparison of redox-induced Fourier-transform infrared (FTIR) difference spectra for the wild type and Glu-C180-->Gln variant. A possible haem propionate involvement has recently been supported by combining (13)C-haem propionate labelling with redox-induced FTIR difference spectroscopy.

Calculated coupling of transmembrane electron and proton transfer in dihemic quinol:fumarate reductase

The quinol:fumarate reductase of Wolinella succinogenes binds a low- and a high-potential heme b group in its transmembrane subunit C. Both hemes are part of the electron transport chain between the two catalytic sites of this redox enzyme. The oxidation-reduction midpoint potentials of the hemes are well established but their assignment in the structure has not yet been determined. By simulating redox titrations, using continuum electrostatics calculations, it was possible to achieve an unequivocal assignment of the low- and high-potential hemes to the distal and proximal positions in the structure, respectively. Prominent features governing the differences in midpoint potential between the two hemes are the higher loss of reaction field energy for the proximal heme and the stronger destabilization of the oxidized form of the proximal heme due to several buried Arg and Lys residues. According to the so-called "E-pathway hypothesis", quinol:fumarate reductase has previously been postulated to exhibit a novel coupling of transmembrane electron and proton transfer. Simulation of heme b reduction indicates that the protonation state of the conserved residue Glu C180, predicted to play a key role in this process, indeed depends on the redox state of the hemes. This result clearly supports the E-pathway hypothesis.

Wolinella succinogenesquinol:fumarate reductase and its comparison toE. colisuccinate:quinone reductase

The three-dimensional structure of Wolinella succinogenes quinol:fumarate reductase (QFR), a dihaem-containing member of the superfamily of succinate:quinone oxidoreductases (SQOR), has been determined at 2.2 A resolution by X-ray crystallography [Lancaster et al., Nature 402 (1999) 377-385]. The structure and mechanism of W. succinogenes QFR and their relevance to the SQOR superfamily have recently been reviewed [Lancaster, Adv. Protein Chem. 63 (2003) 131-149]. Here, a comparison is presented of W. succinogenes QFR to the recently determined structure of the mono-haem containing succinate:quinone reductase from Escherichia coli [Yankovskaya et al., Science 299 (2003) 700-704]. In spite of differences in polypeptide and haem composition, the overall topology of the membrane anchors and their relative orientation to the conserved hydrophilic subunits is strikingly similar. A major difference is the lack of any evidence for a 'proximal' quinone site, close to the hydrophilic subunits, in W. succinogenes QFR.