A Blue Non‐Heme Iron Protein from Desulfovibrio gigas

Abstract

A novel iron-containing blue protein, named neelaredoxin, was isolated from the sulfate-reducing bacterium Desulfovibrio gigas. It is a monomeric protein with a molecular mass of 15 kDa containing two iron atoms/molecule. The N-terminal sequence of neelaredoxin has similarity to the second domain of desulfoferrodoxin, a protein purified from Desulfovibrio vulgaris Hildenborough. This finding supports the hypothesis that the gene coding for desulfoferrodoxin (rbo) might have arisen from a gene fusion [Brumlik, M. J., Leroy, G., Bruschi, M. & Voordouw, G. (1990) J. Bacteriol. 172, 7289–7292]. The visible spectrum exhibits a single band at 666 nm, responsible for the blue color of the protein, which is completely bleached upon reduction with sodium ascorbate. In the oxidized state the EPR spectrum is complex, exhibiting well-resolved features at g = 7.6, 7.0, 5.9, and 5.8 which are assigned to two high-spin (S= 5/2) mononuclear-iron (III) centers with different rhombic distortions (E/D≈ 0.05 and ≈ 0.08). The two iron atoms contribute identically to the visible spectrum as judged from visible redox titrations, from which a reduction potential of + 190 mV was determined for both iron sites at pH 7.5. At high pH the visible and the EPR spectra become pH-dependent with a pK_a above 9: the 666–nm band shifts to 590 nm and the EPR signals are converted into a signal with g_max∼ 4.7. Neelaredoxin is readily reduced both by H₂/hydrogenase/cytochrome c₃ and by NADH/NADH–rubredoxin oxidoreductase.