A bacterioferritin from the strict anaerobe Desulfovibria desulfuricans ATCC 27774

bacterioferritin was isolated from the anaerobic bacterium Desulfovibrio de sulfuricans ATCC 27774, grown with nitrate as the terminal electron accepto r, which is the first example of a bacterioferritin from a strict anaerobic organism. This new bacterioferritin was isolated mainly as a 24-mer of 20 kDa identical subunits, containing 0.5 noncovalently bound heme and 2 iron atoms per monomer. Although its N-terminal sequence is significantly homolo gous with ferritins from other microorganisms and the ligands to the di-iro n ferroxidase center are conserved, it is one of the most divergent bacteri oferritins so far characterized. Also, in contrast to all other known bacte rioferritins, its heme is not of the B type; its chromatographic behavior i s identical to that of iron uroporphyrin. Thus, D. desulfuricans bacteriofe rritin appears to be the second example of a protein unexpectedly containin g this heme cofactor, or a closely related porphyrin, after its finding in Desulfovibrio gigas rubredoxin:oxygen oxidoreductase [Timkovich, R., Burkha lter, R. S., Xavier, A. V., Chen, L., and Le Gall, J. (1994) Bioorg. Chem. 22, 284-293]. The oxidized form of the protein has a visible spectrum chara cteristic of low-spin ferric hemes, exhibiting a weak absorption band at 71 5 nm, indicative of bis-methionine heme axial coordination; upon reduction, the ct-band appears at 550 nm and a splitting of the Soret band occurs, wi th two maxima at 410 and 425 nm. The heme center has a reduction potential of 140 +/- 10 mV (pH 7.6), a value unusually high compared to that of other bacterioferritins (ca. -200 mV).