The heme moiety, found in many iron containing proteins, is a flat polycyclic aromatic ring (porphyrin) with an iron atom in the middle. Although porphyrins are not restricted to containing iron (synthetic cobalt analogs of hemoproteins are common), the vast majority of porphyrin containing proteins have iron as the central metal.
There are three kinds of heme found in nature.
- Heme B is the most common, found in human hemoglobin and myoglobin, and the structure of heme B is given here:
- Heme A differs from Heme B in that a methyl side chain is oxidized to a formyl group, and one of the vinyl side chains has been replaced by an isoprenoid tail. Heme A is found in the enzyme cytochrome c oxidase.
- Heme C differs from Heme B in that the vinyl side chains of the heme group are instead covalently bound to the protein itself. Heme c is found in cytochrome c.
The naming of the cytochromes reflects the kinds of hemes they contain. cytochromes a have type a hemes, Cytochrome bs have type b hemes, etc.
Note: this is a major work in progress