NifDK

NifDK refers to the molybdenum-iron dinitrogenase component of nitrogenase, comprising two subunits encoded by nifD (alpha subunit) and nifK (beta subunit). Together, these proteins form the Mo-Fe protein, the dinitrogenase component responsible for reducing atmospheric nitrogen (N2) to ammonia (NH3) in cooperation with the electron donor component NifH (the Fe protein).

Structurally, NifD and NifK assemble into a functional heterotetramer, typically described as an α2β2 dimer. Each αβ pair provides a site containing the FeMo cofactor, the active center of nitrogen reduction, and the complex interacts with electrons donated from NifH during ATP-dependent turnover. The catalytic cycle involves electron transfer to the MoFe protein, coupled with ATP hydrolysis, enabling multi-electron reductions of N2.

Genetically, nifD and nifK are usually part of the nif operon or gene cluster in diazotrophic bacteria and archaea. Expression is tightly regulated and typically induced under nitrogen-limiting conditions and low oxygen, with regulatory proteins such as NifA and, in some organisms, NifL, modulating transcription in response to environmental cues. Proper assembly with accessory factors and cofactors is essential for activity; defects in NifD or NifK disrupt nitrogenase function and nitrogen fixation.

NifDK is central to the global nitrogen cycle and has been the subject of extensive structural and functional studies. Insights into its mechanisms inform efforts to understand nitrogen fixation in nature and to explore biotechnological applications, including potential strategies to engineer nitrogen fixation pathways in plants or other organisms.