The catalytic reduction of hydrazine (N2H4) to ammonia by a β-98Tyr→His MoFe protein in the absence of the Fe protein or ATP is reported. The reduction of N2 or other substrates (e.g., hydrazine, protons, acetylene) by nitrogenase normally requires the transient association of the two nitrogenase component proteins, the Fe protein and the MoFe protein. The Fe protein, with two bound MgATP molecules, transfers one electron to the MoFe protein during each association, coupled to the hydrolysis of two MgATP. All substrate reduction reactions catalyzed by nitrogenase require delivery of electrons by the Fe protein coupled to the hydrolysis of MgATP. We report that when a single amino acid within the MoFe protein (β-98Tyr) is substituted by His, the resulting MoFe protein supports catalytic reduction of the nitrogenous substrate hydrazine (N2H4) to two ammonia molecules when provided with a low potential reductant, polyaminocarboxylate ligated EuII (E m-1.1 V vs NHE). The wild-type and a number of other MoFe proteins with amino acid substitutions do not show significant rates of hydrazine reduction under these conditions, whereas the β-98His MoFe protein catalyzes hydrazine reduction at rates up to 170 nmol NH 3/min/mg MoFe protein. This rate of hydrazine reduction is 94% of the rate catalyzed by the β-98His or wild-type MoFe protein when combined with the Fe protein, ATP, and reductant under comparable conditions. The β-98His MoFe protein reduction of hydrazine in the absence of the Fe protein showed saturation kinetics for the concentration of reductant and substrate. The implications of these results in understanding the nitrogenase mechanism are discussed.