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J. Biol. Chem., Vol. 283, Issue 7, 3827-3838, February 15, 2008

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Analysis and Molecular Modeling of the Formation, Structure, and Activity of the Phosphatidylserine-Calcium-Phosphate Complex Associated with Biomineralization^*

From the Department of Chemistry and Biochemistry, Graduate Science Research Center, University of South Carolina, Columbia, South Carolina 29208

The nucleational core of matrix vesicles contains a complex (CPLX) of phosphatidylserine (PS), Ca²⁺, and inorganic phosphate (P_i) that is important to both normal and pathological calcification. Factors required for PS-CPLX formation and nucleational activity were studied using in vitro model systems and molecular dynamic simulations. Ca²⁺ levels required for and rates of PS-CPLX formation were monitored by light scattering at 340 nm, assessing changes in amount and particle size. Fourier transform infrared spectroscopy was used to explore changes in chemical structure and composition. Washing with pH 5 buffer was used to examine the role of amorphous calcium phosphate in CPLX nucleational activity, which was assessed by incubation in synthetic cartilage lymph with varied pH values. Addition of 4 Ca²⁺/PS was minimally required to form viable complexes. During the critical first 10-min reaction period, rapid reduction in particle size signaled changes in PS-CPLX structure. Fourier transform infrared spectroscopy revealed increasing mineral phosphate that became progressively deprotonated to . This Ca²⁺-mediated effect was mimicked in part by increasing the Ca²⁺/PS reaction ratio. Molecular dynamic simulations provided key insight into initial interactions between Ca²⁺ and P_i and the carboxyl, amino, and phosphodiester groups of PS. Deduced interatomic distances agreed closely with previous radial distribution function x-ray-absorption fine structure measurements, except for an elongated Ca²⁺–N distance, suggesting additional changes in atomic structure during the critical 10-min ripening period. These findings clarify the process of PS-CPLX formation, reveal details of its structure, and provide insight into its role as a nucleator of crystalline calcium phosphate mineral formation.

Received for publication, September 12, 2007 , and in revised form, December 7, 2007.

^* This work was supported by National Institutes of Health NIAMS Grants AR42359 (to L.. N. Y. W.) and AR18983 (to R. E. W.) and Department of Defense, Office of Naval Research Grant N00014-97-1-0806 (to B. R. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St., Graduate Science Research Center, Columbia, SC 29208. Tel.: 803-777-6626; Fax: 803-777-9521; E-mail: wuthier{at}mail.chem.sc.edu.

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