Quinol:fumarate reductase (QFR) is a membrane protein complex that couples the reduction of fumarate to succinate to the oxidation of quinol to quinone, in a reaction opposite to that catalyzed by the related enzyme succinate:quinone reductase (succinate dehydrogenase). In the previously determined structure of QFR from Wolinella succinogenes, the site of fumarate reduction in the flavoprotein subunit A of the enzyme was identified, but the site of menaquinol oxidation was not. In the crystal structure, the acidic residue Glu-66 of the membrane spanning, diheme-containing subunit C lines a cavity that could be occupied by the substrate menaquinol. Here we describe that, after replacement of Glu-C66 with Gln by site-directed mutagenesis, the resulting mutant is unable to grow on fumarate and the purified enzyme lacks quinol oxidation activity. X-ray crystal structure analysis of the Glu-C66 3 Gln variant enzyme at 3.1-Å resolution rules out any major structural changes compared with the wild-type enzyme. The oxidationreduction potentials of the heme groups are not significantly affected. We conclude that Glu-C66 is an essential constituent of the menaquinol oxidation site. Because Glu-C66 is oriented toward a cavity leading to the periplasm, the release of two protons on menaquinol oxidation is expected to occur to the periplasm, whereas the uptake of two protons on fumarate reduction occurs from the cytoplasm. Thus our results indicate that the reaction catalyzed by W. succinogenes QFR generates a transmembrane electrochemical potential.

Flavin adenine dinucleotide (FAD) and three different flavoproteins in aqueous solution were subjected to redox-triggered Fourier transform infrared difference spectroscopy. The acquired vibrational spectra show a great number of positive and negative peaks, pertaining to the oxidized and reduced state of the molecule, respectively. Density functional theory calculations on the B3LYP/6-31G(d) level were employed to assign several of the observed bands to vibrational modes of the isoalloxazine moiety of the flavin cofactor in both its oxidized and, for the first time, its reduced state. Prominent modes measured for oxidized FAD include nu(C(4)=O) and nu(C(2)=O) at 1716 and 1674 cm(-1), respectively, nu(C(4a)=N(5)) at 1580 cm(-1), and nu(C(10a)=N(1)) at 1548 cm(-1). Measured modes of the reduced form of FAD include nu(C(2)=O) at 1692 cm(-1), nu(C(4)=O) at 1634 cm(-1), and nu(C(4a)=C(10a)) at 1600 cm(-1). While the overall shape of the enzyme spectra is similar to the shape of the spectrum of free FAD, there are numerous differences in detail. In particular, the nu(C=N) modes of the flavin exhibit frequency shifts in the protein-bound form, most prominently for pyruvate oxidase where nu(C(10a)=N(1)) downshifts by 14 cm(-1) to 1534 cm(-1). The significance of this shift and a possible explanation in connection with the bent conformation of the flavin cofactor in this enzyme are discussed.

The cytochrome bc(1) complex from Paracoccus denitrificans and soluble fragments of its cytochrome c(1) and Rieske ISP subunits are characterized by a combined approach of protein electrochemistry and FTIR difference spectroscopy. The FTIR difference spectra provide information about alterations in the protein upon redox reactions: signals from the polypeptide backbone, from the cofactors, and from amino acid side chains. We describe typical modes for conformational changes in the polypeptide and contributions of different secondary structure elements. Signals attributed to the different cofactors can be presented on the basis of selected potential steps. Modes associated with bound quinone are identified by comparison with spectra of quinone in solution at 1656, 1642, and 1610 cm(-1) and between 1494 and 1388 cm(-1), as well as at 1288 and 1262 cm(-1). Signals originating from the quinone bound at the Q(o) site can be distinguished. On the basis of the infrared data, the total quinone concentration is determined to be 2.6-3.3 quinones per monomer, depending on preparation conditions. The balance of evidence supports the double-occupancy model. Interestingly, the amplitude of the band at 1746 cm(-1) increases with quinone content, reflecting a protonation reaction of acidic groups. In this context, the involvement of glutamates and/or aspartates in the vicinity of the Q(o) site is discussed on the basis of recently determined crystal structures.

In this study a combined electrochemical and FTIR spectroscopic approach was applied to monitor the binding of stigmatellin, a Q(o) site inhibitor of the cytochrome bc(1) complex from Saccharomyces cerevisiae. Natural stigmatellin A induced clear shifts in the redox-induced FTIR difference spectra. For data interpretation a stigmatellin derivative (UST) with the conjugated trienes replaced by an aliphatic tail was synthesized, and the carbonyl group shown in crystal structures to interact with His181, the [2Fe-2S] ligand of the Rieske, was specifically (13)C labeled. Electrochemically induced FTIR difference spectra of the inhibitors in CH(3)OD were obtained and revealed signals characteristic for the oxidized and reduced forms of the labeled and unlabeled compounds. On the basis of signals from the inhibitors alone, the binding of the inhibitor to the bc(1) complex was monitored. Direct evidence for the interaction of the carbonyl group with the protein was provided by the observed shift of the nu(C=O) vibrational mode of about 10 cm(-1). In addition, redox-dependent reorganizations of the protein were identified, including protonation changes of acidic residues at 1746 and 1734 cm(-1). The conformational changes observed upon inhibitor binding are discussed with respect to the crystal structures and proposed mechanistic models [Hunte, C., Koepke, J., Lange, C., Rossmanith, T., and Michel, H. (2000) Structure 8, 669-684; Palsdottir, H., Lojero, C. G., Trumpower, B. L., and Hunte, C. (2003) J. Biol. Chem. 278, 31303-31311].