The energetic basis for hydroxyapatite mineralization by amelogenin variants provides insights into the origin of amelogenesis imperfecta

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 116; no. 28; pp. 13867 - 13872
• Main Authors: Tao, Jinhui, Shin, Yongsoon, Jayasinha, Rajith, Buchko, Garry W., Burton, Sarah D., Dohnalkova, Alice C., Wang, Zheming, Shaw, Wendy J., Tarasevich, Barbara J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 09.07.2019
• Subjects: Adsorbates; Adsorption - genetics; Amelogenesis imperfecta; Amelogenesis Imperfecta - genetics; Amelogenesis Imperfecta - metabolism; Amelogenesis Imperfecta - pathology; Amelogenin; Amelogenin - chemistry; Amelogenin - genetics; Amino acid sequence; Amino Acid Sequence - genetics; Amino Acid Substitution - genetics; Amino acids; Amino Acids - chemistry; Amino Acids - genetics; Animals; Atomic force microscopy; Biomineralization - genetics; Degradation; Dental enamel; Durapatite - chemistry; Enamel; Energy Metabolism - genetics; Extracellular matrix; Extracellular Matrix Proteins - chemistry; Extracellular Matrix Proteins - genetics; Genetic transformation; Humans; Hydroxyapatite; Matrix Metalloproteinase 20 - chemistry; Matrix Metalloproteinase 20 - genetics; Mice; Microscopy; Microscopy, Atomic Force; Mineralization; Phase transitions; Phenotypes; Physical Sciences; Protein Conformation; Proteins; Quantitative analysis; Thermodynamics; biomineralization; protein adsorption; amelogenin
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1815654116

Small variations in the primary amino acid sequence of extracellular matrix proteins can have profound effects on the biomineralization of hard tissues. For example, a change in one amino acid within the amelogenin protein can lead to drastic changes in enamel phenotype, resulting in amelogenesis imperfecta, enamel that is defective and easily damaged. Despite the importance of these undesirable phenotypes, there is very little understanding of how single amino acid variation in amelogenins can lead to malformed enamel. Here, we aim to develop a thermodynamic under-standing of how protein variants can affect steps of the biomineralization process. High-resolution, in situ atomic force microscopy (AFM) showed that altering one amino acid within the murine amelogenin sequence (natural variants T21 and P41T, and experimental variant P71T) resulted in an increase in the quantity of protein adsorbed onto hydroxyapatite (HAP) and the formation of multiple protein layers. Quantitative analysis of the equilibrium adsorbate amounts revealed that the protein variants had higher oligomer–oligomer binding energies. MMP20 enzyme degradation and HAP mineralization studies showed that the amino acid variants slowed the degradation of amelogenin by MMP20 and inhibited the growth and phase transformation of HAP. We propose that the protein variants cause malformed enamel because they bind excessively to HAP and disrupt the normal HAP growth and enzymatic degradation processes. The in situ methods applied to determine the energetics of molecular level processes are powerful tools toward understanding the mechanisms of biomineralization.