324 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS: GROUP II
Conclusions to be drawn from the discussions of various calmodulin/target
enzyme binding peptide complexes include the following: (1) Calmodulin
releases the autoinhibition or enhances the catalytic activity of hundreds of
enzymes; (2) calmodulin contains four EF hand helix – loop – helix motifs that
bind calcium, with the four EF hands being distributed in essentially mirror
image N - and C - terminal domains connected by a central linker region;
(3) the calcium - ion binding loops remain virtually unchanged and exposed
to solvent in all complexes allowing easy ingress and egress for Ca 2+ ; (4)
calmodulin ’ s hydrophobic pockets, exposed upon calcium ion binding, are
deep enough to accommodate large aromatic, hydrophobic residues such
as tryptophan and phenylalanine from the target peptides, or long aliphatic
side chains from leucine and isoleucine residues; (5) large areas (2500 to
3000 Å^2 ) of the target enzyme molecule are shielded from solvent through
binding to calmodulin; (6) calmodulin methionine residues are plentiful and
fl exible and contain the polarizable sulfur atom, and met residues are usually
involved in hydrophobic binding to target peptides; (7) additional hydrogen
bonding and electrostatic interactions connect calmodulin and the target
enzyme, and many times these interactions determine the orientation of the
target peptide with respect to calmodulin ’ s N - and C - terminal domains; (8)
the fl exible linker region connecting calmodulin ’ s N - and C - terminal domains
adjusts to different target enzyme sequences throughα - helix unwinding and
extension of the central linker; and (9) in many cases calmodulin ’ s N - terminal
domain, especially helix A, moves more than other parts of the molecule in
accommodating differences in target enzyme binding sequence and secondary
structure.
Vetter and Leclerc also discuss the X - ray crystallographic structure of
calmodulin complexed with the gating domain of a Ca 2+ - activated K + channel
(SK channel). This CaM/SK channel complex, studied by Schumacher et al.^94
(PDB: 1G4Y), exhibits a structure that is very different from those seen previ-
ously. Calmodulin binds to the SK channel through the channel ’ s CaM - binding
domain (CaMBD), located in an intracellular region of the channel. The SK
channel calmodulin - binding domain consists of two long α - helices ( α 1 and α 2)
connected by a loop. The α 1 - helix comprises CaMBD residues 413 – 439, the
loop residues 440 – 445, and the α 2 - helix residues 446 – 488. Two CaMBD units
form a dimer ( α 1, α 2 and α 1 ′ , α 2 ′ ) that interacts with two separate calmodulin
molecules (CaM1 and CaM2) in an extended conformation. Each calmodulin
subunit carries two calcium ions in its N - terminal domain and no calcium ions
in its C - terminal domain. Each CaMBD α 1 - helix interacts with the C - terminal
domain of one of the calmodulin subunits (CaM1, for instance), whereas the
α 2 - helix interacts with the N - terminal domain of the second calmodulin
(CaM2) subunit and the central linker region of the fi rst calmodulin (CaM1)
subunit. The effect is that each calmodulin subunit wraps around three CaMBD
α - helices. Figure 1 of reference 94 shows the structure clearly. The CaMBD
units have an overall positive character (pI 10.5, charge +14.7) and are drawn
to the acidic CaM (pI 3.9) units. However, as with other CaM – target enzyme