ABSTRACT
Mo-nitrogenase is the enzyme responsible for the conversion of atmospheric nitrogen to ammonia. Its active site is the iron-molibdenum cofactor, FeMo-co, which is composed by two incomplete cubes, Fe4S3 and MoFe3S3, connected through three bridging S. N2 adsorption is expected to take place on the FeMo-co, and to be catalytically converted to two NH3 molecules through the successive addition of protons and electrons. Mutant studies have evidenced that the activity of the nitrogenase FeMo-co is significantly influenced by the environment. In particular the mutation of HIS-195 to glutamine inhibits its ability to reduce nitrogen, though leaving intact its capability to adsorb N2. In this work we investigated theoretically the adsorption of N2 and of one hydrogen atom in presence of HIS-195. We performed our simulations with both C and N as central atoms, as it has been recently shown experimentally that N might not be the FeMo-co interstitial atom and C is a reasonable candidate We report energetic results for four different N2 adsorption mechanisms. In particular we identify a favorable adsorption mechanism characterized by a partial opening of the FeMo-co which is here proposed for the first time.
