Mechanisms of Interaction of Biomolecule Phosphate Side Chains with Calcite during Dissolution

In proteins, phosphorylation of amino acid residues confers unique functions, including mineral-binding properties. For example, osteopontin, an abundant phosphoprotein in many biomineralized tissues and structures including bones, teeth, otoconia, and shells, can be variably and extensively phosphorylated. This post-translational modification of osteopontin imparts potent mineralization-regulating functional properties for both calcium phosphate- and calcium carbonate-containing tissues/structures. The local environment in which crystal formation (and dissolution) occurs is also rich in other nonprotein phosphate complexes such as pyrophosphate, polyphosphate (PP), and adenosine triphosphate. Here, we investigated the interaction of various small phospho-molecules with calcite under dissolution conditions. Using atomic force microscopy (AFM), we report on nanotopographic surface alterations resulting from dissolution of the (1 0 4) cleavage surface of calcite exposed to (i) a short-chain PP containing five phosphates (PP5), (ii) the phospho-amino acids P-serine, P-threonine, and P-tyrosine, and (iii) phosphorylethanolamine. We compared CORINA software-measured distances with Ca-Ca spacings characteristic of the step edges visualized experimentally by AFM to provide best-spacing matches on the (1 0 4) calcite acute and obtuse surface step directions during dissolution-this allowed for determination of plausible chiral or achiral molecular footprints for the phospho-molecules docked to Ca atoms at the dissolving calcite surface.

Automated summary

Phosphorylation of amino acids in proteins plays a crucial role in mineralization of tissues and structures; phosphorylation of phosphoproteins like osteopontin regulates mineralization in calcium phosphate and calcium carbonate tissues; small phospho-molecules interact with calcite during dissolution, leading to nanotopographic surface alterations.