hemoproteins

Hemoproteins are a class of proteins that contain a heme prosthetic group, an iron-containing porphyrin ring. The iron in heme can cycle between Fe2+ and Fe3+ states and coordinate ligands such as oxygen, carbon monoxide, or nitric oxide. In most hemoproteins the heme is noncovalently bound, but in cytochrome c the heme is covalently attached to the protein via thioether bonds to cysteine residues (heme c). The iron often has two axial ligands, with the surrounding protein environment tuning reactivity and ligand affinity.

The functions of hemoproteins are diverse. They include transport and storage of oxygen (hemoglobin in blood, myoglobin in muscle), and roles in electron transfer within respiration and other redox processes (cytochromes a, b, and c, among others). Hemoproteins also act as enzymes that catalyze oxidation-reduction reactions, such as catalase and peroxidases, and the heme-containing cytochrome P450 family involved in xenobiotic metabolism. The binding of ligands and the oxidation state of iron govern activity, with features such as the distal histidine stabilizing bound O2 and cooperative binding contributing to hemoglobin’s oxygen transport.

Biosynthesis and degradation are integral to hemoprotein function. Heme is synthesized in mitochondria and cytosol through a multi-step pathway and is inserted into apoproteins. Degradation by heme oxygenase releases biliverdin (converted to bilirubin), frees iron, and produces carbon monoxide. Hemoproteins are essential across organisms for energy production, metabolism, signaling, and defense against oxidative stress.