Abstract
Post-translational modifications are one way that biomineral-associated cells control the function and fate of proteins. Of the ten different types of post-translational modifications, one of the most interesting and complex is glycosylation, or the covalent attachment of carbohydrates to amino acid sidechains Asn, Ser, and Thr of proteins. In this review the author surveys some of the known biomineral-associated glycoproteins and summarizes recent in vitro recombinant protein experiments which test the impact of glycosylation on biomineralization protein functions, such as nucleation, crystal growth, and matrix assembly. These in vitro studies show that glycosylation does not alter the inherent function of the polypeptide chain; rather, it either accentuates or attenuates functionality. In essence, glycosylation gives the cell the “last word” as to what degree a biomineralization protein will participate in the biomineralization process.
Highlights
Over the last forty years there has been a concerted effort to understand how organisms craft biomineralized skeletal structures for survival [1,2,3]
These two proteins are found in avian eggshells [19] and, the polypeptide chains have been sequenced, they have not been fully characterized with regard to their oligosaccharide content or sequence, it is known that they bind to amorphous calcium carbonate (ACC)—containing matrix vesicles—and guide these vesicles to the mineralization front, where calcite crystals form from the ACC particles [19]
From the foregoing, we can observe that glycosylation provides an additional degree of control over extracellular protein function by either accentuating or attenuating the intrinsic functionality of the polypeptide sequence
Summary
Over the last forty years there has been a concerted effort to understand how organisms craft biomineralized skeletal structures for survival [1,2,3]. With regard to the latter, it has been well documented that the genomes of biomineralizing organisms code for families of proteins that are mineral-specific and unique with regard to primary sequence construction and structure [7,8,9,10] The appearance of these proteins in the extracellular matrix during mineral formation is a clear attempt by cells to regulate the nucleation and assembly stages that lead to the final mineral product of the skeletal elements that are necessary for organism survival. Perhaps the most complex post-translational modification process is glycosylation, or the addition of one or more carbohydrate monomers (known as monosaccharides) to specific amino acid sidechains on a protein, converting the polypeptide into a glycoprotein [11,12,13,14,15,16]. Suggestions as to the direction of future studies of glycosylated biomineralization proteins will be offered
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