Nitrogenase
Nitrogenases (EC 1.18.6.1EC 1.19.6.1) are enzymes used by some organisms to fix atmospheric nitrogen gas (N2). There is only one known family of enzymes that accomplishes this process. Whilst the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (
), the activation energy is very high (
). Nitrogenase acts as a catalyst, reducing this energy barrier such that the reaction can take place at ambient temperatures.
Introduction
The enzymatic reduction of dinitrogen to ammonia requires both a reducing agent, such as ferredoxin or flavodoxin, and an input of chemical energy, released from the hydrolysis of ATP, to overcome the activation energy barrier. The enzyme is composed of the heterotetrameric MoFe protein that is transiently associated with the homodimeric Fe protein. Electrons for the reduction of nitrogen are supplied to nitrogenase when it associates with the reduced, nucleotide-bound homodimeric Fe protein. The heterocomplex undergoes cycles of association and disassociation to transfer one electron, which is the rate-limiting step in nitrogen reduction . ATP supplies the energy to drive the transfer of electrons from the Fe protein to the MoFe protein. The reduction potential of each electron transferred to the MoFe protein is sufficient to break one of dinitrogen's chemical bonds, though it has not yet been shown that exactly three cycles are sufficient to convert one molecule of N2 to ammonia. Nitrogenase ultimately bonds each atom of nitrogen to three hydrogen atoms to form ammonia (NH3), which is in turn bonded to glutamate to form glutamine. The nitrogenase reaction additionally produces molecular hydrogen as a side product.
Nitrogenase (flavodoxin)
Nitrogenase (flavodoxin) (EC 1.19.6.1) is an enzyme with system name reduced flavodoxin:dinitrogen oxidoreductase (ATP-hydrolysing). This enzyme catalyses the following chemical reaction
The enzyme is a complex of two proteins containing iron-sulfur centres and molybdenum.
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See also
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