C-type Cytochrome Formation:  Chemical and Biological Enigmas

Abstract

C-type cytochromes are proteins that are essential for the life of virtually all organisms. They characteristically contain heme that is covalently attached via thioether bonds to two cysteines in the protein. In this Account, we describe the challenging chemistry of thioether bond formation and the surprising variety of biogenesis systems that exist in nature to perform the difficult posttranslational heme attachment process. We show what insight has been gained into the various biogenesis systems from in vitro and in vivo experiments and highlight some forthcoming challenges in this field.

A heme-degradation pathway in a blood-sucking insect

“…5F). Thioether linkage may involve the internal (␣) carbon atom of the vinyl, as in the attachment of heme to the cysteine residues of c-type cytochromes (27). However, based on present evidence we cannot exclude a cysteine sulfur atom binding to the terminal (␤) carbon atom of heme vinyl groups.…”

“…Alternatively, one could also speculate that a glutathione molecule may be first added to heme by a specific lyase activity, and, then, ␥-glutamyl residue is removed by a ␥-glutamyl-transpeptidase (29). Lyase activities are involved in the binding of the ␣-carbon of heme vinyl to the cysteine sulfur atom of apocytochrome C (27) and in the convalent attachment of phycobiliproteins to their bilin chromophores (31). An intrinsic lyase activity presented by apophytochromes is responsible for their ␣-thioether linkage to the ethylidene groups of their respective BV-derived chromophores (17) and the ␤-thioether linkage of BV vinyl groups to a bacteriophytochromes (28).…”

A heme-binding domain controls regulation of ATP-dependent potassium channels

“…In the case of the Slo1 channels (12), a cytochrome c-like CXXCH motif has been implicated (12,28). However, this raises immediate questions that do not chime with established patterns of behavior in other heme proteins, because most proteins bind heme reversibly (i.e., noncovalently), whereas cytochrome c uses complex and specialized biosynthetic machinery to bind heme irreversibly (i.e., covalently) through thioether bonds from the Cys residues of the CXXCH motif to the heme vinyl groups (29,30). There is as yet no evidence that ion channels proteins use similarly specialized biosynthetic machinery; thus, it seems unlikely that their heme is covalently attached.…”

Tuning the formation of a covalent haem–protein link by selection of reductive or oxidative conditions as exemplified by ascorbate peroxidase

“…Although this similarity might be coincidental, it could be that this represents the rate of reaction that is obtained as a consequence primarily of a proximity effect. It seems unlikely that a reaction on this timescale would be operative in vivo, and thus catalysed thioether bond formation, by any of the three characterized cytochrome c biogenesis systems [24,25], presumably must rely upon other factors so far unidentified to accelerate the reaction between thiol and vinyl. We have speculated previously that the histidine residue of the CXXCH unit might act as a proton donor in thioether bond formation, including during the in vitro formation of a thioether bond in a c-type cytochrome [26].…”