A Model of Butyrylcholinesterase Based on the X-Ray Structure of Acetylcholinesterase Indicates Differences in Specificity

Abstract

In vertebrates, two enzymes efficiently catalyze acetylcholine (ACh) hydrolysis: acetylcholinesterase (AChE) and butyrylcholinesterase (BChE)1. The principal role of AChE is the termination of impulse transmission at cholinergic synapses2. Although the second enzyme, BChE, is widely distributed, its biological role is unknown3. BChE derives its name from the fact that it hydrolyses butyrylcholine (BCh) at rates similar to or faster than ACh, whereas AChE hydrolyses BCh much more slowly4. AChE and BChE are further distinguished by their differential susceptibility to various inhibitors5. For example, some bisquaternary compounds, which are more potent inhibitors of AChE than their monoquaternary counterparts, bind poorly to BChE6. Human BChE (H-BChE) is of great interest to anaesthesiologists and geneticists, because it is responsible for the breakdown of the short-term muscle relaxant, succinylcholine7, and because of the existence of numerous genetic variants in which the rate of succinylcholine hydrolysis is reduced8. Recent developments in cloning and sequencing of the cholinesterases have revealed striking sequence homology between AChE and BChE1,8–11. Residues 4–534 of T-AChE, which are the ones seen in the X-ray structure12, can be aligned with residues 2–532 of H-BChE10, with 53% identity and no deletions or additions. The residues of the catalytic triad are found in exactly the same positions (S200, E327 and H440 in T-AChE), as are the intra-chain disulfide bonds13,14 This marked structural similarity encouraged us to use the three-dimensional structure of T-AChE12 to model H-BChE. We hoped, thereby, to gain an understanding of how the structural differences between the two enzymes might account for the known differences in specificity between them.