The influence of chelating agents upon the dissimilatory reduction of Fe(III) by Shewanella putrefaciens. Part 2. Oxo- and hydroxo-bridged polynuclear Fe(III) complexes

The susceptibility to dissimilatory redn. of polynuclear oxo- and hydroxo-bridged Fe(III) complexes by S. putrefaciens intact cells and membranes was investigated. These complexes were ligated by the potential tetradentates heidi (H3heidi = N-(2-hydroxyethyl)iminodiacetic acid) or nta (H3nta = nitrilotriacetic acid), or the potential tridentate ida (H2ida = iminodiacetic acid). A no. of defined small complexes with varied nuclearity and soly. properties were employed, as well as undefined species prepd. by mixing different molar ratios of ida or heidi:Fe(III) in soln. The rates of Fe(III) redn. detd. by an assay for Fe(II) formation with ferrozine were validated by monitoring c-type cytochrome oxidn. and re-redn. assocd. with electron transport. For the undefined Fe(III) polymeric species, redn. rates in whole cells and membranes were considerably faster in the presence of heidi compared to ida. This is believed to result from generally smaller and more reactive clusters forming with heidi as a consequence of the alkoxo function of this ligand being able to bridge between Fe(III) nuclei, with access to an Fe(III) reductase located at the cytoplasmic membrane being of some importance. The increases in redn. rates of the undefined ida species with Fe(III) using membranes relative to whole cells reinforce such a view. Using sol. synthetic Fe(III) clusters, slow redn. was noted for an oxo-bridged dimer coordinatively satd. with ida and featuring unligated carboxylates. This suggests that sterically hindering the cation can influence enzyme action. A heidi dimer and heidi multimer (17 or 19 Fe(III) nuclei), which are both of poor soly., were found to be reduced by whole cells, but dissimilation rates increased markedly using membranes. These data suggest that Fe(III) reductase activity may be located at both the outer membrane and the cytoplasmic membrane of S. putrefaciens. Slower redn. of the heidi multimer relative to the heidi dimer reflects the presence of a central hydroxo(oxo)-bridged core contg. nine Fe(III) nuclei within the former cluster. This unit is a poor substrate for dissimilation, owing to the fact that the Fe(III) is not ligated by aminocarboxylate. The faster redn. noted for the heidi dimer in membranes than for a sol. ida monomer suggests that the presence of ligating water mols. may relieve steric hindrance to enzyme attack. Furthermore, redn. of an insol. oxo-bridged nta dimer featuring ligating water mols. in intact cells was faster than that of a sol. monomer coordinatively satd. by nta and possessing an unligated carboxylate. This suggests that steric factors may override soly. considerations with respect to the susceptibility to redn. of certain Fe(III) complexes by the bacterium. [on SciFinder(R)]