Protein primary structure correlates with calcium oxalate stone matrix preference

Abstract

Despite the apparent importance of matrix proteins in calcium oxalate kidney stone formation, the complexity of the protein mixture continues to elude explanation. Based on a series of experiments, we have proposed a model where protein aggregates formed from a mixture containing both strongly charged polyanions and strongly charged polycations could initiate calcium oxalate crystal formation and crystal aggregation to create a stone. These protein aggregates also preferentially adsorb many weakly charged proteins from the urine to create a complex protein mixture that mimics the protein distributions observed in patient samples. To verify essential details of this model and identify an explanation for phase selectivity observed in weakly charged proteins, we have examined primary structures of major proteins preferring either the matrix phase or the urine phase for their contents of aspartate, glutamate, lysine and arginine; amino acids that would represent fixed charges at normal urine pH of 6–7. We verified enrichment in stone matrix of proteins with a large number of charged residues exhibiting extreme isoelectric points, both low (pI<5) and high (pI>9). We found that the many proteins with intermediate isoelectric points exhibiting preference for stone matrix contained a smaller number of charge residues, though still more total charges than the intermediate isoelectric point proteins preferring the urine phase. While other sources of charge have yet to be considered, protein preference for stone matrix appears to correlate with high total charge content.

Roles of heat-shock protein 90 and its four domains (N, LR, M and C) in calcium oxalate stone-forming processes

“…The study based on mass spectrometry and bioinformatics analysis has shown that most of the identified COM-binding proteins exhibit either negatively or positively charged properties [ 50 ]. These charges depend on contents of amino acids, particularly side chains that determine acidic (aspartate and glutamate) or basic (lysine and arginine) property of individual residues [ 59 ]. Interestingly, most of the proteins identified from the stone matrices exhibit isoelectric point (p I ) lower than 5 or higher than 9 [ 59 ].…”

“…These charges depend on contents of amino acids, particularly side chains that determine acidic (aspartate and glutamate) or basic (lysine and arginine) property of individual residues [ 59 ]. Interestingly, most of the proteins identified from the stone matrices exhibit isoelectric point (p I ) lower than 5 or higher than 9 [ 59 ]. Another factor determining protein–crystal interactions is the presence of binding sites for calcium (Ca 2+ ) and oxalate (C 2 O 4 2− ) ions in the protein molecules.…”

Phase Separation of Oppositely Charged Polymers Regulates Bioinspired Silicification

“…Ionic strength, in addition to other chemical and physical properties, is a key parameter in liquid–liquid phase separation of oppositely charged polymers [25, 32–34] . High ionic strength leads to charge screening and weakening of the electrostatic interactions, giving rise to more hydrated and less dense polymer condensates [35] .…”

“…Ionic strength, in addition to other chemical and physical properties, is a key parameter in liquid-liquid phase separation of oppositely charged polymers. [25,[32][33][34] High ionic strength leads to charge screening and weakening of the electrostatic interactions, giving rise to more hydrated and less dense polymer condensates. [35] Therefore, different morphologies of dried silicification products that were observed earlier [16] and in this study, reflect foremost the effect of drying a highly hydrated hybrid structure, rather than differential interactions of silica precursor with the different polymers.…”

Phase Separation of Oppositely Charged Polymers Regulates Bioinspired Silicification

“…Ionic strength, in addition to other chemical and physical properties, is a key parameter in liquid-liquid phase separation of oppositely charged polymers. [25,[32][33][34] High ionic strength leads to charge screening and weakening of the electrostatic interactions, giving rise to more hydrated and less dense polymer condensates. [35] Therefore, different morphologies of dried silicification products that were…”