reactivity upon mutation of this residue to glutamic acid. The third residue that has been shown to be key for MgATP binding is residue 16, serine. Site-specific mutagenesis was used to demonstrate this fact. This has led to a model in which the serine remains coordinated to the Mg2+ ion after phosphate hydrolysis in order to facilitate its association with a different phosphate of the now ADP molecule. MgATP binding also induces significant conformational changes within the Fe protein. Site-directed mutagenesis was employed to create mutants in which MgATP binds but does not induce a conformational change. Comparing X-ray scattering data in the mutants versus in the wild-type protein led to the conclusion that the entire protein contracts upon MgATP binding, with a decrease in radius of approximately 2.0 Å. Other mechanistic details Many mechanistic aspects of catalysis remain unknown. No crystallographic analysis has been reported on substrate bound to nitrogenase. Nitrogenase is able to reduce acetylene, but is inhibited by carbon monoxide, which binds to the enzyme and thereby prevents binding of dinitrogen. Dinitrogen prevent acetylene binding, but acetylene does not inhibit binding of dinitrogen and requires only one electron for reduction to ethylene. Due to the oxidative properties of oxygen, most nitrogenases are irreversibly inhibited by dioxygen, which degradatively oxidizes the Fe-S cofactors. This requires mechanisms for nitrogen fixers to protect nitrogenase from oxygen in vivo. Despite this problem, many use oxygen as a terminal electron acceptor for respiration. Although the ability of some nitrogen fixers such as Azotobacteraceae to employ an oxygen-labile nitrogenase under aerobic conditions has been attributed to a high metabolic rate, allowing oxygen reduction at the cell membrane, the effectiveness of such a mechanism has been questioned at oxygen concentrations above 70 μM (ambient concentration is 230 μM O2), as well as during additional nutrient limitations. Nonspecific reactions In addition to dinitrogen reduction, nitrogenases also reduce protons to dihydrogen, meaning nitrogenase is also a dehydrogenase. A list of other reactions carried out by nitrogenases is shown below: HC≡CH → H2C=CH2 N–=N+=O → N2 + H2O N=N=N– → N2 + NH3 → CH4, NH3, H3C–CH3, H2C=CH2 (CH3NH2) N≡C–R → RCH3 + NH3 C≡N–R → CH4, H3C–CH3, H2C=CH2, C3H8, C3H6, RNH2 O=C=S → CO + H2S O=C=O → CO + H2O S=C=N– → H2S + HCN O=C=N– → H2O + HCN, CO + NH3 Furthermore, dihydrogen functions as a competitive inhibitor, carbon monoxide functions as a