A cobalt containing protein isolated from Desulfovibrio gigas, a sulfate reducer

A protein which contains a cobalt porphyrin was isolated from the sulfate reducer Desulfovibrio gigas. This protein has a molecular weight of approximately 16,700 daltons and is acidic, having an iso-electric point at 3.7. The N-terminal residue was shown to be threonine, and a cobalt analysis gave 0.8 cobalt atoms/molecule, suggesting the presence of a single prosthetic group. The protein has a violet color with absorption bands typical of a metal porphyrin center with maxima at 420 nm, 580 nm with a shoulder at 550 nm. The ratio $\text{A}{}_{420}\text{(γ)}\text{A}{}_{588}\text{(α)}$ is 2.1. The protein has no electron paramagnetic resonance (e.p.r.) spectrum, and as the visible spectrum suggests, it probably contains diamagnetic Co^III porphyrin. However the cobalt centre appears to be protected from reduction by sodium dithionite or sodium borohydride. Att$\text{e}$mpts at ligand substitution with strong nucleophiles such as CN, causes a slight spectral shift to higher wavelenghts. The cobalt porphyrin can be extracted from the protein with an acidified acetone solution, indicating that it is not covalently bound to the protein.