Mössbauer and EPR studies of activated aconitase: development of a localized valence state at a subsite of the [4Fe-4S] cluster on binding of citrate

摘要: During activation of aconitase a ferrous ion is incorporated into [3Fe-4S] cluster to yield structure with [4Fe-4S] core. Using 57Fe or 56Fe for we have studied Mossbauer spectroscopy the beef heart enzyme in presence citrate. Our studies show that environment one iron site (Fea) drastically altered Fea acquired during aconitase. In oxidized [4Fe-4S]2+ state two species enzyme-bound substrate are observed, whereas only observed reduced [4Fe-4S]+ state. The parameters reveal has substantial high-spin character. This most pronounced 1+ where at exhibits localized valence dramatic increase isomer shift upon binding strongly suggests ligand become least five-coordinate and may function as Lewis acid. absence citrate EPR spectra active (g1,2,3 = 2.06, 1.93, 1.86) no hyperfine broadening H2 17O. However, 2.04, 1.85, 1.78) sizable transferred interactions observed; under experimental conditions hydroxyl groups isocitrate well water labeled We did not detect by 17O-labeled carboxyl 16O. Implications mechanism discussed.