Diheme

Diheme is a term used in biochemistry to describe molecules or protein cofactors that contain two heme prosthetic groups. A heme is an iron-containing porphyrin ring capable of reversible redox chemistry, typically cycling between Fe2+ and Fe3+. In diheme systems, the two heme moieties may be covalently bound to a single protein, joined by noncovalent contacts within a multiheme complex, or exist as a pair of cofactors within related proteins. The two iron centers can exchange electrons through the surrounding protein matrix, enabling intramolecular electron transfer or cooperative redox reactions.

Structural variety in diheme systems includes combinations of heme types (often heme b and/or heme c) and arrangements that position the irons for efficient electron transfer or substrate reduction. In some diheme cytochromes, one heme participates in electron transfer from donor to acceptor, while the second serves a catalytic or regulatory role. Spectroscopically, diheme proteins display features of both heme units, with characteristic Soret bands near 400 nm and multiple Q-bands; the two centers can produce split or broadened signals depending on their redox state and coupling.

Biological roles of diheme proteins include participation in respiration, detoxification, and stress responses. A well-studied class is diheme cytochrome c peroxidases, which use two hemes to channel electrons from an electron donor to hydrogen peroxide, reducing it to water. Other diheme proteins function as electron-relay partners in photosynthetic or respiratory chains. The study of diheme systems informs understanding of redox chemistry, protein design, and bioinspired catalysis.