Low stereoselectivity in methylacetylene and cyclopropene reductions by nitrogenase.

Abstract

The stereochemistry of reductions catalyzed by nitiogenase in 2H20 has been investigated by using allene, methylacetylene, and cyclopropene as substrates. Deuterium labeling patterns in the reduction products were determined by mass spectroscopy, infrared s ectroscopy, 2H-decoupled 220-MHz 1H NMR, and IH-decoupfd 30.7-MHz 2H NMR. Reduction of allene gave pure [2,3-H2lpropene Stereochemistry is a characteristic feature of many enzymemediated reactions that often provides mechanistically valuable clues concerning active site-substrate interactions. Nitrogenase catalyzes the ATP-dependent reduction of N2 to NH3, possibly via enzyme-bound N2H2 and N2H4 intermediates (1); stereochemistry arising from geometrical isomerism is possible only with the first postulated intermediate, diazene. The instability of free diazene (cis or trans) has forestalled a direct demonstration of substrate behavior, and it has not proven detectable during nitrogenase-catalyzed N2 fixation. Among the various adventitious nitrog'enase substrates (1), C2H2 has been used to explore reduction stereochemistry. Exposure of C2H2 to active nitrogenase/2H20 assay mixtures reportedly has resulted predominantly or exclusively in formation of [cis-1,2-2H21C2H4 (2-4). A primary kinetic isotope effect has not been observed in these 2H20 reductions. This finding has been cited in support of a concerted transfer of two protons and two electrons to substrate, possibly involving a specifically positioned pair of acids derived from the enzyme protein residues (5). Because of its high symmetry (D0h), C2H2 is a comparatively insensitive stereochemical probe: a nonspecific addition of exterior solvent protons to C2H2 bound to an active site metal center, or other mechanisms, arguably could account for the observed preferential formation of the cis-alkene isomer. Important advantages of C2H2 as a substrate are that it interacts efficiently with nitrogenase [Km(C2H2) Km(N2)] and that it gives a conveniently analyzed product. There are apparent differences in the kinetic behavior of C2H2 and N2, suggestive of incomplete equivalence between the two substrates (6).We have demonstrated (7) that cyclopropene is a substrate of nitrogenase, being reduced to a mixture of propene and cyclopropane whose proportions are remarkably insensitive (8) to in vivo vs. in vitro assay. Cyclopropene is a reversible inhibitor of nitrogenase N2 reduction and is reduced with a Km value as low as that of N2 or C2H2 (8). In contrast, the acyclic C3H4 isomers allene and methylacetylene are poor substrates, judging from their large Km values (9). Acyclic alkenes such as C2H4 or propene do not show detectable substrate activity with nitrogenase (1). The lower symmetry (C2v), rigid ring structure, and structurally differing products available with cyclopropene suggest that it might be useful as a stereochemical probe for reduction by nitrogehase in 2H20. We note that only three We report here product characterization studies comparing 2H20/nitrogenase reductions of all three C3H4 isom...

Klebsiella pneumoniae nitrogenase. Inhibition of hydrogen evolution by ethylene and the reduction of ethylene to ethane

“…However, reduction of propyne in 2H20 yields cis-and trans-dideuteropropene in the ratio approx. 2: 1, indicating cis-addition to bound alkene is not obligatory (McKenna et al, 1979).…”

Nitrogenase reduction of carbon-containing compounds

“…For example, cyclopropene (Table 1) can undergo reduction to cleave the single bond forming propene and reduction of the double bond forming cyclopropane [48,53,54]. The production of both 1,3- and 2,3-dideuteropropene when the reaction is run in D 2 O indicated that there must be an isomerization process of the intermediate derived from the single bond cleavage [48].…”

“…For example, cyclopropene (Table 1) can undergo reduction to cleave the single bond forming propene and reduction of the double bond forming cyclopropane [48,53,54]. The production of both 1,3- and 2,3-dideuteropropene when the reaction is run in D 2 O indicated that there must be an isomerization process of the intermediate derived from the single bond cleavage [48]. Distinguished from cyclopropene reduction, 3,3-difluorocyclopropene can be reduced by 4 e − /H + to yield 2-fluoropropene or by 6 e − /H + to yield propene suggesting that the ring-opening reaction occurs through the complete cleavage of the -C=C-double bond [55].…”

Reduction of cyclopropene by NifV- and wild-type nitrogenases from Klebsiella pneumoniae

“…The reduction of cyclopropene to cyclopropane seems to follow the classical reduction pathway for an alkene or alkyne at a hydridic metal centre (see, e.g., ref. [22]), even to the extent that there is some stereochemical control, since cyclopropene is hydrogenated by A. vinelandii nitrogenase primarily in the cis positions [23].…”