<p>Cholinesterase enzymes are members of the broader alpha/beta hydrolase family and can be dividied into two distinct groups: those that catalyse the hydrolysis of acetylcholine to choline and acetate (acetylcholinesterases <db_xref db="EC" dbkey="3.1.1.7"/>)<reaction>acetylcholine + H<sub>2</sub>O -> choline + acetate</reaction>and those that catalyse the conversion of other acylcholines to a choline and a weak acid (cholinesterases <db_xref db="EC" dbkey="3.1.1.8"/>)<reaction>an acylcholine + H<sub>2</sub>O -> choline + a carboxylate</reaction></p> <p>Acetylcholinesterase also acts on a variety of acetic esters and catalyses transacetylations. It is the most intensively studied of the cholinesterase enzymes due to its key physiological role in the turnover of the neurotransmitter acylcholine [<cite idref="PUB00036069"/>]. This enzyme is found in, or attached to, cellular or basement membranes of presynaptic cholinergic neurons and postsynaptic cholinoceptive cells within the neuromuscular junction. Signal transmission at the neuromuscular junction involves the release of acylcholine, its interaction with the acycholine receptor and hydrolysis, all occuring in a period of a few milliseconds. Rapid hydrolysis of the newly released aceytlcholine is vital in order to prevent continuous firing of the nerve impulses [<cite idref="PUB00036070"/>]. Consistent with its role in this process, acetylcholinesterase has an unusually high turnover number, ensuring that acetylcholine is broken down quickly. There is evidence to suggest that acetylcholinesterase has additional important roles including involvement in neuronal adhesion, the formation of Alzheimer fibrils, and neurite growth [<cite idref="PUB00036071"/>, <cite idref="PUB00036072"/>, <cite idref="PUB00036073"/>]. </p> <p>The 3D structure of acetylcholinesterase and a cholinesterase have been determined [<cite idref="PUB00010129"/>, <cite idref="PUB00029676"/>]. These proteins share the 3-layer alpha-beta-alpha sandwich fold common to members of the alpha/beta hydrolase family. Surprisingly, given the high turnover number of acetylcholinesterase, the active site of these enzymes is located at the bottom of a deep and narrow cleft, named the active-site gorge.</p><p>Some of the proteins in this group are responsible for the molecular basis of the blood group antigens, surface markers on the outside of the red blood cell membrane. Most of these markers are proteins, but some are carbohydrates attached to lipids or proteins [Reid M.E., Lomas-Francis C. The Blood Group Antigen FactsBook Academic Press, London / San Diego, (1997)]. Acetylcholinesterase (<db_xref db="EC" dbkey="3.1.1.7"/>) belongs to the Yt blood group system and is associated with Yt(a/b) antigen. </p>