Abstract
Heliobacterium modesticaldum is a Gram-positive, anaerobic, anoxygenic photoheterotrophic bacterium. Its cytochrome bc complex (Rieske/cyt b complex) has some similarities to cytochrome b 6 f complexes from cyanobacteria and chloroplasts, and also shares some characteristics of typical bacterial cytochrome bc 1 complexes. One of the unique factors of the heliobacterial cytochrome bc complex is the presence of a diheme cytochrome c instead of the monoheme cytochrome f in the cytochrome b 6 f complex or the monoheme cytochrome c 1 in the bc 1 complex. To understand the structure and function of this diheme cytochrome c protein, we expressed the N-terminal transmembrane-helix-truncated soluble H. modesticaldum diheme cytochrome c in Escherichia coli. This 25 kDa recombinant protein possesses two c-type hemes, confirmed by mass spectrometry and a variety of biochemical techniques. Sequence analysis of the H. modesticaldum diheme cytochrome c indicates that it may have originated from gene duplication and subsequent gene fusion, as in cytochrome c 4 proteins. The recombinant protein exhibits a single redox midpoint potential of +71 mV versus NHE, which indicates that the two hemes have very similar protein environments.
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