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Electron Paramagnetic Resonance and Magnetic Circular Dichroism Spectra of the Nitrogenase M Cluster Precursor Suggest Sulfur Migration upon Oxidation: A Proposal for Substrate and Inhibitor Binding
Author(s) -
Rupnik Kresimir,
Tanifuji Kazuki,
Rettberg Lee,
Ribbe Markus W.,
Hu Yilin,
Hales Brian J.
Publication year - 2020
Publication title -
chembiochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.05
H-Index - 126
eISSN - 1439-7633
pISSN - 1439-4227
DOI - 10.1002/cbic.201900681
Subject(s) - nitrogenase , electron paramagnetic resonance , chemistry , magnetic circular dichroism , cluster (spacecraft) , iron–sulfur cluster , crystallography , sulfur , stereochemistry , substrate (aquarium) , circular dichroism , photochemistry , enzyme , spectral line , nitrogen , nitrogen fixation , nuclear magnetic resonance , biochemistry , biology , physics , organic chemistry , astronomy , computer science , programming language , ecology
The active site of the nitrogen‐fixing enzyme Mo‐nitrogenase is the M cluster ([MoFe 7 S 9 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 ([Fe 8 S 9 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 ([Fe 8 S 8 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.