UC Irvine

The active site of the nitrogen-fixing enzyme Mo-nitrogenase is the M cluster ([MoFe₇ S₉ C⋅R-homocitrate]), also known as the FeMo cofactor or FeMoco. The biosynthesis of this highly complex metallocluster involves a series of proteins. Among them, NifB, a radical-SAM enzyme, is instrumental in the assembly of the L cluster ([Fe₈ S₉ C]), a precursor and all-iron core of the M cluster. In the absence of sulfite, NifB assembles a precursor form of the L cluster called the L* cluster ([Fe₈ S₈ C]), which lacks the final ninth sulfur. EPR and MCD spectroscopies are used to probe the electronic structures of the paramagnetic, oxidized forms of both the L and L* clusters, labeled L^Ox and [L*]^Ox . This study shows that both L^Ox and [L*]^Ox have nearly identical EPR and MCD spectra, thus suggesting that the two clusters have identical structures upon oxidation; in other words, a sulfur migrates away from L^Ox following oxidation, thereby rendering the cluster identical to [L*]^Ox . It is proposed that a similar migration could occur to the M cluster upon oxidation, and that this is an instrumental part of both M cluster formation and nitrogenase substrate/inhibitor binding.