Models for protein binding to calcium oxalate surfaces

It is widely believed that proteins rich in Asp, Glu or Gla (γ carboxyglutamic acid) interact strongly with calcium oxalate surfaces and inhibit calcium oxalate crystal growth. An alternative hypothesis would be that the interaction of Asp, Glu and Gla residues with surfaces could facilitate nucleation and crystal aggregation. Prothrombin fragment 1 and bikunin have been studied extensively as inhibitors, β-microglobulin, transferrin and antitrypsin have been found in stone matrix and tubulin has been observed in the attachment of crystals to cell surfaces. The aim of this study is to examine how well carboxylate groups in proteins found either in stone matrix, or proposed as inhibitors, could fit with the calcium ion sub-lattice of both calcium oxalate monohydrate and dihydrate surfaces. The carboxylate groups in the acidic Asp, Glu and Gla residues were marked in the Protein Data Bank structures and matched to calcium oxalate surfaces using the Cerius 3D molecular modeling program. A contact was defined if a carboxylate oxygen atom approached a surface calcium atom in such a way that the separation was less than 6 Å but greater than 2.4 Å, the sum of the ionic radii. If the proteins maintain their 3D structure, the number of contacts was no more than 3 or 4 for all the proteins studied, irrespective of the calcium oxalate surface.