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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. February 2002, Volume 1, Number 2, 59-63

Calculation of Ionic Bridge Contributions to Homospecific Interactions Mediated by Proteoglycans
Traian Sulea and Zeno Simon
Internet Electron. J. Mol. Des. 2002, 1, 59-63

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Abstract:
Adhesion proteoglycans containing the g200 acidic glycan are responsible for speciesspecific cell aggregation in some sponges. Their homospecific cohesion requires a physiological concentration of calcium ions, but does not take place in the presence of magnesium. This suggests that Ca2+-mediated ionic bridges between the carboxylate groups of the glycan domains (g200-arms) are responsible for the homophilic interactions of adhesion proteoglycans. Here we apply computational methods to predict the strength of such ionic interactions and to explain the metal ion specificity in the aggregation of adhesion proteoglycans. Ionic bridge stabilities in water are calculated for model systems using the solvated interaction energy approach, which combines a molecular mechanics force field (AMBER) with a continuum model of solvation (BEM). Solvated interaction energy calculations show a preference for the formation of the Ca2+-mediated ionic bridge between two acetate ions in water in comparison to the Mg2+-mediated interaction, with a difference in binding free energy of 11.7 kcal/mol. Addition of the estimated translational entropy of the metal ion to the calculated solvated interaction energy results in -5.5 kcal/mol per Ca2+-mediated ionic bridge formation between two carboxylate groups fixed on interacting g200-arms. The energetic cost due to the loss of conformational entropy during g200 homodimerization could reach 1200 kcal/mol at room temperature, while the carboxylate content is about 250 groups per g200-arm. Binding free energy calculations applied to model systems reproduced the observed metal ion specificity in the aggregation of adhesion proteoglycans. The strength and the number of Ca2+-mediated ionic interactions between glycan domains are sufficient to overcome the high conformational entropy costs incurred during homophilic cohesion in order to produce proteoglycan aggregation.

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