Oxidizes Fe(2+) at alkaline pH; successively forms ferrihydrite (Fe(3+)(2)O(3) 0.5 H(2)O) then magnetite (Fe(3)O(4)) from an Fe(2+) solution. Binds 2 heme groups via the magnetochrome (MCR) motifs. Optimum pH is 9.0 for oxidation of Fe(2+). Exists in a homodimer-homotetramer equilibrium. Not seen in magnetosome membranes. 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. This bacteria has on average 17 elongated cubo-octahedral shaped magnetosome with a size of 64 X 57 nm which contain membrane-bound crystals of magnetite (Fe(3)O(4)). Belongs to the magnetosome MamP family.