Fan Daming, Lakshminarayanan Rajamani, Moradian-Oldak Janet
University of Southern California, School of Dentistry, Center for Craniofacial Molecular Biology, 2250 Alcazar Street CSA 103, Los Angeles, CA 90033, USA.
J Struct Biol. 2008 Jul;163(1):109-15. doi: 10.1016/j.jsb.2008.04.007. Epub 2008 Apr 24.
The 32 kDa hydrophilic and acidic enamelin, the most stable cleavage fragment of the enamel specific glycoprotein, is believed to play vital roles in controlling crystal nucleation or growth during enamel biomineralization. Circular dichroism and Fourier transform infrared spectra demonstrate that the secondary structure of the 32 kDa enamelin has a high content of alpha-helix (81.5%). Quantitative analysis on the circular dichroism data revealed that the 32 kDa enamelin undergoes conformational changes with a structural preference to beta-sheet with increasing concentration of calcium ions. We suggest that the increase of beta-sheet conformation in the presence of Ca(2+) may allow preferable interaction of the 32 kDa enamelin with apatite crystal surfaces during enamel biomineralization. The calcium association constant (K(a)=1.55 (+/-0.13)x10(3)M(-1)) of the 32 kDa enamelin calculated from the fitting curve of ellipticity at 222 nm indicated a relatively low affinity. Our current biophysical studies on the 32 kDa enamelin structure provide novel insights towards understanding the enamelin-mineral interaction and subsequently the functions of enamelin during enamel formation.
32 kDa的亲水性酸性釉原蛋白是釉质特异性糖蛋白最稳定的裂解片段，据信在釉质生物矿化过程中对控制晶体成核或生长起着至关重要的作用。圆二色性和傅里叶变换红外光谱表明，32 kDa釉原蛋白的二级结构具有高含量的α-螺旋（81.5%）。对圆二色性数据的定量分析表明，随着钙离子浓度的增加，32 kDa釉原蛋白会发生构象变化，且结构更倾向于β-折叠。我们认为，在Ca(2+)存在的情况下β-折叠构象的增加可能使32 kDa釉原蛋白在釉质生物矿化过程中更易于与磷灰石晶体表面相互作用。根据222 nm处椭圆率的拟合曲线计算得出的32 kDa釉原蛋白的钙结合常数（K(a)=1.55 (+/-0.13)x10(3)M(-1)）表明其亲和力相对较低。我们目前对32 kDa釉原蛋白结构的生物物理研究为理解釉原蛋白与矿物质的相互作用以及随后釉原蛋白在釉质形成过程中的功能提供了新的见解。
