Casein Phosphopeptides as a Model for Noncollagenous Protein Control of Collagen Mineralization In Vitro

Noncollagenous proteins (NCPs) present in bone and other mineralized connective tissues play important roles in collagen mineralization. However, their regulation mechanisms remain unclear. While several NCP analogues have been used as models to study the regulation of mineralization, many of them lack phosphate groups, an important feature of NCPs. In this study, we propose the use of casein phosphopeptides (CPPs) in an in vitro model to study the role of phosphorylated NCPs. We found that soluble CPPs can delay precipitation in mineralization solutions and induce the formation of native-like mineralized collagen fibrils. Further, our results show that phosphate groups on collagen-bound CPPs inside the fibrils accelerate intrafibrillar mineralization, while those on the surface of the fibrils promote surface mineral formation. CPPs are a useful analogue for NCPs in studying the roles of soluble and matrix-bound phosphoproteins in collagen mineralization.