STRUCTURE AND DYNAMICS OF THE ACTIVE-SITE GORGE OF ACETYLCHOLINESTERASE - SYNERGISTIC USE OF MOLECULAR-DYNAMICS SIMULATION AND X-RAY CRYSTALLOGRAPHY

The active site of acetylcholinesterase (AChE) from Torpedo californic a is located 20 Angstrom from the enzyme surface at the bottom of a na rrow gorge. To understand the role of this gorge in the function of AC hE, we have studied simulations of its molecular dynamics. When simula tions were conducted with pure water filling the gorge, residues in th e vicinity of the active site deviated quickly and markedly from the c rystal structure; Further study of the original crystallographic data suggests that a bis-quaternary decamethonium (DECA) ion, acquired duri ng enzyme purification, resides in the gorge. There is additional elec tron density within the gorge that may represent small bound cations. When DECA and 2 cations are placed within the gorge, the simulation an d the crystal structure are dramatically reconciled. The small cations , more so than DECA, appear to stabilize part of the gorge wall throug h electrostatic interactions. This part of the gorge wall is relativel y thin and may regulate substrate, product, and water movement through the active site.