THE AROMATIC PATCH OF 3 PROXIMAL RESIDUES IN THE HUMAN ACETYLCHOLINESTERASE ACTIVE-CENTER ALLOWS FOR VERSATILE INTERACTION MODES WITH INHIBITORS

The role of the functional architecture of the human acetylcholinester ase (HuAChE) active centre in accommodating the non-covalent inhibitor s tacrine and huperzine A, or the carbamates pyridostigmine and physos tigmine, was analysed using 16 mutants of residues lining the active-c entre 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), result ed in pronounced changes in stability of all the complexes examined; ( b) effects due to replacements of the five other aromatic residues alo ng 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)) wer e relatively small; (c) effects due to substitution of the carboxylic residues in the gorge (Glu(202), Glu(450)) were moderate. These result s and molecular modelling indicate that the aromatic side chains of re sidues Trp(86), Tyr(133) and Tyr(337) form together a continuous 'arom atic patch' lining the wall of the active-centre gorge, allowing for t he accommodation of the different ligands via multiple modes of intera ction. Studies with HuAChE mutants carrying replacements at positions 86, 133 and 337 indicate that the orientations of huperzine A and tacr ine in the HuAChE complexes in solution are significantly different fr om those observed in X-ray structures of the corresponding complexes w ith Torpedo californica AChE (TcAChE). These discrepancies may be expl ained in terms of structural differences between the complexes of HuAC hE and TcAChE or, more likely, by the enhanced flexibility of the AChE active-centre gorge in solution as compared with the crystalline stat e.