Chapter 15: Structure and functions of acetylcholinesterase and butyrylcholinesterase

Vertebrates possess two cholinesterases, acetylcholinesterase and butyrylcholinesterase. Cholinesterases catalyze a very simple reaction, hydrolysis of the ester bond of acetylcholine. The role of AChE in cholinergic transmission, although admittedly secondary to that of the pre-synaptic machinery responsible for the synthesis and release of acetylcholine, and of the postsynaptic receptors, is crucial for synaptic function. The role of ChE is not clear; this enzyme is, in fact, dispensable, since its absence in humans does not correlate with any physiological abnormality. Despite the fact that they do not, at first sight, appear as glamorous as receptors, cholinesterases display a number of fascinating features, and pose important questions concerning their structure and functions. AChE is one of the fastest enzymes known and possesses an unusual molecular structure. Both AChE and BChE display a repertoire of molecular forms, which differ in their quaternary structure and may be anchored in different ways to synaptic structures. Cholinesterases are thought to exert non-cholinergic functions, e.g. in morphogenesis, during early embryonic development, in the modulation of neuronal activity and in the elimination of various toxic compounds, which may explain their presence outside the context of cholinergic transmission. They are also expressed abnormally in some tumors and in other pathological states. This chapter discusses the following aspects: the atomic structure of cholinesterases and their catalytic mechanism; the structure and biosynthesis of their molecular forms; the possibility that these enzymes participate in cellular interactions in addition to their catalytic activity.