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The structure of bovine intestinal calcium-binding protein (ICaBP) has been
determined crystallographically at a resolution of 2.3 A and refined by a least
squares technique to an R factor of 17.8%. The refined structure includes all
600 non-hydrogen protein atoms, two bound calcium ions, and solvent consisting
of one sulfate ion and 36 water molecules. The molecule consists of two
helix-loop-helix calcium-binding domains known as EF hands, connected by a
linker containing a single turn of helix. Helix-helix interactions are primarily
hydrophobic, but also include a few strategic hydrogen bonds. Most of the
hydrogen bonds, however, are found in the calcium-binding loops, where they
occur both within a single loop and between the two. Examination of the hydrogen
bonding patterns in the calcium-binding loops of ICaBP and the related protein,
parvalbumin, reveals several conserved hydrogen bonds which are evidently
important for loop stabilization. The primary and tertiary structural features
which promote the formation of an EF hand were originally identified from the
structure of parvalbumin. They are modified in light of the ICaBP structure and
considered as they apply to other calcium-binding proteins. The C-terminal
domain of ICaBP is a normal EF hand, with ion binding properties similar to
those of the calmodulin hands, but the N-terminal domain is a variant hand whose
calcium ligands are mostly peptide carbonyls. Relative to a normal EF hand, this
domain exhibits a similar KD for calcium binding but a greatly reduced affinity
for calcium analogs such as cadmium and the lanthanide series. Lanthanides in
particular may be inappropriate models for calcium in this system.
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