Resonance Raman spectroscopy of cytochrome bc1 complexes from Rhodospirillum rubrum: initial characterization and reductive titrations

Molecular Details on Multiple Cofactor Containing Redox Metalloproteins Revealed by Infrared and Resonance Raman Spectroscopies

Vibrational spectroscopy and in particular, resonance Raman (RR) spectroscopy, can provide molecular details on metalloproteins containing multiple cofactors, which are often challenging for other spectroscopies. Due to distinct spectroscopic fingerprints, RR spectroscopy has a unique capacity to monitor simultaneously and independently different metal cofactors that can have particular roles in metalloproteins. These include e.g., (i) different types of hemes, for instance hemes c, a and a3 in caa3-type oxygen reductases, (ii) distinct spin populations, such as electron transfer (ET) low-spin (LS) and catalytic high-spin (HS) hemes in nitrite reductases, (iii) different types of Fe-S clusters, such as 3Fe-4S and 4Fe-4S centers in di-cluster ferredoxins, and (iv) bi-metallic center and ET Fe-S clusters in hydrogenases. IR spectroscopy can provide unmatched molecular details on specific enzymes like hydrogenases that possess catalytic centers coordinated by CO and CN- ligands, which exhibit spectrally well separated IR bands. This article reviews the work on metalloproteins for which vibrational spectroscopy has ensured advances in understanding structural and mechanistic properties, including multiple heme-containing proteins, such as nitrite reductases that house a notable total of 28 hemes in a functional unit, respiratory chain complexes, and hydrogenases that carry out the most fundamental functions in cells.

Photo-induced dynamics of the heme centers in cytochrome bc1

The ultrafast response of cytochromebc₁is investigated for the first time,viatransient absorption spectroscopy.

Resonance Raman spectroscopy of c-type cytochromes

This paper provides an overview of the usefulness of the resonance Raman (RR) spectroscopy in the determination of the structural and electronic properties of heme(s) included in c-type cytochromes. It reviews the mode assignments presently available for heme c and includes recent RR data on the most important subclasses of c-type cytochromes. It also describes the effects of cytochrome c-oxidase and cytochrome c-reductase associations on the heme vibrational modes of the bound cytochrome c.

Structural characterization of isolated mitochondrial cytochrome c1

Resonance Raman spectroscopy (RRS) has been employed to characterize cytochromes c1 isolated from bc1 complexes of beef heart mitochondria and Rhodopseudomonas sphaeroides. The data obtained in this study extend the physical characterization of cytochromes c1 and focus on the effects of the local protein environment on the heme active site. While the general characteristics of the cytochromes c1 are similar to those of smaller soluble cytochromes c, the behavior of several core-size and ligation-sensitive heme modes reveal that significant systematic differences exist between those species. These, most likely, result from changes in the heme axial-ligand interactions.

Characterization of two low-potential cytochromes from bean sprouts

Two cytochromes have been isolated from chlorophyll-free bean sprouts, purified and characterized. The more abundant cytochrome was purified to apparent homogeneity and exhibits visible region absorbance maxima at 416, 520 and 550 nm in the reduced form and at 410 and 530 nm in the oxidized form. Although Resonance Raman spectra of this cytochrome closely resemble those of c-type cytochromes, pyridine hemochromogen analysis suggests that this cytochrome may contain a variant of heme c as its prosthetic group. The cytochrome has an apparent molecular mass of 12.5 kDa, an isoelectric point > 9.0 and a midpoint oxidation-reduction potential (Em) of -130 mV at pH 8.0. The less abundant of the two cytochromes, which was not completely purified, exhibits absorbance maxima at 438 and 560 nm in the reduced form and at 411 nm in the oxidized form and was shown to contain heme c as a prosthetic group. This cytochrome, which may also contain FAD, has an apparent molecular mass of approx. 38 kDa, an isoelectric point > 9.0 and Em = -300 mV. Preliminary results indicate that both cytochromes can form electrostatically-stabilized complexes with ferredoxin, suggesting the possibility that one or both of the cytochromes may participate in low-potential, non-photosynthetic electron transfer pathways involving ferredoxin.

The cytochrome bc 1 complexes of photosynthetic purple bacteria

Complete nucleotide sequences are now available for the pet (fbc) operons coding for the three electron carrying protein subunits of the cytochrome bc 1 complexes of four photosynthetic purple non-sulfur bacteria. It has been demonstrated that, although the complex from one of these bacteria may contain a fourth subunit, three subunit complexes appear to be fully functional. The ligands to the three hemes and the one [2Fe-2S] cluster in the complex have been identified and considerable progress has been made in mapping the two quinone-binding sites present in the complex, as well as the binding sites for quinone analog inhibitors. Hydropathy analyses and alkaline phosphatase fusion experiments have provided considerable insight into the likely folding pattern of the cytochrome b peptide of the complex and identification of the electrogenic steps associated with electron transport through the complex has allowed the orientation within the membrane of the electron-carrying groups of the complex to be modeled.

Protein conformational perturbations affect the photoreduction of native cytochrome c peroxidase (III) at alkaline pH

Ferric cytochrome c peroxidase (CCP) undergoes a ligation-state transition from a pentacoordinate, high-spin (5c/hs) heme to a hexacoordinate, low-spin (6c/1s) heme when titrated over a pH range of 7.30-9.70. This behavior is similar to that exhibited by the ferrous form of the enzyme. However, the photodissociation of the low-spin, axial ligand, exhibited by ferrous CCP at alkaline pH, is not observed for ferric CCP. Instead, a photoinduced reduction of the ferric heme is apparent in the pH range 7.90-9.70. In the absence of O2 and redox mediators such as methyl viologen (MV2+), the reoxidation of the photoreduced enzyme is very slow (tau 1/2 approximately 3 min). F(-)-bound CCP(III) (6c/hs) displays similar pH-dependent photoreduction. Horseradish peroxidase, however, does not. The formation of 6c/1s heme coincides with the onset of appreciable photoreduction (between laser pulses, > 60 ms) of CCP (III) at alkaline pH, suggesting a global protein conformational rearrangement within or around its heme pocket. Photoreduction of alkaline CCP(III) most likely involves intramolecular electron transfer (ET) from the aromatic residue in the proximal heme pocket to the photoexcited heme. We speculate that the kinetics of electron transfer are affected by changes in the orientation of Trp-191.

Characterization of the pet operon of Rhodospirillum rubrum

The three genes of the pet operon, coding, respectively, for the Rieske iron-sulfur protein, cytochrome b and cytochrome c 1 components of the cytochrome bc 1 complex in the photosynthetic bacterium Rhodospirillum rubrum have been sequenced. The amino acid sequences deduced for these three peptides from the nucleotide sequences of the genes have been confirmed, in part, by direct sequencing of portions of the three peptides separated from a sample of the purified, detergent-solubilized complex. These sequences show considerable homology with those previously obtained for the pet operons of other photosynthetic bacteria. Northern blots of R. rubrum mRNA have established that the operon is transcribed as a single polycistronic message, the start site of which has been determined by both primer extension and nuclease protection. Photosynthetic growth of R. rubrum was shown to be inhibited by antimycin A, a specific inhibitor of cytochrome bc 1 complexes, and antimycin A-resistant mutants of R. rubrum have been isolated. Preliminary results suggest that it may be possible to express the R. rubrum pet operon in a strain of the photosynthetic bacterium Rhodobacter capsulatus from which the native pet operon has been deleted.

Structural characterization of heme sites in spinach cytochrome b6f complexes: a resonance Raman study

Resonance Raman spectra of cytochrome b6f complexes isolated from spinach chloroplasts have been obtained. Selective resonance enhancements and partial reductions of the complex by redox mediators were used to isolate and identify the contributions of heme b6 and heme f sites to the observed spectra. Corresponding spectra for turnip cytochrome f have also been obtained. Power-dependent photoreduction was observed in cytochrome f of the complex as well as in the isolated cytochrome f during the course of the Raman experiments.

The Rhodospirillum rubrum cytochrome bc1 complex: redox properties, inhibitor sensitivity and proton pumping

A detergent-solubilized, three-subunit-containing cytochrome bc1 complex, isolated from the photosynthetic bacterium R. rubrum, has been shown to be highly sensitive to stigmatellin, myxothiazol, antimycin A and UHDBT, four specific inhibitors of these complexes. Oxidation-reduction titrations have allowed the determination of Em values for all the electron-carrying prosthetic groups in the complex. Antimycin A has been shown to produce a red shift in the alpha-band absorbance maximum of one of the cytochrome b hemes in the complex and stigmatellin has been shown to alter both the Em and EPR g-values of the Rieske iron-sulfur protein in the complex. Western blots have revealed antigenic similarities between the cytochrome subunits of the R. rubrum complex and those of the related photosynthetic bacteria, Rb. capsulatus and Rb. sphaeroides. The R. rubrum complex has been incorporated into liposomes. These liposomes exhibit respiratory control and are able to couple electron transfer from quinol to cytochrome c to proton translocation across the liposome membrane in a manner consistent with a Q-cycle mechanism. It can thus be concluded that neither electron transport nor coupled proton translocation by the cytochrome bc1 complex requires more than three subunits in R. rubrum.