α-Lys(424) Participates in Insertion of FeMoco to MoFe Protein and Maintains Nitrogenase Activity in Klebsiella oxytoca M5al

Our previous investigation of substrates reduction catalyzed by nitrogenase suggested that α-Ile(423) of MoFe protein possibly functions as an electron transfer gate to Mo site of active center-“FeMoco”. Amino acid residue α-Lys(424) connects directly to α-Ile(423), and they are located in the same α-helix (α423-431). In the present study, function of α-Lys(424) was investigated by replacing it with Arg (alkaline, like Lys), Gln (neutral), Glu (acidic), and Ala (neutral) through site-directed mutagenesis and homologous recombination. The mutants were, respectively, termed 424R, 424Q, 424E, and 424A. Studies of diazotrophic cell growth, cytological, and enzymatic properties indicated that none of the substitutions altered the secondary structure of MoFe protein, or normal expression of nifA, nifL, and nifD. Substitution of alkaline amino acid (i.e., 424R) maintained acetylene (C(2)H(2)) and proton (H(+)) reduction activities at normal levels similar to that of wild-type (WT), because its FeMoco content did not reduce. In contrast, substitution of acidic or neutral amino acid (i.e., 424Q, 424E, 424A) impaired the catalytic activity of nitrogenase to varying degrees. Combination of MoFe protein structural simulation and the results of a series of experiments, the function of α-Lys(424) in ensuring insertion of FeMoco to MoFe protein was further confirmed, and the contribution of α-Lys(424) in maintaining low potential of the microenvironment causing efficient catalytic activity of nitrogenase was demonstrated.