High Quality Content by WIKIPEDIA articles! Heme C (or haem C) differs from heme B in that the two vinyl side chains of heme B are replaced by covalent, thioether linkages to the apoprotein. These linkages do not allow the heme C to easily dissociate from the holoprotein, cytochrome c, compared with the more easily dissociated heme B that may dissociate from the holoprotein, the heme-protein complex, even under mild conditions. This allows a very wide range of cytochrome c structure and function, with the myriad of c type cytochromes acting primarily as electron carriers. Cytochromes c, existing as dozens to hundreds of structural and functional subtypes, are found in a wide variety of cell types, even in those organisms living under extreme conditions. While heme B plays a variety of roles in cells, heme C is almost always located in proteins functioning in electron transport reactions most often associated with cell, mitochondria or chloroplast membranes. Heme C, along with the closely structurally related Heme B, are the most common heme types observed in functioning hemeproteins.