322 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS: GROUP II
basic residues are near the N - terminal domain, they form salt bridges with
glutamate and aspartate residues in calmodulin ’ s C - terminal domain and when
the basic residues (as in rCaMKK) are near its C - terminal end, the salt bridges
form with the same glu and asp residues in calmodulin ’ s C - terminal domain.
The second obvious difference is that the rCaMKK peptide is only partially
α - helical (11 residues from rat CaMKK residues 444 – 454) and contains an
additional well - defi ned β - hairpin - like loop at its C - terminal end (rat CaMKK
residues 455 – 459). Many of calmodulin ’ s methionine residues are involved in
binding the rCaMKK α peptide, as was the case for systems discussed previ-
ously. Electrostatic interactions responsible for forming salt bridges and ori-
enting the rCaMKK α peptide ’ s C - terminal end with calmodulin ’ s C - terminal
end are the peptide basic arg455, lys456, and arg457 (Table 6.9 positions 12,
13, 14) residues and calmodulin ’ s glu14, glu120, glu123, and glu127 residues. In
summary, while the hydrophobic and electrostatic attractions between
rCaMKKα peptide and calmodulin appear similar to the binding modes dis-
cussed previously, the secondary structure of the rCaMKK α peptide deviates
substantially from those of smMLCK and CaMKII α. Signifi cantly and surpris-
ingly, the rCaMKK α peptide ’ s orientation in calmodulin ’ s binding tunnel is
reversed from those of smMLCK and CaMKII α.
An X - ray crystallographic structure of a Ca 2+ - CaM/CaMKK complex pub-
lished by Ikura and co - workers in 2001 confi rms the binding and peptide ori-
entation characteristics for this system.^92 In this case, the calmodulin - binding
cCaMKK peptide is derived from a nematode Ca(2+)/calmodulin - dependent
kinase peptideCaenorhabditis elegans ( C. elegans , PDB: 1IQ5). Although the
anchor residues differ from those described for rCaMKK, the structures are
quite similar. The peptide residues 337 – 349 form an α - helix, while a β - hairpin
loop is formed by residues 350 – 357. Leu337 (Table 6.9 , L1) and ile341 (Table
6.9 , I5) have hydrophobic interactions with calmodulin ’ s N - terminal domain,
while phe352 (Table 6.9 , F16), 14 residues away from the primary leu337 N -
domain anchor) binds hydrophobically to calmodulin ’ s C - terminal domain.
The phe352 residue resides in the β - hairpin loop and probably would not be
in a C - domain anchor position if it were part of a continuing α - helical struc-
ture. The loop structure allows the cCaMKK peptide to fold back and places
the phe352 residue in position to form the hydrophobic bonds necessary to
form the CaM collapsed structure in the same manner discussed above for
rCaMKKα. Hydrogen bonding interactions were found between calmodulin
and cCaMKK residues for glu87 (CaM) – arg336 (cCaMKK), glu87 – thr339, and
lys75 – thr339 (via a water molecule). Salt bridges form to orient cCaMKK ’ s
C - terminal end to CaM ’ s C - terminal domain: glu11 – arg349, for instance.
These authors compare the CaM/cCaMKK (PDB: 1IQ5) structure to the
CaM/smMLCK (PDB: 1CDL) and CaM/CaMKII α (PDB: 1CDM) structures
reported previously. They fi nd that differences can be found in comparisons
of calmodulin N - terminal (residues 5 – 73) and C - terminal (residues 84 – 146)
domain orientation when CaM is bound to the different target peptides. When
the N - terminal and C - terminal calmodulin domains are superimposed, essen-