Macromolecules in mollusc shells and their functions in biomineralization

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Abstract
Mollusc shells are used as a model for studying ‘organic-matrix -mediated’ biomineralization, in which crystals are nucleated and grow in a pre-formed structural framework composed of proteins and polysaccharides. In particular, the possibility that the organic matrix functions as a template for crystal formation by epitaxial growth, is examined. In general, individual organic matrix sheets are composed of a thin layer of β-chitin sandwiched between layers of proteins adopting the antiparallel β-sheet conformation. The protein polypeptide chains are oriented perpendicular to the chitin fibrils. The matrix surfaces contain acidic proteins and polysaccharides. X -ray and electron diffraction patterns of matrices and mineral crystals from the nacreous layers of a bivalve, a gastropod and the cephalopod, Nautilus , show that the chitin fibres and the protein polypeptide chains are aligned with the a and b aragonite crystallographic axes, respectively. This strongly suggests that the mineral formed epitaxially upon the matrix surface. However, as the degree of orientation of the organic constituents is much less than the mineral constituents, it is postulated that the site of nucleation of the mineral crystals comprises only a small part of the matrix structure and is itself composed of well oriented macromolecules, probably acidic proteins. Two acidic proteins, which may constitute part of the nucleation site for calcite in a bivalve, were identified by comparing all the acidic proteins in the calcite layer with those in the aragonite layer. The two proteins were present in the calcite layer only, and in addition were found to have an aspartic-acid -containing amino acid sequence, not present in any of the other soluble matrix proteins. The concept of a matrix composed of a structural core coated with acidic macromolecules, some of which constitute a nucleation site, may well be applicable to other mineralized tissues.

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