Electron-paramagnetic-resonance spectroscopy and related techniques in the study of nitrogenase.

Abstract

Nitrogenase comprises two proteins, each of which has very similar properties when isolated from any of the diazotrophs studied so far. Both exhibit e.p.r. signals in the resting state and during enzyme turnover. The larger one is generally called the MoFe protein, although other names are sometimes used (e.g. dinitrogenase; Hageman & Burris, 1978), and the most active preparations contain 2 Mo, 32 f 3 Fe and 30 S2atoms in an a2P2 molecule of 220 000 daltons. The smaller protein, the Fe protein (or dinitrogenase reductase; Hageman & Burris, 1978), is generally believed t o contain 4 Fe and 4 S2atoms in a y2 molecule of 66 000 daltons, although Haaker et al. (1984) have recently suggested that these metal and sulphide analyses may be in error. In this paper the nomenclature of Eady et al. (1972) will be used for the two proteins as isolated from individual diazotrophs, so that they are referred to by using a capital letter for the genus and a lower case letter for the species, followed by the number 1 for the MoFe protein and 2 for the Fe protein; thus, for example, MoFe protein isolated from Klebsiella pneumoniae is Kpl and the Fe protein isolated from Azotobacter vinelandii is Av2. The enzyme catalyses the reduction of N, t o 2 N H 3 (eqn. 1) with concomitant reduction of H+ t o H2 and hydrolysis of MgATP t o MgADP plus inorganic phosphate.