Molybdenum (Mo) isabsorbed in the form of molybdate ions from the soil. Molybdenum is a component of several enzymes, including nitrogenase and nitrate reductase both of which participate in nitrogen metabolism, and thus, it helps in biological nitrogen fixation.
The reduction of nitrogen to ammonia by living organisms is known as biological nitrogen fixation. The enzyme nitrogenase found in prokaryotes catalyses the reduction of N2 to ammonia and plays a vital role in the global nitrogen cycle. Nitrogenase is a complex enzyme that consists of two proteins, a molybdenum iron protein (Mo-Fe) that binds N2 and a smaller iron protein that acts as an electron donor to molybdenum iron protein. The enzyme requires anaerobic conditions as it is highly sensitive to molecular oxygen. To protect these enzymes, the nodule contains an oxygen scavenger called leg-haemoglobin.
Reduction of nitrate to nitrite is carried out by the agency of an inducible enzyme called nitrate reductase. The enzyme is a molybdoflavoprotein (with a cofactor Mo). It requires a reduced coenzyme (NADH or NAD) for its activity. Without it, the synthesis of proteins is blocked and plant growth ceases. This also delays the flowering of many plants.