Controlling the functionality of cytochrome c(1) redox potentials in the Rhodobacter capsulatus bc(1) complex through disulfide anchoring of a loop and a beta-branched amino acid near the heme-ligating methionine
A. Osyczka et al., Controlling the functionality of cytochrome c(1) redox potentials in the Rhodobacter capsulatus bc(1) complex through disulfide anchoring of a loop and a beta-branched amino acid near the heme-ligating methionine, BIOCHEM, 40(48), 2001, pp. 14547-14556
The cytochrome c(1) subunit of the ubihydroquinone:cytochrome c oxidoreduct
ase (bc(1) complex) contains a single heme group covalently attached to the
polypeptide via thioether bonds of two conserved cysteine residues. In the
photosynthetic bacterium Rhodobacter (Rba.) capsulatus, cytochrome cl cont
ains two additional cysteines, C144 and C167. Site-directed mutagenesis rev
eals a disulfide bond (rare in monoheme c-type cytochromes) anchoring C144
to C167, which is in the middle of an 18 amino acid loop that is present in
some bacterial cytochromes c, but absent in higher organisms. Both single
and double Cys to Ala substitutions drastically lower the +320 mV redox pot
ential of the native form to below 0 mV, yielding nonfunctional cytochrome
bc(1). In sharp contrast to the native protein, mutant cytochrome cl binds
carbon monoxide (CO) in the reduced form, indicating an opening of the heme
environment that is correlated with the drop in potential. In revertants,
loss of the disulfide bond is remediated uniquely by insertion of a beta -b
ranched amino acid two residues away from the heme-ligating methionine 183,
identifying the pattern beta XM, naturally common in many other high-poten
tial cytochromes c. Despite the unrepaired disulfide bond, the beta XM reve
rtants are no longer vulnerable to CO binding and restore function by raisi
ng the redox potential to +227 mV, which is remarkably close to the value o
f the beta XM containing but loop-free mitochondrial cytochrome c(1). The d
isulfide anchored loop and beta XM motifs appear to be two independent but
nonadditive strategies to control the integrity of the heme-binding pocket
and raise cytochrome c midpoint potentials.