Calcium Biochemistry
149
Structural Organization
The holoenzyme
is an oligomeric complex of 6 to 12 sub-
units with a total molecular mass ranging
from 300 to 700 kDa, which can be either
a homomultimer or a heteromultimer.
According to the amino acid sequences,
the different isoforms are closely related
to each other and reveal the segmental
organization typical for CaM-dependent
protein kinases, consisting of catalytic, reg-
ulatory, variable, and association domains
on the same polypeptide chain.
The overall structure of the catalytic
domain of CaMKII is likely to be bilobal,
as expected for all CaM-dependent protein
kinases, on the basis of the crystal structure
of CaMKI. Catalysis takes place within a
cleft between the two lobes orienting the
consensus sequence of the substrate in
such a way that the basic residue(s) in
proximity to the serine/threonine residue
that
should
be
phosphorylated
could
interact with the conserved polar residues
located on the surface of the large lobe.
The small lobe contains a lysine residue
that is essential for ATP binding, that is,
if mutated to a methionine renders the
kinase inactive.
Regulation of the Catalytic Activity
Since
CaMKII has a broad range of substrates,
the consensus sequence for substrate
determination is less strict than that de-
scribed for MLCK before. Basically, an
arginine at the P
3 position, the phos-
phorylated serine/threonine being P(0), is
the only essential requirement for a mul-
tifunctional kinase such as CaMKII. In
addition, hydrophobic residues are pre-
ferred at P
5andP
+
1. Downstream of
the catalytic domain, the autoinhibitory,
the CaM binding, and the association
domains are located, interspersed with
variable domains.
As discussed in detail before, CaM ac-
tivates its targets by wrapping around the
binding domain thereby releasing the en-
zyme from its autoinhibitory state. In the
case of CaMKII, the three-dimensional
structure of the complex between CaM and
a peptide representing the CaM binding
domain of CaMKII has been determined
by X-ray crystallography. This complex
also shows the general feature observed
for similar CaM–peptide complexes, that
is, CaM is collapsed to a globular struc-
ture engulFng the helical peptide that
is Fxed by numerous hydrophobic and
electrostatic interactions. An important
difference between the interactions of CaM
with CaMKII and MLCK, respectively, is
the different afFnity of CaM for the two
enzymes, that is, the afFnity for CaMKII is
much lower (
K
act
20–100 nM) than that for
MLCK (
K
act
1 nM). However, the afFnity
for CaMKII can be substantially enhanced
(
>
1000-fold) by autophosphorylation of
Thr286 resulting in a signiFcant lower-
ing of the CaM off-rate from the complex,
a phenomenon termed
calmodulin trapping
that might have some important implica-
tions for the function of the enzyme as
discussed below.
CaMKII and Neuronal Plasticity
Cyclic cel-
lular depolarization or hormonal stimuli
can trigger a series of calcium pulses inside
ce
l
ls
.Inneurons
,theseCa
2
+
oscillations
can be used to modulate synaptic activity
and strength with subsequently increased
release or resynthesis of neurotransmitters
or other cellular activities. The coordinator
of at least some of these activities appears
to be CaMKII; however, the question is, ‘‘Is
CaMKII able to decode Ca
2
+
oscillations,
and if yes, how?’’
The hippocampus has been used to
study processes in which high-frequency
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