Based upon the structure of the Humicola insolens endoglucanase V (now known as Cel45) [2, 3] it was concluded that Asp121 (in an HxD motif) acted as the general acid (implied by its hydrogen bonding to the glycosidic oxygen of a ligand in the +1 subsite) and that the most likely general base is Asp10 (in a YxD motif), appropriately positioned "below" the sugar plane. As with many inverting enzymes the general base assignment is less secure than that of the acid.