However if only domain iii is important in

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However, if only Domain III is important in recognition, it would be ex- pected that the simple tris(catecholato)-rhodium(III) complex would be an equally good inhibitor. In fact, even at concentrations in which the rhodium-catechol complex was in very large excess, no inhibition of iron uptake was observed, suggesting that Domain II is important in the recognition process. The role of Domain II in the recognition process was probed by using a rhodium dimethyl amide of 2,3-dihydroxybenzene (DMB) as a catechol ligand, with one more carbonyl ligand than in the tris(catecholato)-rhodium(III) com- plex. Remarkably, this molecule shows substantially the same inhibition of en- terobactin-mediated iron uptake in E. coli as does rhodium MECAM itself. Thus, in addition to the iron-catechol portion of the molecule, the carbonyl groups 23
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24 OMS ~OH ~OH catechol TRIMCAM Figure 1.16 MECAM and related enterobactin analogues.
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II. BIOLOGICAL SYSTEMS OF METAL STORAGE, TRANSPORT, AND MINERALIZATION 25 (Domain II) adjacent to the catechol-binding subunits of enterobactin and syn- thetic analogs are required for recognition by the ferric-enterobactin receptor. In contrast, when a methyl group was attached to the "top" of the rhodium ME- CAM complex, essentially no recognition occurred. In summary, although the structure of the outer-membrane protein receptor of E. coli is not yet known, the composite of the results just described gives a sketch of what the ferric-enterobactin binding site must look like: a relatively rigid pocket for receiving the ferric-catecholate portion of the complex, and proton donor groups around this pocket positioned to hydrogen bond to the carbonyl oxygens of the ferric amide groups. The mechanisms of iron release from enterobactin, though followed phenomenologically, are still not known in detail. 2. Zinc, copper, vanadium, chromium, molybdenum, and cobalt As described in an earlier section, transport problems posed by the six ele- ments listed in the heading are somewhat simpler (with the exception of chro- mium) than those for iron. One very interesting recent development has been the characterization of sequestering agents produced by plants which complex a number of metal ions, not just ferric ions. A key compound, now well-charac- terized, is mugeneic acid (Figure 1.17).45 The structural and chemical similari- C3 C3 Figure 1.17 Structure and a stereo view of mugeneic acid. See Reference 42.
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26 1 I TRANSITION-METAL STORAGE, TRANSPORT, AND BIOMINERALIZATION ties of mugeneic acid to ethylenediaminetetraacetic acid (EDT A) have been noted. Like EDTA, mugeneic acid forms an extremely strong .~omplex with ferric ion, but also forms quite strong complexes with copper, zinc, and other transition- metal ions. The structure of the cobalt complex (almost certainly essentially identical with that of the iron complex) is shown in Figure 1.18. Like the sid- erophores produced by microorganisms, the coordination environment accom- modated by mugeneic acid is essentially octahedral. Although the coordination properties of
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