Involved in redox-control of magnetite formation; oxidizes Fe(2+) to Fe(3+) or to mixed-valent Fe(2+)-Fe(3+) oxide minerals (PubMed:25775527). May control magnetite crystal size and number (Probable). Overproduction of MamP leads to more crystals than normal during exponential growth of normal size; in stationary phase crystal numbers become wild-type (PubMed:25048532). Binds 2 heme groups via the 2 magnetochrome (MCR) motifs. E(0) is -89 +/- 11 mV for Fe(3+)-Fe(2+) at pH 7.5. Homodimer. Not seen in magnetosome membranes. Constitutively expressed, levels are high for 24 hours after innoculation then decrease in stationary phase (up to 144 hours) (at protein level) (PubMed:25048532). Part of the probable 18 gene mamAB operon (Probable). The dimer forms a pocket of about 8 X 15 Angstroms, lined with acidic residues, which may bind 2 Fe(2+) ions. Subject to proteolytic cleavage which requires both MamE and MamO. Cells have a weak magnetic response and make magnetosome membranes. Has fewer but larger magnetite crystals (PubMed:20212111). Makes small, flaky magnetite crystals, does not alter subcellular localization of MamC, MamF, MamI or MmsF (PubMed:25775527). Deletion of genes mamH to mamV (amb0961 to amb0978) gives cells with no magnetosomes and no magnetic response (PubMed:20212111). This bacteria makes up to 20 cubo-octahedral magnetosomes of about 45 nm in diameter which contain membrane-bound crystals of magnetite (Fe(3)O(4)). Expression of just the minimal mamAB gene cluster (amb0961 to amb0978), including this gene, is sufficient to form a minimal magnetosome chain with small magnetite particles. Belongs to the magnetosome MamP family.