DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

240 In the 1950s, a series of aromatic carbamates was synthesized
and found to have substantial selective toxicity against insects and to
be potent anti- ChE agents (Ecobichon, 2000).
SECTION II
NEUROPHARMACOLOGY
Structure of Acetylcholinesterase. AChE exists in two general
classes of molecular forms: simple homomeric oligomers of catalytic
subunits (monomers, dimers, and tetramers) and heteromeric associations
of catalytic subunits with structural subunits (Massoulié,
2000; Taylor et al., 2000). The homomeric forms are found as soluble
species in the cell, presumably destined for export or for association
with the outer membrane of the cell, typically through an
attached glycophospholipid. One heteromeric form, largely found in
neuronal synapses, is a tetramer of catalytic subunits disulfide- linked
to a 20,000-Da lipid- linked subunit and localized to the outer surface
of the cell membrane. The other heteromeric form consists of
tetramers of catalytic subunits, disulfide linked to each of three
strands of a collagen- like structural subunit. This molecular species,
whose molecular mass approaches 10 6 Da, is associated with the
basal lamina of junctional areas of skeletal muscle.
Molecular cloning revealed that a single gene encodes vertebrate
AChEs (Schumacher et al., 1986; Taylor et al., 2000). However,
multiple gene products arise from alternative processing of the
mRNA that differ only in their carboxyl- termini; the portion of the
gene encoding the catalytic core of the enzyme is invariant. Hence,
the individual AChE species can be expected to show identical substrate
and inhibitor specificities.
A separate, structurally related gene encodes butyrylcholinesterase,
which is synthesized in the liver and is primarily
found in plasma (Lockridge et al., 1987). The cholinesterases define
a superfamily of proteins whose structural motif is the α, β
hydrolase- fold (Cygler et al., 1993). The family includes several
esterases, other hydrolases not found in the nervous system, and surprisingly,
proteins without hydrolase activity such as thyroglobulin
and members of the tactin and neuroligin families of proteins (Taylor
et al., 2000).
The three- dimensional structures of AChEs show the active
center to be nearly centrosymmetric to each subunit, residing at the
base of a narrow gorge ~20 Å in depth (Bourne et al., 1995; Sussman
et al., 1991). At the base of the gorge lie the residues of the catalytic
triad: Ser203, His447, and Glu334 in mammals (Figure 10–1). The
catalytic mechanism resembles that of other hydrolases; the serine
hydroxyl group is rendered highly nucleophilic through a chargerelay
system involving the carboxylate anion from glutamate, the
imidazole of histidine, and the hydroxyl of serine (Figure 10–2A).
During enzymatic attack of ACh, an ester with trigonal geometry,
a tetrahedral intermediate between enzyme and substrate is
formed (Figure 10–2A) that collapses to an acetyl enzyme conjugate
with the concomitant release of choline. The acetyl enzyme is
124
72
286
74
86
203
297
202
447
337 449
295
Figure 10–1. The active center gorge of mammalian acetylcholinesterase. Bound acetylcholine is shown by the dotted structure depicting
its van der Waals radii. The crystal structure of mouse cholinesterase active center, which is virtually identical to human AChE,
is shown (Bourne et al., 1995). Included are the side chains of (a) the catalytic triad, Glu334, His447, Ser203 (hydrogen bonds are
denoted by the dotted lines); (b) acyl pocket, Phe295 and Phe297; (c) choline subsite, Trp86, Glu202, and Tyr337; and (d) the peripheral
site: Trp286, Tyr72, Tyr124, and Asp74. Tyrosines 337 and 449 are further removed from the active center but likely contribute
to stabilization of certain ligands. The catalytic triad, choline subsite, and acyl pocket are located at the base of the gorge, while the
peripheral site is at the lip of the gorge. The gorge is 18-20 Å deep, with its base centrosymmetric to the subunit.
334