Catalytic cycle, reduction nitrogenase

Figure 13. Scheme showing the eight states of the MoFe-protein during the catalytic cycle of nitrogenase. States which evolve H2, bind N2 and NHj are indicated. The elementary steps involved in the reduction of MoFe-protein by the Fe-protein are shown at the foot of the Figure. 

Scheme 1. The catalytic cycle for the reduction of N2 by the Mo nitrogenase. Eq represents the resting state of the MoFe protein of K. pneumoniae and species E -E represent intermediate forms of this protein following sequential one-electron reduction steps. The arrows between each state represent complex formation between the Fe protein and MoFe protein, electron transfer, and protonation, followed by protein dissociation. N2 binds to species 3, accounting for the stoichiometry of Eq. (1) the displacement of N2 from this species accounts for the competitive inhibition of N2 reduction by H2 (see Ref. 50 for a detailed presentation of this scheme).

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... 

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