Hemes

Heme is an iron-containing porphyrin prosthetic group that occurs in many proteins across living organisms. The core is a protoporphyrin IX macrocycle coordinated to a single iron ion, usually in the ferrous (Fe2+) or ferric (Fe3+) state. In most proteins the iron is bound by four nitrogen atoms of the porphyrin and by two axial ligands provided by the surrounding protein, often including a histidine and a molecule such as oxygen, carbon monoxide, or NO.

Heme plays essential roles in oxygen transport, storage, electron transfer, and catalysis. In hemoglobin and myoglobin, it binds and releases oxygen; in cytochromes, it participates in electron transfer chains; in peroxidases and catalases it catalyzes redox reactions. The iron center enables reversible ligand binding and participation in one- or two-electron redox chemistry, enabling diverse biological functions.

Biosynthesis and degradation: Heme is synthesized through a multistep pathway in which ferrochelatase inserts iron into protoporphyrin IX to form heme. In animals, parts of the pathway occur in mitochondria and cytosol. Heme is degraded by heme oxygenase to biliverdin (which is reduced to bilirubin), free iron, and carbon monoxide.

Types and variants: Heme b is the most common form in vertebrate hemoproteins; heme a is a modified form found in cytochrome c oxidase; heme c is covalently attached to cysteine residues via thioether bonds in c-type cytochromes. Modifications and ligands influence redox potential and ligand affinity.

Clinical relevance: defects in heme biosynthesis underlie porphyrias; altered heme availability affects metabolism, signaling, and drug responses. In plural usage, hemes refers to the multiple heme groups embedded in different proteins.