Functional architecture of the AChE active center appears to be characterized by both structural "rigidity", necessary to stabilize the catalytic triad as well as by flexibility in accommodating the different, high affinity AChE ligands. These seemingly conflicting structural properties of the active center are demonstrated through combination of structural methods with kinetic studies of the enzyme and its mutant derivatives with plethora of structurally diverse ligands and in particular with series of stereoselective covalent and noncovalent AChE ligands. Thus, steric perturbation of the acyl pocket precipitates in a pronounced stereoselectivity toward methylphosphonates by disrupting the stabilizing environment of the catalytic histidine rather than through steric exclusion demonstrating the functional importance of the "rigid" environment of the catalytic machinery. The acyl pocket, the cation-binding subsite (Trp86) and the peripheral anionic subsite were also found to be directly involved in HuAChE stereoselectivity toward charged chiral phosphonates, operating through differential positioning of the ligand cationic moiety within the active center. Residue Trp86 is also a part of the "hydrophobic patch" which seems flexible enough to accommodate the structurally diverse ligands like tacrine, galanthamine and the two diastereomers of huperzine A. Also, we have recently discovered further aspects of the role of both the unique structure and the flexibility of the "hydrophobic patch" in determining the reactivity and stereoselectivity of HuAChE toward certain carbamates including analogs of physostigmine. In these cases the ligands are accommodated mostly through hydrophobic interactions and their stereoselectivity delineates precisely the steric limits of the pocket. Hence, the HuAChE stereoselectivity provides a sensitive tool in the in depth exploration of the functional architecture of the active center. These studies suggest that the combination of "rigidity" and flexibility within the HuAChE gorge are an essential element of its molecular design.