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PubMed2006

The menaquinol-oxidizing cytochrome bc complex from Thermus thermophilus: protein domains and subunits.

A recently resolved respiratory complex III, isolated from the extreme thermophile Thermus thermophilus, is discussed in terms of cofactor and subunit composition, and with respect to the origin of its protein modules. The four polypeptides, encoded by a single operon, share general homologies to canonical complexes both of the bc and b6f type, but exhibit some unexpected features as well. Evidence for high thermostability of the isolated protein and for its quinol substrate specificity is derived from EPR and kinetic measurements. A functional integration of this complex into an aerobic electron transfer scheme, connecting known dehydrogenase activities to the terminal oxidase branches of Thermus is outlined, as well as the specific principles of redox protein interactions prevailing at high temperature. Findings from this enzyme are linked to present knowledge on other menaquinol oxidizing bc complexes.

Amino Acid Sequence↗
PubMed1999

Substitution of the sixth axial ligand of Rhodobacter capsulatus cytochrome c1 heme yields novel cytochrome c1 variants with unusual properties.

The cytochrome (cyt) c1 heme of the ubihydroquinone:cytochrome c oxidoreductase (bc1 complex) is covalently attached to two cysteine residues of the cyt c1 polypeptide chain via two thioether bonds, and the fifth and sixth axial ligands of its iron atom are histidine (H) and methionine (M), respectively. The latter residue is M183 in Rhodobacter capsulatus cyt c1, and previous mutagenesis studies revealed its critical role for the physicochemical properties of cyt c1 [Gray, K. A., Davidson, E., and Daldal, F. (1992) Biochemistry 31, 11864-11873]. In the homologous chloroplast b6f complex, the sixth axial ligand is provided by the amino group of the amino terminal tyrosine residue. To further pursue our investigation on the role played by the sixth axial ligand in heme-protein interactions, novel cyt c1 variants with histidine-lysine (K) and histidine-histidine axial coordination were sought. Using a R. capsulatus genetic system, the cyt c1 mutants M183K and M183H were constructed by site-directed mutagenesis, and chromatophore membranes as well as purified bc1 complexes obtained from these mutants were characterized in detail. The studies revealed that these mutants incorporated the heme group into the mature cyt c1 polypeptides, but yielded nonfunctional bc1 complexes with unusual spectroscopic and thermodynamic properties, including shifted optical absorption maxima (lambdamax) and decreased redox midpoint potential values (Em7). The availability and future detailed studies of these stable cyt c1 mutants should contribute to our understanding of how different factors influence the physicochemical and folding properties of membrane-bound c-type cytochromes in general.

Amino Acid Substitution↗
PubMed2004

Kinetics of interprotein electron transfer between cytochrome c6 and the soluble CuA domain of cyanobacterial cytochrome c oxidase.

Cytochrome c6 is a soluble metalloprotein located in the periplasmic space and the thylakoid lumen of many cyanobacteria and is known to carry electrons from cytochrome b6f to photosystem I. The CuA domain of cytochrome c oxidase, the terminal enzyme which catalyzes the four-electron reduction of molecular oxygen in the respiratory chains of mitochondria and many bacteria, also has a periplasmic location. In order to test whether cytochrome c6 could also function as a donor for cytochrome c oxidase, we investigated the kinetics of the electron transfer between recombinant cytochrome c6 (produced in high yield in Escherichia coli by coexpressing the maturation proteins encoded by the ccmA-H gene cluster) and the recombinant soluble CuA domain (i.e., the donor binding and electron entry site) of subunit II of cytochrome c oxidase from Synechocystis PCC 6803. The forward and the reverse electron transfer reactions were studied by the stopped-flow technique and yielded apparent bimolecular rate constants of (3.3 +/- 0.3) x 10(5) M(-1) s(-1) and (3.9 +/- 0.1) x 10(6) M(-1) s(-1), respectively, in 5 mM potassium phosphate buffer, pH 7, containing 20 mM potassium chloride and 25 degrees C. This corresponds to an equilibrium constant Keq of 0.085 in the physiological direction (DeltarG'0 = 6.1 kJ/mol). The reduction of the CuA fragment by cytochrome c6 is almost independent on ionic strength, which is in contrast to the reaction of the CuA domain with horse heart cytochrome c, which decreases with increasing ionic strength. The findings are discussed with respect to the potential role of cytochrome c6 as mobile electron carrier in both cyanobacterial electron transport pathways.

Animals↗