| Biomineralization is the genetically controlled process by which organisms build composite structures and tissues of macromolecules and inorganic solids. The calcium carbonate shells of marine molluscs have desirable crystalline and mechanical properties that have not been replicated in synthetic materials, and have served as one of the foremost model systems for investigating biogenic mineralization. Our studies have isolated and characterized four proteins (AP7, AP24, AP8-alpha and -beta involved in molecular control of crystal formation in the shell of the California red abalone (Haliotis rufescens ). Biochemical characterization of protein structure has included the determination of amino acid composition and sequence, hydrophilicity, glycosylation state, mass, secondary structure and radius of hydration. To investigate the function of the purified shell proteins, modified calcium carbonate crystals grown in the presence of the proteins were also examined. Crystals were visually inspected by scanning electron microscopy, and lattice structure determined by X-ray diffraction and Raman microprobe spectroscopy. As acidic macromolecules are associated with all calcium-based biominerals, they are the prime candidates for molecular control over crystallization. Therefore, the in situ activity of the most acidic of the purified abalone shell proteins, AP8-alpha and -beta, was followed in real time during calcium carbonate crystal growth by atomic force microscopy. While the AP7 and AP24 proteins were found to interact with the crystal surface via their N-terminal sequences and frustrate crystal growth, molecular scale resolution of AP8 activity revealed that the acidic proteins are capable of inducing stereoselective morphological modifications and growth acceleration at the asymmetric crystal surface. Based on these unprecedented observations, a novel model is proposed in which the AP8 proteins act as surfactants through complementary mechanisms that affect both the kinetics and thermodynamics of ion attachment to the growing crystals. |
