Figure 3.

Two models of catalytic mechanism of hGIIA are depicted: (A) In the triad model, the water molecule becomes activated by the adjacent histidine N^δ1 atom, and acts as a nucleophile to directly attack the carbonyl of the substrate; (B) In the calcium-coordinated oxyanion model, a second water molecule is linked to the N^δ1 atom through the adjacent water molecule and also coordinated to the calcium ion. The water molecule and the calcium ion polarise the carbonyl group to initiate the cleavage, and the activation energy required is lower than the triad model. Adapted from Berg et al. [18].