Negative cooperativity in the nitrogenase Fe protein electron delivery cycle

Significance Nitrogenase catalyzes N 2 reduction to ammonia, the largest N input into the biogeochemical nitrogen cycle. This difficult reaction involves delivery of electrons from the Fe protein component to the catalytic MoFe protein component in a process that involves hydrolysis of two ATP per electron delivered. MoFe contains two catalytic halves, each of which binds an Fe protein. The prevailing picture has been that the two halves function independently. Here, it is demonstrated that electron transfer (ET) in the two halves exhibits negative cooperativity: Fe→MoFe ET in one-half partially suppresses ET in the other. These findings thus show that conformational coupling in nitrogenase not only gates ET within each half, as shown previously, but introduces negative cooperativity between the two halves.