On the structure-function relationship of nitrogenase M-cluster and P-cluster pairs

It A proposed that the M-cluster cage (Kim-Rees model) in active N₂-ase can exert shape-selective molecular-sieve effects in molecular recognition of exogenous substrates, by providing inside multinuclear active-sites the cavity for N₂, C₂H₂, cyclopropene, and N₂O reduction, with [Mo-3Fe]-site available only for N₂ reduction: on the other handn-RC— CH,n-RC— N,n-RN-C , C—N^– and N₃^–, are bound outside the cavity at the [2Fe]-site left by the labilizable ligand Y. A terminal carboxylate of the Mo-bound (R)-homocitrate is just in position to protect a H₂-evolution site on the P-cluster pair from CO inhibition, and also to take part in mediating a P-cluster-to-Mo-site H⁺-relay system (involving two hydrogen-bonded H₂O) specifically required for N. reduction. The nonreducibility of CO at the [Mo-3Fe]-site is also explained. Experimental support for molecular-sieve effects of M-cluster cage has been obtained from the observed decrease in ethene-cis-d: selectivity by competitive inhibition of HC—CH reduction in D₂O by N—N.Dedicated to Jiaxi Lu on the occasion of his 80th birthday.