The location of substrate attachment to the complex has yet to be elucidated. It is believed that the Fe atoms closest to the interstitial carbon participate in substrate activation, but the terminal molybdenum is also a candidate for nitrogen fixation. X-ray crystallographic studies utilizing MoFe-protein and carbon monoxide (CO), which is isoelectronic to dinitrogen, demonstrated that carbon monoxide is binding to the Fe2-Fe6-edge of FeMoco. Additional studies showed simultaneous binding of two CO-molecules to FeMoco, providing a structural basis for biological Fischer-Tropsch-type chemistry. Se-incorporation studies in combination with time-resolved X-ray crystallography evidenced major structural rearrangements in the FeMoco-structure upon substrate binding events.