FIG 3.

Spectroscopic and catalytic features of the wild-type and variant MaNifH. (A) EPR spectra and (B) CO₂-reducing activities of wild-type and variant MaNifH. EPR spectra were collected at 10 K. The wild-type and R98H and R98G variant MaNifH are dimers of ∼60 kDa and contain 3.7 ± 0.1, 3.9 ± 0.4, and 3.8 ± 0.2 nmol Fe per nmol protein, respectively. Like the wild-type MaNifH, the R98H and R98G variants display the same [Fe₄S₄]⁺ characteristic, S = 1/2 EPR signal in the dithionite-reduced state (A), yet they display disparate activities in CO₂ reduction (B). The hydrocarbon/CO ratios (calculated based on total nmol of reduced carbons) of the wild-type MaNifH and R98H variant are 2.7 and 1.9, respectively, suggesting a shift from hydrocarbon formation to CO formation in the latter case.