bio

Nitrogenase

The only enzyme in biology that can break the triple bond in nitrogen gas, opening the route from inert N₂ to the ammonia life can use. It is almost absurdly expensive: roughly sixteen ATP per molecule of N₂ reduced, and its iron-and-molybdenum reaction centre is destroyed permanently by oxygen, so nitrogen-fixing organisms must both pay the energy bill and build an oxygen barrier around the enzyme. This is why nitrogen still limits growth almost everywhere despite making up nearly four-fifths of the air: the problem was never scarcity, it was cost.