Figure 7.

Formation and relaxation of E₂. In-line: The “on-path” two-step, ATP-dependent addition of two H⁺/e^– to MoFe protein to form E₂. Off-line: Representation of the exergonic (free energy, +|ΔG_h|) “off-path” relaxation of E₂, liberating H₂ and directly regenerating E₀ without intervention of Fe protein, and of the energetically (free energy, +|ΔG_h|) and kinetically forbidden reverse of this process; E₀′ is a putative intermediate state that causes the reaction of E₀ not to be the microscopic reverse of the release of H₂ from E₂ (see text).