Abstract
Cytochromes c are widespread respiratory proteins characterized by the covalent attachment of heme. The formation of c-type cytochromes requires, in all but a few exceptional cases, the formation of two thioether bonds between the two cysteine sulfurs in a –CXXCH– motif in the protein and the vinyl groups of heme. The vinyl groups of the heme are not particularly activated and therefore the addition reaction does not physiologically occur spontaneously in cells. There are several diverse post-translational modification systems for forming these bonds. Here, we describe the complex multiprotein cytochrome c maturation (Ccm) system (in Escherichia coli comprising the proteins CcmABCDEFGH), also called System I, that performs the heme attachment. System I is found in plant mitochondria, archaea and many Gram-negative bacteria; the systems found in other organisms and organelles are described elsewhere in this minireview series.
Highlights
The post-translational modification that covalently attaches heme to the –CXXCH– motif of cytochromes c is a protein-catalysed process, of which there are several types
Much has been learned about the cytochrome c maturation (Ccm) apparatus, many aspects remain to be unravelled, and the recent discovery of System I* raises new and unexpected questions
The idea of a unique type of bond between heme and the side chain of an absolutely essential histidine in CcmE has been well developed over the last 10 years. If this is so important for the function of CcmE, it is unexpected that this essential histidine is replaced by an cysteine in some Ccm systems
Summary
Cytochromes c are widespread respiratory proteins characterized by the covalent attachment of heme. The formation of c-type cytochromes requires, in all but a few exceptional cases, the formation of two thioether bonds between the two cysteine sulfurs in a –CXXCH– motif in the protein and the vinyl groups of heme. The vinyl groups of the heme are not activated and the addition reaction does not physiologically occur spontaneously in cells. There are several diverse post-translational modification systems for forming these bonds. We describe the complex multiprotein cytochrome c maturation (Ccm) system (in Escherichia coli comprising the proteins CcmABCDEFGH), called System I, that performs the heme attachment. System I is found in plant mitochondria, archaea and many Gram-negative bacteria; the systems found in other organisms and organelles are described elsewhere in this minireview series
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