Unusually Acidic Proteins in Biomineralization

Calcium carbonate biominerals are the most abundant mineral on the surface of the Earth. In eukaryotes, all biologically controlled calcium carbonate minerals are associated with a minor organic matrix, which displays several essential functions: crystal nucleation, control of crystal shape, and crystal growth inhibition. In addition, the matrix may be involved in enzymatic functions and may mediate cell-cell and cell-matrix interactions. The matrix is a mixture of proteins, glycoproteins, complex carbohydrates, proteoglycans, glycosaminoglycans and, sometimes, lipids. The biochemical properties of this matrix have been studied in numerous cases. One peculiarity shared by most (if not all) matrices associated with calcium carbonate biominerals is the presence of unusually acidic proteins; very often, these are rich in aspartic acid residues. The nature and biochemical properties of these proteins, their study and their unusual behaviour in solution remain topics of debate. In this chapter, we review our present knowledge of these unusually acidic proteins associated with calcium carbonate biomineralizations in selected eukaryotic phyla.