MgATP-dependent electron transfer

MgATP hydrolysis and electron transfer between the two proteins seems not to be direct and the order of reactions may depend on the precise conditions of the experiment at low temperature, electron transfer seems to be reversible (see Ref. 12) for a discussion). One innovation is incorporation of data in which the release of inorganic phosphate was monitored. With other MgATP hydrolyzing enzymes, this step is often the work step in which the energy released by MgATP hydrolysis is utilized. With nitrogenase this step takes place before the dissociation of the two proteins 106). 

Figure 7 The Fe protein cycle of the molybdeniun nitrogenase. The cycle describes the MgATP-dependent electron transfer from the Fe protein (F) to one afi half of the MoFe protein (M), which is accompanied by the hydrolysis of 2MgATP to 2MgADP and 2Pj. Subscripts ox = oxidized and red = reduced...

Scheme 2. MgATP-dependent electron transfer between components of V nitrogenase, The pre-steady-state electron transfer reactions between the Fe protein and the VFe protein of V nitrogenase of A, chroococcum have been analyzed in terms of this scheme (52). Ac2 represents the Fe protein and Acl represents the VFe protein of this system. This scheme is analogous to that used in the detailed study of Mo nitrogenase (see Hef. 50).

Nitrogenase, as must now become clear, is a complex enzyme of two component proteins which requires ATP, a reductant, a reducible substrate, Mg " " as an activator, and an anaerobic environment to function. To this complexity must be added the difficulty that the component proteins have no enzymatic half reactions . There are, perhaps, four main questions to decide about the mechanism (1) the role(s) of the two component proteins (2) the role(s) of ATP (3) the nature of the active site(s) and (4) the mechanism of N2 reduction. Despite the complexities and difficulties mentioned above, progress in the last 15 years has partly answered all these questions. The Fe protein mediates an ATP-dependent electron transfer from the donor to the MoFe protein which contains the active site. MgATP binds and induces a conformational change in the Fe protein which lowers its redox potential. FeMoco, the molybdenum cofactor, which may be part of the active site of N2 reduction, has been isolated and partly characterized, while an intermediate in N2 reduction has recently been discovered (Thorneley ct al., 1978). The next part of this chapter describes the evidence for these claims. This evidence involves the noncatalytic reactions of the individual proteins, their... 

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