Circular dichroism: A powerful tool for studying biomineralization promoter proteins
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Introduction Mineralized tissue, such as eggshells, mollusk shells, teeth, or bone, forms through highly regulated deposition of inorganic material on an organic matrix. The minerals involved in biomineralization are mostly precipitates of calcium phosphate and calcium carbonate. They appear as different crystal modifications in different tissues, such as hydroxylapatite (Ca10(PO4)6(OH)2) in teeth and bone, or calcite, aragonite, or vaterite (different polymorphs of CaCO3) in mollusk shells or eggshells. As both hypo- and hypermineralization result in non-viable materials, biomineralization has to provide very distinct arrangements of crystallites. This happens through a precise mechanism, which may be either cell-controlled or cell-independent.1 In cell-controlled processes, ions or preformed precipitates of inorganic material are delivered in lipid vesicles to the extracellular space and released after subsequent rupture. In cellindependent processes, minerals form on biological matrices with dedicated biomineralization promoter proteins (BPPs)
as a template to trigger the nucleation of nanometer-sized crystals.2,3 The modulation of BPP-mediated processes likely involves conformational changes that lead to a shift in secondary structure, the three-dimensional fold of the amino acid chain that constitutes the protein. Various factors have an influence on the equilibrium between two or more conformations in cell-independent biomineralization (Scheme 1). It has been shown for various BPPs that they are intrinsically unordered and highly charged.3 Therefore, it is not surprising that their conformation is sensitive to pH. Furthermore, as a mineral cation binds to charged domains of the active conformation of a BPP, it also stabilizes this conformation. Some proteins take up some degree of β-sheet secondary structure, a highly ordered protein fold in which aligned amino acid strands interact through hydrogen bonding. Some studies report aggregation of BPPs upon the addition of bivalent cations such as Ca2+—this could either be mediated through ionic cross-linking or through β-sheet domains, which are generally known to have a propensity for driving
Melika Sarem, Institute for Macromolecular Chemistry, University of Freiburg; Helmholtz Virtual Institute on Multifunctional Biomaterials for Medicine, Kantstr. 55, 14513 Teltow, Germany; Bioss, Centre for Biological Signalling Studies, University of Freiburg, Germany; [email protected] Steffen Lüdeke, Institute of Pharmaceutical Sciences, The University of Freiburg, Germany; [email protected] DOI: 10.1557/mrs.2015.116
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MRS BULLETIN • VOLUME 40 • JUNE 2015 • www.mrs.org/bulletin
© 2015 Materials Research Society
CIRCULAR DICHROISM: A POWERFUL TOOL FOR STUDYING BIOMINERALIZATION PROMOTER PROTEINS
Scheme 1. Hypothetical structural prerequisites for proteinpromoted nucleation. Conformational transitions may be driven by binding of bivalent cations and other environmental factors, such as pH or charged surfaces. Protein assembly/
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