Diastereomer-dependent substrate reduction properties of a dinitrogenase containing 1-fluorohomocitrate in the iron-molybdenum cofactor.

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Abstract
In vitro synthesis of the iron-molybdenum cofactor (FeMo-co) of dinitrogenase using homocitrate and its analogs allows the formation of modified forms of FeMo-co that show altered substrate specificities (N2, acetylene, cyanide, or proton reduction) of nitrogenase [reduced ferredoxin:dinitrogen oxidoreductase (ATP-hydrolyzing), EC 1.18.6.1]. The (1R,2S)-threo- and (1S,2S)-erythro-fluorinated diastereomers of homocitrate have been incorporated in vitro into dinitrogenase in place of homocitrate. Dinitrogenase activated with FeMo-co synthesized using threo-fluorohomocitrate reduces protons, cyanide, and acetylene but cannot reduce N2. In addition, proton reduction is inhibited by carbon monoxide (CO), a characteristic of dinitrogenase from NifV-mutants. Dinitrogenase activated with FeMo-co synthesized using erythro-fluorohomocitrate reduces protons, cyanide, acetylene, and N2. In this case proton reduction is not inhibited by CO, a characteristic of the wild-type enzyme. Cyanide reduction properties of dinitrogenase activated with FeMo-co containing either fluorohomocitrate diastereomer are similar, and CO strongly inhibits cyanide reduction. Dinitrogenases activated with FeMo-co containing homocitrate analogs with a hydroxy group on the C-1 position are much less susceptible to CO inhibition of cyanide reduction. However, proton and cyanide reduction by dinitrogenase containing FeMo-co activated with (1R,2S) threo-isocitrate is only one-third that of dinitrogenase activated with the racemic mixture of isocitrate and shows strong CO inhibition of substrate reduction. These results suggest that CO inhibition of proton and cyanide reduction occurs when the hydroxyl group on the C-1 position of analogs is "trans" to the C-2 carboxyl group (i.e., in the threo conformation). When racemic mixtures of these analogs are used in the system, it seems that the erythro form is preferentially incorporated into dinitrogenase. Finally, carbonyl sulfide inhibition of substrate reduction by dinitrogenase is dependent on the homocitrate analog incorporated into FeMo-co.