Behind every hard, mineral crystalline material in nature, there's a bevy of soft proteins guiding its formation and contributing to its strength and resilience. The hard calcium carbonate shells of oysters rely on a set of proteins that perform several key functions. They trigger dissolved calcium carbonate to crystallize out of solution, direct crystal growth into an ordered arrangement, and attach themselves to the crystals and each other to form a strongly bonded, properly shaped material. Although these proteins are made up of hundreds of amino acid building blocks, several amino acids appear to play major roles. Acidic amino acids, particularly aspartate, play a major role in crystal formation and binding protein and crystals together. Sulfur-containing amino acids, particularly cysteine, play a major role in binding proteins to each other to add strength and hold its shape.Edit Summary
"The mollusk shell is a hard tissue consisting of calcium carbonate crystals and an organic matrix. The nacre of the shell is characterized by a stacked compartment structure with a uniformly oriented c axis of aragonite crystals in each compartment. Using a calcium carbonate–binding assay, we identified an acidic matrix protein, Pif, in the pearl oyster Pinctada fucata that specifically binds to aragonite crystals...Both Pif 80 and Pif 97 are acidic with calculated isoelectric point (pI) values of 4.99 and 4.65, respectively. Pif 97 consists of 525 amino acid residues and has two conserved domains, a von Willebrand type A (VWA) for protein-protein interaction domain and a chitin-binding domain similar to that of Peritrophin A. Pif 97 contains a high proportion of charged amino acid residues, Asp (14.9%), Glu (6.5%), Lys (11.1%), and Arg (5.0%), and many Cys residues (23 residues) that might form disulfide bridges to confer a rigid three-dimensional conformation. In contrast, Pif 80 consists of 460 amino acid residues and has no conserved domains. Pif 80 has more charged amino acid residues [Asp (28.5%), Glu (4.1%), Lys (18.7%), and Arg (10.9%)] than does Pif 97. Pif 80 has 17 repeats of a four-amino-acid motif, Asp-Asp-Arg (Lys)–Lys (Arg), scattered throughout its sequence and a cluster of acidic amino acid residues (Asp2-Glu-Asp7) near the center of the molecule. The high ratio of Asp in Pif 80 may play a role in aragonite-binding, considering that acidic amino acid residues are associated with the regulation of crystal polymorph and that poly-(Asp) has been shown to induce aragonite formation." (Suzuki et al. 2009:1388).