Figure 24.

Possible CO binding configurations on the M- or V-clusters. A reflects a μ₂-CO species bridged between two Fe centers of the catalytic cofactor, designated ‘lo-CO.’ This state was observed in both Mo- and V nitrogenase. B and C reflect potential ‘hi-CO’ species that were observed in Mo- and V-nitrogenase, respectively. B reflects two end-on-bound CO molecules (with uncertainty in the status of the bridging sulfide, as depicted with dashed lines), while C was proposed to be a combination of the μ₂-CO moiety (i.e. ‘lo-CO’) in addition to an end-on bound CO (designated ‘extra CO’).