Final E5 to E8 Steps in the Nitrogenase Mechanism for Nitrogen Fixation.

Abstract

Nitrogenase converts nitrogen in the air to ammonia. It is often regarded as the second most important enzyme in nature after photosystem II. The mechanism for how nitrogenase is able to perform the difficult task of cleaving the strong bond in N₂ is debated. It is known that for every electron that is donated to N₂, two ATP are hydrolyzed. In the experimentally suggested mechanism, the activation occurs after four reductions of the ground state, but there is no suggestion for how the enzyme uses the hydrolysis energy to perform catalysis. In the theoretical mechanism, it is suggested that hydrolysis is used to reduce the electron donor. In previous papers, the steps leading to the activation of N₂ in the so-called E₄ state has been investigated, using both the experimental and theoretical mechanism, showing that only the theoretical one leads to agreement with EPR observations for E₄. In the present paper, the four steps following E₄, leading to the release of two ammonia molecules, are described using the same methodology as used in the previous studies.