Interactions between the peripheral site and the acylation site in acetylcholinesterase

Acetylcholinesterase (AChE) hydrolyzes its physiological substrate acetylcholine at one of the highest known catalytic rates. Two sites of ligand interaction have been identified: an acylation site or A-site at the base of the active site gorge, and a peripheral site or P-site at its mouth. Despite a wealth of information about the AChE structure and the role of specific residues in catalysis, an understanding of the catalytic mechanism and the role of the P-site has lagged far behind. In recent years we have clarified how the P- and A-sites interact to promote catalysis. Our studies have revealed that the P-site mediates substrate trapping and that ligand binding to the P-site can result in steric blockade of the A-site as well as allosteric activation. We have demonstrated this activation only for the acylation step of the catalytic reaction, but others have proposed that it involves the deacylation step. To investigate this point, we have measured the reaction of carbamoyl esters (carbamates) with AChE. With these slowly hydrolyzed substrates, the carbamoylation (acylation) and decarbamoylation (deacylation) steps can be resolved and analyzed separately. Carbamoylcholine is one of the closest structural analogs of acetylcholine, and we monitored these steps in continuous mixed assays with acetylthiocholine as a reporter substrate. At high concentrations of carbamoylcholine, decarbamoylation was inhibited but no activation of carbamoylation was observed. However, high concentrations of acetylthiocholine had no effect on the decarbamoylation rate constants. We concluded that the binding of acetylthiocholine to the P-site does not activate deacylation reactions.