Nitrogenases () are
enzymes used by some organisms to fix atmospheric
nitrogen gas (N
2). 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.