Nitrogen Fixation, a Molybdenum-Requiring Process

Nitrogen fixation is the most important input of biologically available nitrogen in Earth's ecosystems and is a metabolic ability possessed only by some Prokaryotes. To date four classes of nitrogenase enzymes have been characterized. Three nitrogenases are homologous enzymes with similar protein subunit composition and metal cofactor structure; these are the Mo-nitrogenase, V-nitrogenase, and Fe-only nitrogenase. How these three systems evolved and which of them first appeared on Earth is still under debate. The best studied system is the Fe-Mo-co based although several comparative analyses have been performed in past years.