Ferrihemin

Ferrihemin is a term used in biochemistry to denote ferric heme, the iron(III) protoporphyrin IX prosthetic group found in certain hemoproteins and synthetic complexes. In this form the central metal is Fe3+, coordinated within the porphyrin ring and often bound by additional ligands at axial positions. The ferric oxidation state imparts different redox and ligand-binding properties compared with ferrous heme (Fe2+). In many proteins, the iron is six-coordinate, with an axial ligand on either side of the porphyrin plane; in synthetic or modified systems, ligands such as water or chloride may occupy axial sites.

Biological roles for ferrihemin center on redox chemistry and catalytic activity rather than on reversible oxygen transport, which is typically associated with ferrous heme. Ferric heme participates in diverse enzymatic reactions, including peroxidation, electron transfer, and oxidative catalysis, where redox cycling between Fe3+ and Fe2+ enables substrate processing. In heme-containing enzymes, the ferric state can act as a resting or intermediate state that becomes more reactive upon reduction or activation by substrates and co-factors.

Occurrence and terminology can vary; ferrihemin is encountered in older literature or in contexts distinguishing ferric from ferrous heme. The term is sometimes used interchangeably with ferriheme or, in clinical and research settings, with hemin—the Fe3+ protoporphyrin IX chloride used as a reagent and therapeutic agent. Hemin is a prominent example of ferrihemin in practice, illustrating the biomedical relevance of ferric heme chemistry. See also heme, hemoprotein, hemin.