An enzyme that reduces nitrogen to ammonia can also reduce carbon monoxide to hydrocarbons.

A Radical Mechanism: The bacterial enzyme nitrogenase plays a key role in the global nitrogen cycle by catalyzing the reduction of N ₂ to ammonia. At the heart of the enzyme is a metal-sulfur cluster that contains an interstitial light atom recently identified as a carbide. The identification raised questions concerning the role of the carbide in the enzyme mechanism and how it is inserted into the metal cluster. Wiig et al. (p. 1672 ; see the Perspective by Boal and Rosenzweig ) now show that the carbide derives from S-adenosylmethionine (SAM) and is inserted into the core by the radical SAM enzyme NifB.

In a small-scale reaction, vanadium-dependent nitrogenase has previously been shown to catalyze reductive catenation of carbon monoxide (CO) to ethylene, ethane, propylene, and propane. Here, we report the identification of additional hydrocarbon products [α-butylene, n-butane, and methane (CH₄)] in a scaled-up reaction featuring 20 milligrams of vanadium-iron protein, the catalytic component of vanadium nitrogenase. Additionally, we show that the more common molybdenum-dependent nitrogenase can generate the same hydrocarbons from CO, although CH₄ was not detected. The identification of CO as a substrate for both molybdenum- and vanadium-nitrogenases strengthens the hypothesis that CO reduction is an evolutionary relic of the function of the nitrogenase family. Moreover, the comparison between the CO-reducing capacities of the two nitrogenases suggests that the identity of heterometal at the active cofactor site affects the efficiency and product distribution of this reaction.

Delicate dance becomes a ballet: The enzyme nitrogenase uses adenosine triphosphate and several unusual iron-sulfur cofactors to pump electrons into typically inert dinitrogen (N ₂ ), providing protons along the way. Previous work has shown that sulfur atoms in the iron-molybdenum cofactor (FeMoCo) are labile and suggests that replacement of one of the sulfurs by N ₂ is integral to the mechanism of N ₂ binding and reduction. Through the elimination of excess reducing agent during preparation, Kang et al. determined structures of Mo-nitrogenase in a resting conformation. Unexpectedly, they found that all three sulfurs at the outer edge of FeMoCo appear to be labile, with one subunit even having two of three sulfurs replaced by light, diatomic ligands. Biochemical and spectroscopic data indicate that the protein is active, holds tightly bound N ₂ , and is in the expected oxidation state. These results may prompt a reassessment of the possible mechanisms of N ₂ reduction and the role of dynamic belt ligands in FeMoCo. Science , this issue p. 1381