ABSTRACT

Cytochromes bl9 b557, 6557 5, and 6559 are unusual proteins that are also called bacterioferritins [241-243]. They contain both a binuclear iron center and a lowspin six-coordinate heme b, with bis-Met axial ligation (Fig. 23) [244-246]. The tertiary and quaternary structure of these proteins is very similar to that of ferritins forming a nearly spherical shell composed of 24 identical protein chains and 12 hemes, thus suggesting that their probable function is iron uptake. The building blocks of bacterioferritins are protein dimers constituted by two identical subunits binding a single heme group through two corresponding methionines. Each subunit is constituted by four nearly parallel a helices linked through the binuclear metal binding site. Iron is stored as a hydrated ferric oxide mineral in the central part of the resulting shell.