The high-resolution X-ray crystallographic structure of the ferritin (EcFtnA) of Escherichia coli; comparison with human H ferritin (HuHF) and the structures of the Fe3+ and Zn2+ derivatives11Edited by R. Huber

Thermodynamic Analysis of Ferrous Ion Binding toEscherichia coliFerritin EcFtnA

“…The third binding event exhibits endothermic heats of reaction with lower affinity but a binding stoichiometry of ∼ 28 Zn 2+ /protein corresponding to saturation of all B sites (∼1 Zn 2+ /site). This interpretation is in agreement with the X-ray crystallographic data of the Zn 2+ derivative of EcFtnA, showing full occupancy of the A and B sites of the protein by Zn 2+ (11,29) with no Zn 2+ found at the C site. This is also consistent with Zn 2+ being an inhibitor of iron oxidation in EcFTnA presumably because Zn 2+ competes with Fe 2+ ions for the dinuclear ferroxidase sites (10,26 (14,15,18,19).…”

“…Third, previous Zn 2+ and Tb 3+ competition experiments showed that iron oxidation occurs at the A and B sites of variant E130A with very little iron at the C site, a result consistent with our ITC data and with A and B sites being the primary sites of Fe 2+ binding and oxidation (10). Fourth, the X-ray structure of WT EcFtnA showed similar structural positions of the A-and B-site residues in the Fe 3+ -free and Fe 3+ -bound structures, indicating very little or no residue reorientation of the Fe 3+ -binding structure (11). In contrast, the Zn 2+ -binding structure showed more movement of side chains at the di-iron center.…”

“…Both EcBFR and EcFtnA rapidly oxidize iron at their ferroxidase centers (6)(7)(8), whereas some of the residues critical for fast iron oxidation are missing in EcFtnB and its function remains unknown (7,9). EcBFR has the typical di-iron ferroxidase center found in mammalian ferritins, whereas EcFtnA has an additional third iron binding site (C site) that is unique to this protein (7,10,11 and Figure 1). Thus, in principle, a total of 72 Fe 2+ can possibly bind to EcFtnA, one in each of the 24 sites of the A, B, and C trinuclear center.…”

2.6 Å resolution crystal structure of the bacterioferritin from Azotobacter vinelandii

“…First, the Fe1 atom in AvBF has both a low occupancy (0.54 on average) and a long distance to Fe2. The lower occupancy of Fe1 implicates that three steps might have continuously occurred to the dinuclear center: oxidation of Fe1 and Fe2, rupture of any l-oxo or hydroxyl-bridge connecting them, and sequential migration of Fe1 to the inner nucleation site (possibly composed of Asp50, Glu47, and Asp129), which might have left some dinuclear sites occupied by Fe 3þ only at the site of Fe2 as suggested by Stillman et al [23]. From the perspective of oxidation, the Fe1-Fe2 distance of AvBF would be as short as that of the 'isolated' oxidized structure of DdBF, 3.71…”

“…In general, the dinuclear iron site in AvBF may represent a special partially oxidized state, to some extent, different from the 'cycled' oxidized state observed for DdBF [11], and is likely an outcome of the alternative oxidation and reduction. The low occupancy of Fe1 resulted from its oxidation during crystal installation for diffraction, while the long Fe1-Fe2 distance was from the reduction processes, including crystallization with excess of sodium dithionite as well as reduction by synchrotron radiation similar to the case of the iron derivative of the non-heme ferritin from Escherichia coli [23]. Furthermore, the important difference between the dinuclear iron site of AvBF and that of the 'cycled' DdBF comes from the fact that in the latter case the data were measured from flash-frozen crystals [11].…”