Figure 1.

Chemical structures of the various heme types in eukaryotes. Starting with the heme b precursor (protoheme), heme c is made through covalent attachment to the sulfhydryl moieties of either cytochrome c or complex III subunit cytochrome c₁; this process is assisted by the cytochrome c lyases. Heme a is synthesized from heme b through the sequential action of heme o synthase and heme a synthase enzymes, going through the intermediate, heme o.