Modulation of Heme Redox Potential in the Cytochrome c₆ Family

Abstract

Cytochrome c6A is a unique dithio-cytochrome of green algae and plants. It has a very similar core structure to that of bacterial and algal cytochromes c6 but is unable to fulfill the same function of transferring electrons from cytochrome f to photosystem I. A key feature is that its heme midpoint potential is more than 200 mV below that of cytochrome c6 despite having His and Met as axial heme-iron ligands. To identify the molecular origins of the difference in potential, the structure of cytochrome c6 from the cyanobacterium Phormidium laminosum has been determined by X-ray crystallography and compared with the known structure of cytochrome c6A. One salient difference of the heme pockets is that a highly conserved Gln (Q51) in cytochrome c6 is replaced by Val (V52) in c6A. Using protein film voltammetry, we found that swapping these residues raised the c6A potential by +109 mV and decreased that of c6 by almost the same extent, -100 mV. X-ray crystallography of the V52Q protein showed that the Gln residue adopts the same configuration relative to the heme as in cytochrome c6 and we propose that this stereochemistry destabilizes the oxidized form of the heme. Consequently, replacement of Gln by Val was probably a key step in the evolution of cytochrome c6A from cytochrome c6, inhibiting reduction by the cytochrome b6f complex and facilitating establishment of a new function.

Outer membrane cytochrome c, OmcF, from Geobacter sulfurreducens: High structural similarity to an algal cytochrome c6

“…The extra histidine close to the heme propionate groups might explain the significant redox-Bohr effect observed in the case of OmcF S . The amino acid sequences, structures, and redox potentials of cytochrome c 6 molecules from photosynthetic organisms, algae and cyanobacteria, are very similar to each other as reviewed by Dikiy et al 10 and Worrall et al 4 Although the structure of OmcF S is similar to that of cytochrome c 6 OmcF S is a more basic protein and has a much lower redox potential. In addition, although cytochrome c 6 molecules are soluble proteins, OmcF is predicted to be attached to the membrane.…”

“…3,4 The structures of seven cytochromes c 6 have been previously determined. [4][5][6][7][8][9][10] Further, a c 6 -like cytochrome (PetJ2) of unknown function was recently identified in Synechoccus sp. PCC 7002 with a reduction potential of 1148 mV and high pI.…”

Structural Basis for the Oxidation of Protein-bound Sulfur by the Sulfur Cycle Molybdohemo-Enzyme Sulfane Dehydrogenase SoxCD

“…4A). As cytochrome c-type proteins have a ubiquitous fold, a large number of homologous structures to SoxD 1 is found within the protein data base with the closest homologues 1CED, a cytochrome c6 from Monoraphidium braunii (52), 2VO8, a cytochrome c6 from Phormidium laminosum (53), and 2ZBO, a cytochrome c6 from Hizikia fusiformis (Fig. 4B).…”

The role of a disulfide bridge in the stability and folding kinetics of Arabidopsis thaliana cytochrome c6A

“…The vector pGEMPlc6 (Amp r ) harbouring the wild-type (wt) gene of Phormidium laminosum (Pl) cyt c 6 [6] was introduced by transformation into E. coli BL21(DE3) cells containing the pEC86 (Cam r ) plasmid. This plasmid contains genes to express the necessary proteins to assist with covalent heme attachment [39].…”

“…However, for cyt c 6A and cyt c 6B current evidence confirms that they are likely to be functionally distinct from photosynthetic cyt c 6 due to their E m s being > 200 mV lower. This property makes them incapable of functioning in the photosynthetic pathway in the same way as cyt c 6 [6][7][8].…”