The Coral Protein CARP3 Acts from a Disordered Mineral Surface Film to Divert Aragonite Crystallization in Favor of Mg-Calcite

Stony corals construct their aragonite skeleton by calcium carbonate precipitation, in a process recently suggested to be biologically controlled. Amorphous calcium carbonate and small amounts of calcite are also reported recently, however, their functional role is unknown. Coral acid-rich proteins (CARPs) are extracted from the coral skeleton and are shown to be active in calcium carbonate precipitation in vitro. However, individual function of these proteins in coral mineralization is not known. Here, the regulatory activity of the aspartate-rich CARP3 protein is examined. The whole protein and two peptides representing its acidic domain and its variable domain are used in CaCO₃ precipitation reactions from Mg-rich solutions. The biomolecules alter crystallization pathways, promoting Mg-calcite in place of aragonite, with the acidic peptide capable of eradicating aragonite formation. The activity of CARP3 and its representative peptides is exerted from disordered CaCO₃ mineral phases, coating the crystals formed, as shown by 2D ¹H–¹³C heteronuclear correlation nuclear magnetic resonance (NMR) measurements, localizing organic protons in atomic proximity to disordered carbonate carbons. The structures of the protein and individual domains as derived from NMR measurements and folding calculations and their amino acid compositions are discussed in the context of their observed activity and its implication to mineralization in hard corals.