THE CYTOCHROME C(3) SUPERFAMILY - AMINO-ACID-SEQUENCE OF A DIMERIC OCTAHAEM CYTOCHROME C(3) (M(R)26000) ISOLATED FROM DESULFOVIBRIO-GIGAS

Cytochrome c(3) (M(r) 26 000) isolated from Desulfovibrio gigas is a d imeric cytochrome consisting of two identical subunits of 109 amino ac ids, each of which contains four haem groups. On the basis of its amin o acid sequence, this cytochrome clearly belongs to the cytochrome c(3 ) superfamily, and will be classified in class III of the c-type cytoc hromes as defined by Ambler [(1980) in From Cyclotrons to Cytochromes (Robinson, A. B. and Kaplan, N. O., eds.), pp. 263-279, Academic Press , London]. It contains ten cysteine and nine histidine residues in eac h subunit, and eight cysteines and eight histidines linked to the four haem groups were found to be invariant on alignment of all known cyto chrome c(3) sequences. Two intermolecular disulphide bridges have been determined between cysteine residues 5 and 46 of the two monomers. Cy tochrome c(3) (M(r) 26000) from D. gigas is clearly different from cyt ochrome c(3) (M(r) 13000) from the same strain, with which it shows on ly 27% sequence identity. Compared with cytochrome c(3) (M(r) 26000) f rom D. desulfuricans Norway, the three-dimensional structure of which has been determined, 26.95% of the residues have been conserved. In th e enzyme from D. desulfuricans Norway, hydrophobic interactions have b een described across the dimer interface. Residues involved in similar interactions seem to be well conserved in the equivalent D. gigas cyt ochrome. This sequence provides structural data to allow specification of this new subclass of polyhaem cytochromes. Furthermore, D. gigas c ytochrome c(3) (M(r) 26000) is the first polyhaem cytochrome shown to contain two disulphide bridges linking two identical subunits, which c ould induce more rigid folding. The folding and the evolution of this family of polyhaem cytochromes are discussed.