CALCIUM-DEPENDENT MOLECULES 307
that the dumbbell - shaped solution structure of all apo - calmodulins discussed
here are quite different from the globular appearance of the X - ray structure
apo - calmodulin discussed above (PDB: 1QX5, Figure 6.22B ), although the
central linker region remains similar as far as its unwound state is concerned.
In all cases, removal of Ca 2+ ions from the structure changes the conformation
of calmodulin to close the hydrophobic pockets that would bind to target
enzymes. Figure 6.23 also shows the secondary structure of a Ca 2+ - saturated
calmodulin (PDB: 1X02). This structure, determined in 2006, made use of a
new NMR technique called stereo - array isotope labeling (SAIL).^78 The tech-
nique uses a stereospecifi c and regiospecifi c pattern of stable isotopes that is
optimized for the NMR spectra being collected. The SAIL technique offers
sharpened lines, spectral simplifi cation without loss of information, and the
ability to rapidly collect the structural restraints required to solve a high -
quality solution structure for proteins twice as large as commonly solved by
NMR. The reference 78 authors state that the SAIL technique makes larger
proteins such as the 17 - kDa calmodulin molecule accessible to more detailed
solution structure determination.
Several other research groups determined solution structures of the N -
terminal or C - terminal regions of apo - and Ca 2+ - calmodulin.^79 The Yazawa
(reference 79a ) group in Japan presented solution structures for Ca 2+ - free and
Ca2+ - saturated N - terminal yeast calmodulin (PDB: 1F54, apo, and 1F55, Ca 2+ -
saturated). The binding of two calcium ions caused large rearrangement of the
Figure 6.23 Comparison of three different apo - calmodulin NMR structures (PDB:
1DMO, 1CFD, 1LKJ) and one Ca 2+ - saturated calmodulin (PDB: 1X02). Visualized
using CambridgeSoft Chem3D Ultra 10.0 with notations in ChemDraw Ultra 10.0.
(Printed with permission of CambridgeSoft Corporation.)
PDB: 1DMO
apo-CaMala1lys148lys148ala1ser1ala1lys146lys148PDB: 1CFD
apo-CaMPDB: 1LKJ
apo-CaMPDB: 1X02
Ca2+-saturated