Vanadium nitrogenase, which catalyzes the conversion of nitrogen to ammonia, can also reduce carbon monoxide to form ethylene, ethane, and propane, report Chi Chung Lee, Yilin Hu, and Markus W. Ribbe of the University of California, Irvine (Science 2010, 329, 642). Nitrogenase enzymes are produced by certain species of bacteria and allow the microbes to grow with only N2 as a nitrogen source; the enzymes can have different metal-based cofactors, depending on what’s available in their environment. In the absence of N2, the enzymes typically produce hydrogen. But Lee and colleagues noticed that in a CO atmosphere the H2 production drops by about one-third for the vanadium enzyme. Mass spectrometry and isotope-labeling experiments uncovered that the enzyme is converting CO to C2H4, C2H6, and C3H8. Molybdenum nitrogenase, in contrast, does not react with CO. The researchers propose that the vanadium enzyme may more closely resemble an ancestral form of nitrogenase with broad catalytic capability and that the molybdenum enzyme evolved to emphasize N2 fixation once photosynthesis had developed to provide an alternative method of carbon fixation.