Redox-Bohr and Other Cooperativity Effects in the Nine-heme Cytochrome c from Desulfovibrio desulfuricans ATCC 27774

Abstract

The nine-heme cytochrome c is a monomeric multiheme cytochrome found in Desulfovibrio desulfuricans ATCC 27774. The polypeptide chain comprises 296 residues and wraps around nine hemes of type c. It is believed to take part in the periplasmic assembly of proteins involved in the mechanism of hydrogen cycling, receiving electrons from the tetraheme cytochrome c 3 . With the purpose of understanding the molecular basis of electron transfer processes in this cytochrome, we have determined the crystal structures of its oxidized and reduced forms at pH 7.5 and performed theoretical calculations of the binding equilibrium of protons and electrons in these structures. This integrated study allowed us to observe that the reduction process induced relevant conformational changes in several residues, as well as protonation changes in some protonatable residues. In particular, the surroundings of hemes I and IV constitute two areas of special interest. In addition, we were able to ascertain the groups involved in the redoxBohr effect present in this cytochrome and the conformational changes that may underlie the redox-cooperativity effects on different hemes. Furthermore, the thermodynamic simulations provide evidence that the N-and C-terminal domains function in an independent manner, with the hemes belonging to the N-terminal domain showing, in general, a lower redox potential than those found in the C-terminal domain. In this way, electrons captured by the N-terminal domain could easily flow to the C-terminal domain, allowing the former to capture more electrons. A notable exception is heme IX, which has low redox potential and could serve as the exit path for electrons toward other proteins in the electron transfer pathway..

Crystal Structure of the 16 Heme Cytochrome from Desulfovibrio gigas: A Glycosylated Protein in a Sulphate-reducing Bacterium

“…The electron flow in the C-terminal arm of DgHmcA would then be contrary to what has been proposed for Dd9HcA alone. 38 The architecture of HmcA in a V shape could allow the bacteria to have, in the same protein, different electrostatic environments that would allow the interaction with different electron donors, therefore enhancing the electron transport from the periplasm to the cytoplasm.…”

“…38 In DgHmcA it is more pronounced for hemes II, V, XII, XIV and XV. In the high-spin heme (XV), the propionate groups are very exposed to the solvent, and one is hydrogen bonded to Lys437.…”

A<i>β</i>-Like Peptide Displayed on Bacteriophage T7 Catalyzes Chromate and Uranyl Reduction

“…From the phage cDNA library screening, we did not identify any genes from G. sulfurreducens that have been reported to play an important role in bioremediation, such as hydrogenase enzymes or cytochromes [2,4,6,35,36]. Very unexpectedly small peptides encoded by DNA sequence from non-coding regions of the G. sulfurreducens and S. oneidenesis genomes were shown to reduce chromate and uranium.…”

Simulation of multihaem cytochromes

“…These same approaches were extended to a more challenging system, consisting in a multihaem cytochrome with nine identical haems wraped around by a chain of 296 residues, belonging to D. desulfuricans ATCC 27774. Structures were obtained at pH 7.5 in the fully oxidised and the fully reduced states, and the binding equilibrium of protons and electrons was simulated [50] in order to investigate the redox-Bohr (referred above) and other cooperativity effects. As in the case described before, this cytochrome displays small conformational changes in charged groups upon reduction that originate, for some of the haems, redox titration curves that hint at the existence of positive cooperativity in reduction, suggesting that these effects are widespread in the multihaem cytochrome family.…”