Human cholinesterases demonstrated markedly different behavior. The only difference in the structure of the extended active site is the identity of the residue 331 (hAChE) or 322 (hBChE) (Figure ). Asn322 in hBChE directly interacts with Glu441, while the more buried water molecule of the pair (HOH1109 in 1XLW) does not, instead also forming an h-bond with Asn322. This results in the upper oxygen of Glu441 forming three h-bonds, and the lower one only one (Figure , h-bond to HOH1117 not shown). hAChE, in contrast, has Val331 in place of Asn. The buried water molecule switches to interact with Glu450, and it forms two h-bonds with each of its oxygens (Figure ). This influences the rotation of the carboxylate group and the basicity of its individual oxygens.