Abstract
Cytochrome c3 (M(r) 26000) isolated from Desulfovibrio gigas is a
dimeric cytochrome consisting of two identical subunits of 109 amino
acids, each of which contains four haem groups. On the basis of
its amino acid sequence, this cytochrome clearly belongs to the
cytochrome c3 superfamily, and will be classified in class III of
the c-type cytochromes 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 each subunit, and eight cysteines and eight histidines
linked to the four haem groups were found to be invariant on alignment
of all known cytochrome c3 sequences. Two intermolecular disulphide
bridges have been determined between cysteine residues 5 and 46
of the two monomers. Cytochrome c3 (M(r) 26,000) from D gigas is
clearly different from cytochrome c3 (M(r) 13,000) from the same
strain, with which it shows only 27% sequence identity. Compared
with cytochrome c3 (M(r) 26,000) from D. desulfuricans Norway, the
three-dimensional structure of which has been determined, 26.95%
of the residues have been conserved. In the enzyme from D. desulfuricans
Norway, hydrophobic interactions have been described across the
dimer interface. Residues involved in similar interactions seem
to be well conserved in the equivalent D. gigas cytochrome. This
sequence provides structural data to allow specification of this
new subclass of polyhaem cytochromes. Furthermore, D. gigas cytochrome
c3 (M(r) 26,000) is the first polyhaem cytochrome shown to contain
two disulphide bridges linking two identical subunits, which could
induce more rigid folding. The folding and the evolution of this
family of polyhaem cytochromes are discussed.
Users
Please
log in to take part in the discussion (add own reviews or comments).