Protein electrostatic surface distribution can determine whether calcium oxalate crystal growth is promoted or inhibited

Acidic proteins found in mineralized tissues act as nature's crystal engine ers, where they play a key role in promoting or inhibiting the growth of mi nerals such as hydroxyapatite and calcium oxalate. Despite their importance in such fundamental physiological processes as bone and tooth formation, h owever, there is remarkably little known of the protein structure-function relationships that govern crystal recognition. We have taken a model system approach to elucidate some of the relationships between protein surface ch emistry and secondary crystal growth of biological minerals. We show here t hat the distribution of electrostatic surface charge on our model protein, Protein G, determined whether the secondary growth of calcium oxalate, the principal mineral phase of kidney stones, was promoted or inhibited when th e proteins were preadsorbed at low and equivalent surface coverages of <10% . The native Protein G, which contains 10 surface carboxylates, increased t he rate of calcium oxalate growth from aqueous solution under constant comp osition conditions up to 97%, whereas a site-directed mutant with six of th e surface charges removed inhibited the growth rate by 60%. The adsorption isotherms of both proteins were determined and suggested that the differenc es in electrostatic surface properties also lead to differences in protein orientation on the crystal surface. These results demonstrate that differen ces in electrostatic surface potential of proteins can directly determine w hether secondary calcium oxalate growth is promoted or inhibited, and a mod el is proposed that suggests the distribution of carboxylate residues deter mines the interrelated binding orientation and exposed surface chemistry of the adsorbed Protein G.