Spectroscopic characterization of 57Fe-reconstituted rubrerythrin, a non-heme iron protein with structural analogies to ribonucleotide reductase.

Rubrerythrin, a contraction of rubredoxin and hemerythrin, is the trivial name given to a non-heme iron protein isolated from Desulfovibrio vulgaris (Hildenborough). This protein, whose physiological function is unknown, was first characterized by J. LeGall et al. [(1988) Biochemistry 28, 1636] as being a homodimer of subunit M(r) = 21,900 with four Fe per homodimer distributed as two rubredoxin-type FeS4 centers and one hemerythrin-type diiron cluster. Subsequent analysis of the amino acid sequence of the rubrerythrin gene [Kurtz, D. M., Jr., & Prickril, B.C. (1991) Biochem. Biophys. Res. Commun. 181, 137] revealed an internal homology which suggested that each subunit can accommodate one diiron cluster. Here, we report a procedure for reconstitution of the as-isolated D. vulgaris rubrerythrin with 57Fe. The reconstituted protein was characterized by optical, electron paramagnetic resonance, and Mössbauer spectroscopies. The results indicate successful incorporation of 57Fe into the two types of sites and strongly suggest that each subunit of rubrerythrin can indeed accommodate one diiron cluster as well as one rubredoxin-type center. Combined with amino acid sequence analysis, the spectroscopic characterization further suggests that the rubrerythrin subunit contains a diiron site whose structure is more closely related to that in ribonucleotide reductase than to that in hemerythrin.