The 'aromatic patch' of three proximal residues in the human acetylcholinesterase active centre allows for versatile interaction modes with inhibitors

The role of the functional architecture of the human acetylcholinesterase (HuAChE) active centre in accommodating the non-covalent inhibitors tacrine and huperzine A, or the carbamates pyridostigmine and physostigmine, was analysed using 16 mutants of residues lining the active-centre gorge. Despite the structural diversity of the ligands, certain common properties of the complexes could be observed: (a) replacement of aromatic residues Tyr(133), Tyr(837) and especially Trp(86), resulted in pronounced changes in stability of all the complexes examined; (b) effects due to replacements of the five other aromatic residues along the active-centre gorge, such as the acyl pocket (Phe(295) Phe(297)) Or at the peripheral anionic site (Tyr(124), Trp(286), Tyr(341)) were relatively small; (c) effects due to substitution of the carboxylic residues in the gorge (Glu(202), Glu(450)) were moderate. These results and molecular modelling indicate that the aromatic side chains of residues Trp(86), Tyr(133) and Tyr(337) form together a continuous 'aromatic patch' lining the wall of the active-centre gorge, allowing for the accommodation of the different ligands via multiple modes of interaction. Studies with HuAChE mutants carrying replacements at positions 86, 133 and 337 indicate that the orientations of huperzine A and tacrine in the HuAChE complexes in solution are significantly different from those observed in X-ray structures of the corresponding complexes with Torpedo californica AChE (TcAChE). These discrepancies may be explained in terms of structural differences between the complexes of HuAChE and TcAChE or, more likely, by the enhanced flexibility of the AChE active-centre gorge in solution as compared with the crystalline state.