Iron-protein interface

The structure of ferritin is the most complete paradigm for bioinorganic chemistry because of three features the protein coat, the iron-protein interface, and the iron core. ... 

Iron-Protein Interface Formation of the iron core appears to be initiated at an Fe-protein interface where Fe(ll)-0-Fe(III) dimers and small clusters of Fe(IIl) atoms have been detected attached to the protein and bridged to each other by oxo/hydroxo bridges. Evidence for multiple nucleation sites has been obtained... 

Ferritin consists of (1) an apoferritin protein coat, (2) an iron-protein interface, and (3) an inner iron core [7]. The apoferritin coat, or shell, contains 24 subunits arranged as an icosahedron with a molecular weight (MW) of 440 kDa. The subunits are of two related types, designated as H and L, with MW of 21000 and 19000, respectively. These homologous subunits have similar secondary and tertiary structures with a 55% identity and can polymerize together in different proportions to form many hybrid molecules, or isoferritins. 

The Iron/Proteln Interface. Interactions of Iron with the protein coat of ferritin are most easily characterized In the early stages of core formation when most. If not all, of the Iron present Is In contact with the protein coat. In the complete core, bulk Iron Is Inorganic. To date, the protein coat has been little examined early In Iron core formation except In terms of effects on the Iron environment. Studies of the Iron early In core formation will be discussed later. 

Another interesting cluster conversion is the joining of two Fe2S2 clusters in a protein to form a single Fe4S4 cluster at the interface between a dimeric protein. Such a cluster is present in the nitrogenase iron protein (Fig. 24-2) and probably also in biotin synthase.294 The clusters in such proteins can also be split to release the monomers. 

The cluster is coordinated at the tip of the cluster binding subdomain. Fe" (Fe-2) is close to the surface of the protein with its histidine ligands fully exposed to the solvent, whereas Fe " (Fe-1) is buried within the protein and surrounded by the three loops forming the cluster binding subdomain. However, in NDO the histidine ligands are not solvent accessible, but buried at the interface between the Rieske domain and the catalytic domain both histidine ligands form hydrogen bonds with acidic side chains in the catalytic site close to the catalytic iron. 

Several binding sites for Tb3+ or Cd2+ ions have been identified in the interior of the apoferritin protein shell, some of which may be iron-binding sites (Harrison et ai, 1989 Granier et ah, 1998). In HoSF and HoLF, two sites were identified on the inner surface of the B helix at the subunit dimer interface (Figure 6.15, Plate 11) which bind two Cd2+ ions. One involves Glu-57 and Glu-60 as ligands and the other Glu-61 and Glu-64 (Granier et al., 1998). In H-chain ferritins the first pair of Glu-57 and Glu-60 are both replaced by His and only a single Tb3+ is found bound to Glu-61 and Glu-64 (Lawson et al, 1991). 

Even though the iron atoms are separated in haemoglobin by about 25 A, communication between them is still able to occur and this has been postulated to involve a trigger mechanism (Perutz, 1971). The trigger is the movement of the proximal histidine as dioxygen binds to (or is released from) the Fe(n) and results in interconversion between the T and R structures. This movement causes a conformational change which is transmitted through the protein to the other iron sites. X-ray studies indicate that relative shifts of up to 6 A at subunit interfaces occur between the T and R states (Perutz, 1978). 

The two protein subunits found in the holoenzyme, each containing an iron-sulfur cluster, fold into interconnected a-helices and p-pleated sheet that, at their interface, ligate the 4Fe-4S cluster through four cysteine residues, two from each... 

The P-cluster, located at the interface of MoFe-protein s a- and (3-subunits, is believed to function as the electron transfer mediator between Fe-protein and the N2 reduction site at the M center. The P-cluster is contained within a hydrophobic environment and located approximately 10 A below the MoFe-protein surface. Three cysteine side chains from each subunit bind to iron ions in the P-cluster. The cluster is now known to exist in Pox and PN forms in active enzyme, both with stoichiometry FegS7. The PN form, with its octahedrally coordinated central sulfur, has the structure shown in Figure 6.6. As can be seen in Table 6.3, the PN form contains all ferrous irons, corresponding to the P (5 = 0) state, whereas the Pox form corresponds to the P2+ (5=3 or 4) form. 

QH2 to the Rieske iron-sulfur protein, binds at QP, near the 2Fe-2S center and heme bL on the P side. The dimeric structure is essential to the function of Complex III. The interface between monomers forms two pockets, each containing a QP site from one monomer and a QN site from the other. The ubiquinone intermediates move within these sheltered pockets. 

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