Spatzal, Perez et al. were able to insert a selenium atom into the S2B position of the FeMo-cofactor, and track how it moves during catalysis. Iron (Fe) atoms are shown in orange, sulfur (S) atoms in yellow, carbon in gray, molybdenum in cyan, and oxygen in red. The overall reaction is also shown. Reduction of each nitrogen molecule (N2), requires eight electrons and at least eight protons (H+), yielding two ammonia molecules (NH3) and one hydrogen molecule (H2). This nitrogen fixation reaction is coupled to the hydrolysis of 16 adenosine triphosphate (ATP) molecules (Burgess and Lowe, 1996; Howard and Rees, 2006; Hoffman et al., 2014).