Cytochrome c_6B of Synechococcus sp. WH 8102 – Crystal structure and basic properties of novel c₆-like family representative

Highlights: • Crystal structure of cytochrome c_6B from Synechococcus sp. WH 8102 was solved. • Basic biophysical properties of cytochrome c_6B were determined. • Cytochrome c_6B exhibits similar architecture to cytochrome c₆. • Organization of heme binding pocket of cytochrome c_6B differs from that of c₆. • Midpoint potential of cytochrome c_6B is significantly lower than of cytochrome c₆. - Abstract: 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 c_6A 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 c_6B, c_6C and c_M 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 c_6B 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 c_6B has yet to be determined its properties probably exclude the participation of this protein in electron trafficking between b₆f complex and photosystem I