Structural and Functional Analyses of the Acidic and Basic Amino Acid Repeat Sequence (DDRK) in Pif 80 from Pinctada fucata on the Aragonite Crystal Surface Using NMR

Abstract

Shellfish and crustaceans use calcium carbonate (CaCO3) as a protective shell and exoskeleton. CaCO3 is a major biomineral component in invertebrates. The Pinctada fucata shell contains two crystal polymorphs, with outer prismatic and inner nacreous layers. Diverse shell organic materials play significant roles in microstructure formation with organic–inorganic interactions. Pif 80 has a repeated acidic and basic amino acid low-complexity region that specifically binds to aragonite crystals. In this study, we aimed to elucidate the microstructure formation mechanism by analyzing the interaction between the low-complexity region and CaCO3. We used solution NMR to evaluate the structure and activity of the Pif 80 acidic region. We designed synthesis methods for aragonite nanoparticles that can be dispersed and rotated in solution. The results revealed that the “GDDRKDDRKG” peptide has the appropriate structure for aragonite surface binding, wherein the four carboxy groups of the Asp side chain are in the same plane, supported by Arg and Lys residues. The saturation transfer difference also supported this structure. These results suggest that the Pif 80 acidic region binds the aragonite crystal surface via Asp side chains with basic residues, providing new insights into the function of acidic and basic amino acids in biomineral formation.