Journal of Lipid Research, Vol 33, 985-994, Copyright © 1992 by Lipid Research, Inc.
Calcium binding to phospholipid: structural study of calcium glycerophosphate
M Inoue, Y In and T Ishida
Osaka University of Pharmaceutical Sciences, Japan.
To consider possible interaction of the phospholipid membrane with calcium
ions, crystal structures of calcium dl-alpha- and beta- glycerophosphates
(alpha- and beta-CaGs, respectively) were investigated by X-ray diffraction
methods. After many attempts, relatively large single crystals of beta-CaG
were prepared from the aqueous solution containing HCl, while crystals of
CaHPO4.2H2O were obtained from alpha-CaG solution under the same
crystallization conditions. The crystal structure of beta-CaG is
orthorhombic with space group Pna2(1) and cell dimensions of a = 8.251(1),
b = 13.038(3), c = 25.483 (10) A, V = 2741.5 (13) A3 and Z = 16 [four
molecules (A to D) in an asymmetric unit]. Molecules of A to D took, as a
whole, similar extended conformations, although A and B were different from
C and D in the orientation about a glycerol C-C bond. Four independent
beta-glycerophosphates commonly act as two types of bidentate ligands,
where one is the coordination to the calcium ion by the glycerol O(1) and
phosphate O(22) atoms, and the other by the phosphate O(22) and O(23)
atoms, thus forming the calcium coordination of a distorted square plane,
respectively. Each of four independent calcium ions forms the same
coordination geometry of a distorted pentagonal bipyramid. Infinite double
layers consisting of alternate A/B molecules and of alternative C/D ones
and sandwiching calcium ions were arranged face-to- face along the
b-direction and were piled up in the a-direction, thus forming the stacked
bilayer unit with the thickness of d002 = 12.75 A. The elaborate networks
of calcium coordinations and hydrogen bondings were formed among the layers
and stabilized the crystal structure. Based on the structural parameters of
the present beta-CaG crystal, a possible interaction model of phospholipid
with calcium ions was proposed.
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