Iron uptake binding mechanism

Transferrin iron uptake via receptor-mediated endocytosis has clearly appeared fairly late in evolution, when we consider that the bilobal iron-binding protein is found only as far back as insects . As we have seen in the preceding chapters, iron-uptake mechanisms involving the synthesis of more or less specific siderophores have evolved together with strategies implying the solubilization of insoluble ferric iron by the combined effects of pH and reduction, and even the development of receptor proteins capable of taking up transferrin-, lactoferrin- or haem-bound iron from specific hosts. 

Figure 11.1 Schematic representation of iron uptake mechanisms, (a) The transferrin-mediated pathway in animals involves receptor-mediated endocytosis of diferric transferrin (Tf), release of iron at the lower pH of the endocytic vesicle and recycling of apoTf. (b) The mechanism in H. influenzae involves extraction of iron from Tf at outer membrane receptors and transport to the inner membrane permease system by a periplasmic ferric binding protein (Fbp). From Baker, 1997. Reproduced by permission of Nature Publishing Group.

Once the siderophore-iron complexes are inside the bacteria, the iron is released and utilized for vital cell functions. The iron-free hydroxamate siderophores are commonly re-excreted to bring in an additional iron load (Enterobactin is at least partially degraded by a cytoplasmic esterase This cycle is repeated until specific intracellular ferric uptake regulation proteins (Fur proteins) bind iron, and signal that the intracellular iron level is satisfactory, at -which point ne-w siderophore and siderophore-receptor biosynthesis are halted and the iron-uptake process stops. This intricate feedback mechanism allows a meticulous control over iron(III) uptake and accumulation against an unfavorable concentration gradient so as to maintain the intracellular iron(III) level within the required narrow window. Several excellent reviews concerning siderophore-iron transport mechanisms have been recently published i.3,i6, is,40,45,60-62 ... 

Fig. 5. Equilibrium binding and chelation mechanism for intestinal regulation and control of iron uptake with a mucosal directed, active transport system...

A significant difference between BFR and H-chain ferritins is the mechanism of iron uptake and core formation. In H-chain ferritins, initial iron uptake takes place by ferrous iron being oxidized at the ferroxidase center and moving into the core, and once a sizeable core has been established, autooxidation at the core surface takes over from the ferroxidase center reaction (see Section 7.2). In BFR, iron uptake takes place in three distinct phases. Phase I is the binding of two ferrous ions per ferroxidase center (48 per protein). Phase 2 corresponds to rapid oxidation at the ferroxidase center according to reactions (7) and (8), where z represents the charge on the protein. ... 

The uptake of siderophore-iron complexes by Gram-negative bacteria is energy dependent and occurs via specific outer membrane proteins. In the periplasmic space, it binds to its cognate periplasmic binding protein and is then actively transported across the cytoplasmic membrane by an ATP-trans-porter protein. Three principal mechanisms for transport through the outer membrane have been described ... 

Iron transport was studied using the Fe " - and H-complexes of parabactin (25). After a quick uptake of 10% of both labels there was a continuing steady uptake of Fe " while the amount of remained constant. This could either mean that after binding to the cell surface Fe only is transferred into the cell ( taxi mechanism ) or there is a fast re-export of the ligand. A decision in favor of the... 

The schizokinen-mediated Fe " transport in Bacillus megaterium was studied by double labelling with e and (8). At 37°C, uptake of Fe and of are parallel during the first 30 sec, then that of e continues until it levels off after 2 min, while that of [ H]-schizokinen drops to a low constant level. At 0°C, uptake of both labels reaches this low level which is obviously due to the binding of the ferri-siderophore to the cell surface. At 37°C, transport into the cell, release of iron, and re-export of the ligand follow. Apparently a shuttle mechanism takes place, cf. the experimental results obtained with parabactin (Sect. 3.2) indicative of a taxi mechanism. 

Transferrin plays a major role in the transport and cellular uptake of thorium (Peter and Lehmann 1981). Thorium can be displaced from transferrin by an excess of iron, but it is not known whether thorium and iron bind to the same sites on the transferrin molecule. It has also been determined that thorotrast (Th02 colloid) blocks the uptake of labelled protein by the RES in female rabbits and in both male and female rats (Hyman and Paldino 1967). The mechanism of the blockade is not clear. Sex differences were found in rabbits but not in rats. The particle size of the Thorotrast colloid influences its effect on the uptake of protein only particles larger than 1 pm will interfere with uptake of protein by the RES. 

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