Scheme 1. Simplified Lowe-Thorneley kinetic scheme for nitrogen fixation applicable to all three nitrogenase isozymes. The catalytic intermediates of a MoFe (or VFe, or FeFe) protein are denoted E_n, where n = 0–8 is the number of [e⁻/H⁺] that have been delivered to the catalytic FeMo-(or FeV-, or FeFe-) cofactor.