The structural principles underlying molybdenum insertase complex assembly

Within the cell, the trace element molybdenum (Mo) is only biologically active when complexed either within the nitrogenase‐specific FeMo cofactor or within the molybdenum cofactor (Moco). Moco consists of an organic part, called molybdopterin (MPT) and an inorganic part, that is, the Mo‐center. The enzyme which catalyzes the Mo‐center formation is the molybdenum insertase (Mo‐insertase). Mo‐insertases consist of two functional domains called G‐ and E‐domain. The G‐domain catalyzes the formation of adenylated MPT (MPT‐AMP), which is the substrate for the E‐domain, that catalyzes the actual molybdate insertion reaction. Though the functions of E‐ and G‐domain have been elucidated to great structural and mechanistic detail, their combined function is poorly characterized. In this work, we describe a structural model of the eukaryotic Mo‐insertase Cnx1 complex that was generated based on cross‐linking mass spectrometry combined with computational modeling. We revealed Cnx1 to form an asymmetric hexameric complex which allows the E‐ and G‐domain active sites to align in a catalytic productive orientation toward each other.