Hydrazine as a substrate and inhibitor of Azotobacter vinelandii nitrogenase

Hydrazine has been tested as a substrate and inhibitor of nitrogenase from Azotobacter vinelandii. It is a linear noncompetitive inhibitor of acetylene reduction, with K_il = K_is = 80 mM at pH 8.0. Carbon monoxide is a linear noncompetitive inhibitor of hydrazine reduction with K_ii = K_is = 2 × 10^?4atm. The inhibition of acetylene reduction by hydrazine is unaffected by the presence of hydrogen, and hydrogen does not inhibit the reduction of hydrazine. Hydrazine can completely suppress hydrogen evolution, while not inhibiting phosphate hydrolysis. The apparent K_m for hydrazine reduction varies with pH, reaching a limiting value of about 25 mM at high pH. The apparent K_i for hydrazine inhibition of hydrogen evolution reaches a similar limiting value at high pH. By varying the concentration of ATP it is possible to alter the relative allocation of electrons to acetylene or hydrazine. Hydrazine is a relatively more potent inhibitor of acetylene reduction at low levels of ATP. It is concluded that hydrazine is able to react effectively with a less reduced state of the enzyme from A. vinelandii than is acetylene or dinitrogen.