Chemistry Reference
In-Depth Information
years of treatment [72-74]. GSK-3β was first linked to bipolar disorder in 1996 by the discovery that lithium is a
direct inhibitor of GSK-3β [75]. They found that lithium inhibited GSK-3β with an IC50 of approximately 2 mM,
slightly larger than the therapeutic concentration range of lithium in serum, which is approximately 0.5-1.5 mM
[75]. Subsequently, GSK-3β was shown to be inhibited by lithium both in intact cells [76] and in mammalian brain
in vivo
[44].
Among the multiple effects in the brain due to lithium, there is the inhibition of GSK-3β induced tau
phosphorylation. Lithium reversibly reduced tau phosphorylation at therapeutic concentrations, and even at high
concentrations did not alter the neuronal morphology [77]. In addition, lithium prevents the neuronal death induced
by the fibrillary -amyloid. Neuroprotection appears to be mainly due to GSK-3β inhibition [78]. Another ionic
compound, zinc, has been shown to non-competitively inhibit GSK-3 and induce an increase in glucose transport
activity in mouse adipocytes [79]. However, because of the non-selective nature of lithium and zinc, the metabolic
actions of these ions cannot be ascribed solely to inhibition of GSK-3β. For this reason, several classes of highly
selective GSK-3β inhibitors have been developed.
At present, several GSK-3 inhibitors have been described, with IC50 values in the nanomolar range (Table
1
and
Fig.
2
) [48] whereas most of the observed effects are
in vitro
and cellular studies.
Pyrazines, pyrimidines, bisindolemaleimides, hymenialdisine, paullones, indirubins, and anilinomaleimides have
been developed as ATP competitive GSK-3β inhibitors, while thiadiazolidinones were reported to be the first ATP
noncompetitive GSK-3β inhibitors. Soon after crystallization of GSK-3β [59,60,82], the enzyme was cocrystallized
with some inhibitors, which provided an understanding of their mechanism of interaction within the ATP-binding
pocket.
The 2.4 Å crystal structure of GSK-3β with the non-hydrolyzable ATP analog adenyl imidodiphosphate (AMP-
PNP), pdb code: 1PYX, provides a logical starting point for understanding how ATP-mimetic inhibitors interact
with GSK-3β [82] (Fig.
3
). AMP-PNP binds in the cleft formed between the N- and C-terminal lobes of GSK-3β,
with the adenine group making hydrogen bonds with the hinge residues Asp133 and Val135. In addition to the polar
interactions, the adenine group also makes hydrophobic interactions with GSK-3β residues Ile62, Val70, Ala83,
Val110, Leu132, Tyr134 and Leu188 (Fig.
2
).
The ribose group of AMP-PNP interacts with GSK-3β through a single hydrogen bond between O3' and the
carbonyl oxygen of Glu185. No direct hydrogen bonds are observed between O2' and GSK-3 β residues.
This is in contrast to what was observed for several other protein kinase complexes with nucleotides, in which a
hydrogen bond is formed with a conserved residue near the hinge.
In GSK-3β, this residue is Thr138 and although it seems possible for a threonine to make this type of hydrogen
bond, it is not the case in this structure.
Instead, the O of Thr138 is directed away from the ribose group, making a hydrogen bond with the backbone
nitrogen of Arg141.
Lys85 hydrogen bonds to the α- and β-phosphates of AMP-PNP and also forms a salt-bridge with a kinase-
conserved glutamate (Glu97) from the C-helix.
“Mg1” coordinates four oxygen atoms; one from each of the α- and -phosphates and one from each of the kinase
conserved residues Asn186 and Asp200. “Mg2” shows octahedral coordination of six oxygen atoms; one from each
of the β- and -phosphates, both carboxylate oxygen atoms from Asp200 and two water molecules.
In addition, the kinase conserved residue Lys183 (catalytic loop) extends up towards the -phosphate making
hydrogen bonds with a -phosphate oxygen and a carboxylate oxygen of another kinase conserved residue Asp181
(catalytic loop).
