CALCIUM-DEPENDENT MOLECULES 317
tides in binding to calmodulin. It should be noted that calmodulin contains an
unusually high number of methionine residues and that all of these are involved
in van der Waals interactions with incoming target peptides. It is proposed that
one reason calmodulin is able to recognize and bind many different target
peptides (of variable length and primary sequence) lies in the fl exibility of
the unbranched methionine side chain that, through minor conformational
changes, adjusts to capture the target.
Later in 1992, Quiocho and co - workers published the 2.4 Å X - ray crystal-
lographic structure of calmodulin complexed with the CaM - binding region of
chicken smooth muscle light chain kinase (smMLCK, PDB: 1CDL). 86a The
peptide amino acid sequence is found in Table 6.9. The peptide, known as
smMLCK, spans residues 796 – 815 of smooth muscle light chain kinase. The
peptide also serves as an autoinhibitor of smMLCK. The fi rst 12 amino acids
appear to fulfi ll this function, while residues trp4 (1) and 8 – 17 (5 – 14) are pri-
marily involved in CaM binding. The CaM – peptide complex exhibits a compact
ellipsoidal structure with overall dimensions of approximately 50 × 30 × 25 Å.
The NMR structure described in reference 85 (PDB: 2BBM) and the X - ray
structure described in reference 86a (PDB: 1CDL) are quite similar, with
calmodulin surrounding the peptide helical coil. The extra length of the 2BBM
peptide — 26 residues versus 18 in the 1CDL structure — is evident in Table 6.9
and Figure 6.27. The fl exible central linker loop portion in the Ca 2+ - CaM:
smMLCK structure (PDB: 1CDL) spans residues 73 – 77, in contrast to the
longer fl exible loop in the NMR Ca 2+ - CaM: skMLCK structure (PDB: 2BBM)
that spans residues 74 – 82. In the X - ray structure (PDB: 1CDL), the calmodulin
N - and C - terminal domains are related by a pseudo - twofold axis that runs
through the center of the structure relating helices A and E, B and F, C and
G, and D and H. The bound peptide, α - helical for almost its entire length, lies
in the tunnel running diagonally along the long axis of the calmodulin ellip-
soid. N - and C - terminal calmodulin hydrophobic pockets interface with the
hydrophobic side of the helical peptide. See Figure 6.27.
In agreement with the NMR structure PDB: 2BBM discussed above, the
calmodulin – peptide interactions occur in an antiparallel conformation — that
is, N - terminal peptide residues trp4 (1) and thr7 (4) interact with the C -
terminal calmodulin pocket while C - terminal peptide residues ala13 (10), ile14
(11), and leu17 (14) reside in the N - domain pocket. The residue numbers in
parentheses correspond to the reference 87 numbers in parentheses in
Table 6.9 (In reference 86a , there is a third peptide numbering system which
this author has ignored.) Of the approximate 185 contacts of < 4 Å between
calmodulin and peptide, about 80% are van der Waals contacts (hydrophobic)
and the remainder electrostatic — salt bridges between peptide basic and
calmodulin acidic side chains and hydrogen bonds. As with the NMR structure
of reference 85 , PDB: 2BBM, all methionine residues of calmodulin are
involved in hydrophobic bonding with the peptide. See Figure 6.27.
Arg16 (13) of the peptide chain forms electrostatic (hydrogen bonding and
charge - coupling) interactions with the central loop bend and residues in