Fe-protein cycle

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

Fig. 8. The Fe protein cycle of molybdenum nitrogenase. This cycle describes the transfer of one electron from the Fe protein (F) to one afi half of the MoFe protein (M) with the accompEmying hydrolysis of 2MgATP to 2MgADP + 2Pf. The rate-determining step is the dissociation of F (MgADP)2 from M,rf. Subscript red = reduced and ox = oxidized.

Figure 12 The Fe protein cycle for Klebsiella pneumoniae Kpl is the MoFe protein Kp2 is the Fe protein.

Duyvis, M. G., Wassink, H., and Haaker, H. (1998) Nitrogenase of Azotobacter vinelandii kinetic analysis of the Fe protein cycle, Biochemistry 37, 17345-17354. 

The most comprehensive model for the function of molybdenum nitrogenase in the reduction of N2 is that of Lowe and Thomeley, which was developed almost two decades ago. This model describes two aspects of nitrogenase catalysis, the Fe protein cycle and the MoFe protein cycle. 

Fe Protein Cycle Electron Transfer from the Fe Protein to the MoFe Protein in the Nitrogenase Complex... 

Figure 7 shows what is beheved to occur during the one-electron Fe protein cycle of the molybdenum nitrogenase, the process in which the Fe protein serves as an obligate electron donor for the MoFe protein." 4 >5T58 jjj to... 

Figure 9 The MoFe protein cycle of the molybdeniun nitrogenase." The cycle describes a plausible sequence of events during the reduction of N2 to 2NH3 and H2. M represents one functional half of the MoFe protein, which is composed of one afi subunit pair plus one FeMo cofactor and one P cluster. Subscripts 0 to 7 indicate the number of electrons transferred to M from the Fe protein via the Fe protein cycle. It is plausibly assumed that each transferred electron is neutralized by the addition of one proton. Each dotted arrow corresponds to one completed Fe protein cycle (shown in Figure 7)...

Figure 9 shows the MoFe protein cycle, which consists of eight one-electron transfer steps, each of which corresponds to one Fe protein cycle.The reactions shown in Figure 9 are consistent with the overall stoichiometry of the enzymatic N2 reduction, that is, N2 + 8H" + 8e - 2NH3 -E H2. ... 

The kinetics of nitrogenase was elucidated many years ago and comprises two cycles, the Fe-protein cycle and the actual conversion of Nj to NHj in the MoFe protein (Figure 6.2). The electrons needed for the reduction of N2 are provided by the Fe protein. To this end, the Fe-protein in its reduced state ([Fc4S4] ) temporarily associates with the MoFe-protein, and one electron is transferred from each Fe-protein to the corresponding catalytic unit. The relatively large distance between the nucleotide binding sites and the protein complex interface (>10 A) requires a structural switch mechanism within the protein matrix that initiates the concomitant hydrolysis of two MgATP... 

Figure 6.2 Kinetics of the nitrogenase reaction as established by Thorneley and Lowe. Each dotted arrow represents one Fe-protein cycle.

Fig. 8. The interconnecting Fe-protein and MoFe-proteins cycles. Fe represents the Fe protein MoFe is the MoFe protein (ox) and (red) are oxidized and reduced states, respectively MgATP and MgADP are the magnesium salts of adenosine triphosphate and diphosphate, respectively. The catalyzed reduction of all known substrates requires at least two electrons, therefore, the Fe-protein cycle (the left half of the figure) must be completed at least twice for the MoFe protein to catalyze substrate reduction.

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