Title
EPR and MCD spectra of the nitrogenase M cluster precursor suggest sulfur migration upon oxidation: A proposal for substrate and inhibitor binding
Author
Kresimir Rupnik, Kazuki Tanifuji, Lee Rettberg, Markus Walter Ribbe, Yilin Hu, Brian Hales
Year
2019
Journal
ChemBioChem
Abstract
The active site of the nitrogen fixing enzyme, Mo‐nitrogenase, is the M cluster ([MoFe7S9C·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 ([Fe8S9C]), a precursor and an 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 ([Fe8S8C]), which lacks the final 9th 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 LOx and [L*]Ox, respectively. This study shows that both LOx and [L*]Ox have nearly identical EPR and MCD spectra, suggesting that the two clusters have identical structures upon oxidation; in other words, a sulfur migrates away from LOx following oxidation, rendering the cluster identical to [L*]Ox. It is proposed that a similar migration could occur to the M cluster upon oxidation and that it is an instrumental part of both M cluster formation and nitrogenase substrate/inhibitor binding.
Instrument
J-715
Keywords
Circular dichroism, Magnetic circular dichroism, Protein structure, Biochemistry