308 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS: GROUP II
four N - terminal α - helices and exposed the hydrophobic surface as observed
for vertebrate calmodulin. While the observed overall conformation of yeast
and vertebrate peptide backbone are similar, several signifi cant conforma-
tional differences were observed between the two proteins. The reference 79a
authors believe the differences originate from the 38% disagreement in amino
acid sequences between yeast and vertebrate CaM. The β - sheet in apo - CaM,
PDB: 1F54 is strongly twisted compared with that in the N - domain of verte-
brate CaM, but turns to the normal more stable conformation upon calcium -
ion binding. The change in β - sheet conformation may also explain the more
cooperative nature of Ca 2+ binding for yeast CaM. There are changes in the
amino acid residues found on the hydrophobic surface of yeast calmodulin
(leu51 substitutes for the normal vertebrate met51, leu71 for met71, ile55 for
val55). These changes are thought to be one of the reasons for the poor activa-
tion of target enzymes by yeast CaM. A Swedish group (reference 79b )
reported on the Ca 2+ - free and Ca 2+ - saturated C - terminal calmodulin (PDB:
1CMF, apo, and PDB: 1CMG, Ca 2+ - saturated). Structural comparison of apo -
CaM with Ca 2+ - saturated CaM showed that Ca 2+ binding caused major rear-
rangements of the secondary structure elements with changes in inter - residue
distances of up to 15 Å and exposure of the hydrophobic interior of the four -
helix bundle in the Ca 2+ - saturated form (PDB: 1CMG).
The data in Table 6.7 , as found in reference 68 , appears to disagree with
other interhelical angle data in the literature. For instance, some of the angle
data for the calcium - ion free calmodulin disagree about 180 ° from that in Table
6.7 , probably indicating different methods used by researchers to calculate
the angles. Table 6.7 is included here to indicate how different methods —
NMR and X - ray — plus variable calmodulin sources and constructs, affect
the interhelical bond angles. See also Figures 6.22 and 6.23 for structural
comparisons.
6.3.2.3 Calmodulin Interactions with Drug Molecules. Calmodulin inter-
acts with drug molecules containing aromatic rings. These molecules bind in
calmodulin ’ s hydrophobic pockets that are exposed upon calcium binding.
These drugs must have at least two essential features: (1) a large hydrophobic
region consisting of two or three aromatic rings and (2) a positively charged
amino group that is at least three atoms away from the ring structure. The
aromatic ring region will be held in calmodulin ’ s hydrophobic peptide - binding
pocket, and the amino group will be attracted to negatively charged glutamic
acid residues present in calmodulin. The glu residues involved in drug binding
are usually found in helices A and G. Trifl uoperazine (TFP; see Figure 6.24 ) is
an anti - psychotic drug and calmodulin antagonist — that is, TFP binds and
inhibits attachment of target enzymes to calmodulin.
TFP has been widely studied in this regard, and a number of structural
studies have indicated the binding of one, two, three, or four TFP molecules
to calmodulin in the hydrophobic pockets where target enzymes would also
bind. Most studies in solution have indicated that CaM has two high - affi nity