Structure of cytochrome c′: a dimeric, high-spin haem protein

Abstract

The bacterial cytochromes c′ (ref. 1) occur in various photosynthetic and denitrifying bacteria, where they are presumed to function in electron transport². Most cytochromes c′ are isolated as dimeric molecules composed of two identical subunits of ∼14,000 molecular weight (MW), with each polypeptide chain incorporating a protohaem IX prosthetic group covalently bound through thioether linkages formed by condensation of two cysteine side chains with the haem vinyl groups³. Although a similar mode of covalent haem attachment is found in members of the well studied mitochondrial cytochrome c family⁴, the cytochromes c′ are, in contrast, high-spin haem proteins (hence the prime designation) which bind neutral ligands⁵, characteristic properties generally associated with members of the oxygen-binding globin family⁶. Here we present a preliminary structural description of the cytochrome c′ derived from the photosynthetic, purple non-sulphur bacterium Rhodospirillum molischianum, and show that it bears little structural resemblance to members of either the cytochrome c or globin structural families. The cytochrome c′ monomer structure is, instead, principally organized as a left-twisted, 4-α-helical bundle, a structural motif previously observed in several other sequentially and functionally unrelated proteins^{7 13}.