Ferroxidase activity of recombinant Desulfovibrio vulgaris rubrerythrin
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Rubrerythrin (Rr) is the trivial name given to a non-heme iron protein of unknown function which has been found in anaerobic sulfate-reducing bacteria. Rr is unique in containing both rubredoxin-type FeS4 and diiron-oxo sites in the same protein. The results described here demonstrate for the recombinant protein that: (a) Rr catalyzes oxidation of Fe2+ to Fe3+ by O2, i.e., Rr has ferroxidase activity, (b) both FeS4 and diiron domains of the Rr protein are required for ferroxidase activity, (c) with excess Fe2+ and O2 the initial rate of this oxidation appears to be first order in [Rr] and independent of starting [Fe2+] above 30 μM, (d) the Fe3+ is produced in a form which is capable of rapid incorporation into the iron-binding site of ovotransferrin, and (e) the ferroxidase activity of Rr is comparable to that of published ferroxidase activities of apoferritins on a subunit basis. Ferroxidase activity of Rr was monitored either by the rate of increase in absorbance at 315 nm (which lies near an isosbestic point for oxidized and reduced Rr) or by using apoovotransferrin as Fe3+ acceptor, and measuring the rate and extent of diferric transferrin formation at 460 nm. No polyironoxyhydroxide aggregates appeared to associate with Rr after the ferroxidase reaction. A truncated form of Rr containing only the diiron domain had little or no ferroxidase activity. Rr could function as one component of a set of enzymes which channels the reaction products of O2 and Fe2+ onto a non-toxic pathway during transient exposure of the bacteria to air.
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