Iron essentiality

The two sites also differ in their pH stability towards iron release. Experiments on serum transferrin showed that one site loses iron at a pH near 6.0, and the other at a pH nearer 5.0 (203, 204), giving a distinctly biphasic pH-induced release profile (Fig. 28). The acid-stable A site was later shown to be the C-terminal site (202). It is this differential response to pH, together with kinetic effects (below), that enables N-terminal and C-terminal monoferric transferrins to be prepared (200). Although the N-terminal site is more labile, both kinetically and to acid, the reasons are not necessarily the same the acid stability may depend on the protonation of specific residues (Section V.B) and is likely to differ somewhat from one transferrin to another in response to sequence changes. The biphasic acid-induced release of iron seen for transferrin is not shared by lactoferrin. Although biphasic release from lactoferrin, in the presence at EDTA, has been reported (205), under most conditions both sites release iron essentially together at a pH(2.5-4.0) several units lower than that for transferrin (Fig. 28). 

The effect of pH differs for the two sites of transferrin and differs between transferrin and lactoferrin. When titrated with acid, in the absence of chelators, serum transferrin loses iron over the pH range 6.0 to 4.0 release is biphasic (Fig. 28), with iron lost from the more acid-labile N-lobe site first (Section IV.D.l). Lactoferrin, on the other hand, is distinctly more stable in acid, with release occurring 2 pH units lower, over the pH range 4.0 to 2.5, and the two sites losing iron essentially together. 

To evaluate the importance of the factors discussed above for the discrimination of CO and O2 binding, a variety of five-coordinate model heme complexes have been synthesized. Since addition of ligands such as imidazoles or pyridines to four-coordinate PFe complexes favors the formation of six-coordinate, diamagnetic complexes, as discussed in Section 4.1.2 (i.e. the open coordination site at the iron, essential to mimic reversible O2 binding in model hemes, is blocked by the second ligand). 

Fe Iron Essential to all organisms. Cofactor in many enzymes, heme proteins L (>1000) L... 

Iron is a major component of the Earth s crust, but its own chemistry greatly limits utilization and also sets the basis for its toxicity. The capacity of readily exchanging electrons in aerobic conditions makes iron essential for fundamental cell functions, such as DNA synthesis, transport of oxygen and electrons, and cell respiration. However, because humans have no means to control iron excretion, excess iron, regardless of the route of entry, accumulates in parenchymal organs and threatens cell viability. 

Similar results on the chemical state of the corrosion products in the reactor water were reported from other BWR circuits, e. g. the reactor water of the British Steam Generating Heavy Water Reactor (SGHWR) (Bridle et al., 1986), where the insoluble iron essentially appeared as a-Fe203, containing minor amounts of Fe304. In this case, approximately 50% of the Co present in the reactor water was attached to the suspended solids, with the remainder being in a ionic dissolved state similar ratios between the particulate and the dissolved state were obtained for Co. 

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