Biology Reference
In-Depth Information
by a balanced chromosomal translocation is associated in a large Scottish family
with major psychiatric illness including schizophrenia, bipolar disorder, and severe
recurrent depression (Blackwood and Muir
2004
). DISC1 mutant mice display
altered behavioral phenotypes suggestive of PDE4 dysregulation (Clapcote et al.
2007
). DISC1 has been shown to interact with residues on the catalytic domain of
PDE4D, and with regions of UCR2 upstream of the gating helix (Murdoch et al.
2007
). This has led us to the hypothesis that DISC1 binding stabilizes the closed
conformation, by simultaneously binding to both UCR2 and the catalytic domain.
Consistent with this proposal, Millar et al. have shown that PKA phosphorylation
disrupts DISC1 binding to PDE4B, which results in an activation of enzyme activity
(Millar et al.
2005
). This activation may occur as a response to increased cAMP
concentration, providing an additional level of local control of PDE4 activity, in
this example, perhaps at individual neuronal synapses.
Finally, the signaling scaffold protein RACK1 (receptors for activated C-kinase)
has been shown to bind the N-terminal region of PDE4D5 as well as the C-terminus
(Yarwood et al.
1999
). The residues important for the binding of RACK1 to the
C-terminus of PDE4D5 were mapped by peptide array scanning to the C-terminal
regulatory helix and the face of the PDE4D catalytic domain (Bolger et al.
2006
).
PDE4D5 complexes with RACK1 showed no change in
K
M
or
V
max
toward cAMP
(Yarwood et al.
1999
) suggesting that RACK1 may hold the C-terminal regulatory
helix in an open conformation. RACK1 may also hinder UCR2 closure, as binding
of RACK1 slightly attenuates the sensitivity of PDE4D5 to inhibition by rolipram
(Yarwood et al.
1999
).
4 Active Site-Directed Competitive Inhibitors Bind
an Open PDE4 Conformer
We suggest that the two active sites in the PDE4 dimer act independently when
UCR2 and the C-terminal regulatory helix are in the open conformation (Fig.
3
). As
a consequence, active site-directed PDE4 inhibitors that purely compete with
cAMP at the active site display simple Michaelis-Menten kinetics of enzyme
inhibition. The design of competitive PDE4 inhibitors has converged on com-
pounds that exploit a binding pose similar to cAMP (Fig.
4
). Competitive PDE4
inhibitors generally contain three binding elements, which mimic those found in the
cyclic nucleotide substrate. These are a hydrogen bond acceptor, a planar aromatic
ring system, and functional groups that coordinate with catalytic site metals (mag-
nesium or zinc). All PDE inhibitors have a central planar ring that stacks in a clamp
(P clamp) formed by highly conserved hydrophobic residues that sandwich the
inhibitor in the active site (Card et al.
2004
). All PDE inhibitors also hydrogen bond
to an invariant glutamine (Q switch) at the bottom of the hydrophobic clamp. In
PDE4D, Phe538 and Ile502 form the walls of the hydrophobic clamp with Gln535
corresponding to the invariant glutamine. These features of the active site are
