Enzymological basis of nitrogen fixation

where the first is the limit chemical stoichiometry of one H2 formed for every N2 reduced. At standard state, the dintirogen reduction is exothermic and remains so at cellular concentrations with ferredoxins or flavodoxins as electron donors. Nevertheless, substrate reduction requires ATP hydrolysis where the limit stoichiometry usually is considered to be 2 ATP per electron (16 ATP total to satisfy the first equation). Two questions are to be evaluated: how are the two equations linked; and, how relevant are the in vitro conditions to in vivo?