Insights into the relationship between the haem-binding pocket and the redox potential of c6 cytochromes: four atomic resolution structures of c6 and c6-like proteins from Synechococcus sp. PCC 7002.

The structure of cytochrome c6C from the mesophilic cyanobacterium Synechococcus sp. PCC 7002 has been determined at 1.03 Šresolution. This is the first structural report on the recently discovered cyanobacterial cytochrome c6-like proteins found in marine and nitrogen-fixing cyanobacteria. Despite high similarity in the overall three-dimensional fold between cytochromes c6 and c6C, the latter shows saliently different electrostatic properties in terms of surface charge distribution and dipole moments. Its midpoint redox potential is less than half of the value for typical c6 cytochromes and results mainly from the substitution of one residue in the haem pocket. Here, high-resolution crystal structures of mutants of both cytochromes c6 and c6C are presented, and the impact of the mutation of specific residues in the haem-binding pocket on the redox potential is discussed. These findings contribute to the elucidation of the structure-function relationship of c6-like cytochromes.

Cytochrome c(6B) of Synechococcus sp. WH 8102--crystal structure and basic properties of novel c(6)-like family representative.

Cytochromes c are soluble electron carriers of relatively low molecular weight, containing single heme moiety. In cyanobacteria cytochrome c6 participates in electron transfer from cytochrome b6f complex to photosystem I. Recent phylogenetic analysis revealed the existence of a few families of proteins homologous to the previously mentioned. Cytochrome c6A from Arabidopsis thaliana was identified as a protein responsible for disulfide bond formation in response to intracellular redox state changes and c550 is well known element of photosystem II. However, function of cytochromes marked as c6B, c6C and cM as well as the physiological process in which they take a part still remain unidentified. Here we present the first structural and biophysical analysis of cytochrome from the c6B family from mesophilic cyanobacteria Synechococcus sp. WH 8102. Purified protein was crystallized and its structure was refined at 1.4 Å resolution. Overall architecture of this polypeptide resembles typical I-class cytochromes c. The main features, that distinguish described protein from cytochrome c6, are slightly red-shifted α band of UV-Vis spectrum as well as relatively low midpoint potential (113.2±2.2 mV). Although, physiological function of cytochrome c6B has yet to be determined its properties probably exclude the participation of this protein in electron trafficking between b6f complex and photosystem I.