CALCIUM-DEPENDENT ALPHA-HELICAL STRUCTURE IN OSTEOCALCIN
CALCIUM-DEPENDENT ALPHA-HELICAL STRUCTURE IN OSTEOCALCIN
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DOI：
10.1021/bi00539a038
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发表时间：
1982-01-01
期刊：
影响因子：
2.9
通讯作者：
CARR, SA
中科院分区：
文献类型：
--
作者：
HAUSCHKA, PV;CARR, SA
骨钙素是一种丰富的骨钙结合蛋白，含有维生素K依赖的3个残基。-羧谷氨酸（Gla）是其49种（人、猴和牛）或50种（鸡）氨基酸中的一种。在体内和体外，玻璃侧链直接参与Ca2+离子的结合和骨钙素在羟基磷灰石（HA）表面的吸附。当Ca2+结合时，骨钙素表现出主要的构象变化。无金属鸡骨钙素是一种随机线圈，只有8%的残留在α中。由圆二色性显示的螺旋。在生理水平的Ca2+存在的情况下，38%的蛋白质采用。α。-螺旋构象，其过渡中点为0.75 mM Ca2+，以快速、可逆的方式发生，需要完整的二硫桥，当Gla残基脱羧为Glu时，成比例地减弱，对1.5 M NaCl不敏感，可以被其他阳离子模仿。Tyr荧光、紫外差谱和对四硝基甲烷的反应性证实了构象的变化。同源猴骨钙素也表现出Ca2+依赖性结构。整合骨钙素序列的预测计算得出了该蛋白的结构模型，其主要特征包括2个相反的。α。-由9-12个残基组成的螺旋结构域，由。连接。转向并由Cys23.sbd稳定。Cys29二硫键。阳离子绑定允许实现完整的。alpha。-螺旋势通过部分中和在螺旋域的高阴离子电荷。在整个进化过程中，Gla在17、21和24位的周期性出现是高度保守的，并且所有Gla侧链都位于一个α的同一面。螺旋间距约为。5.4 .ANG。，与钙离子在HA晶格中的原子间分离非常相似。螺旋骨钙素对HA的亲和力显著增加；因此，Ca2+诱导的结构转变可能执行与骨代谢相关的信息作用。
Osteocalcin is an abundant Ca2+-binding protein of bone containing 3 residues of vitamin K dependent .gamma.-carboxyglutamic acid (Gla) among its 49 (human, monkey and cow) or 50 (chicken) amino acids. Gla side chains participate directly in the binding of Ca2+ ions and the adsorption of osteocalcin to hydroxyapatite (HA) surfaces in vivo and in vitro. Osteocalcin exhibits a major conformational change when Ca2+ is bound. Metal-free chicken osteocalcin is a random coil with only 8% of its residues in the .alpha. helix as revealed by circular dichroism. In the presence of physiological levels of Ca2+, 38% of the protein adopts the .alpha.-helical conformation with a transition midpoint at 0.75 mM Ca2+ in a rapid, reversible fashion which requires an intact disulfide bridge, is proportionally diminished when Gla residues are decarboxylated to Glu, is insensitive to 1.5 M NaCl and can be mimicked by other cations. Tyr fluorescence, UV difference spectra and Tyr reactivity to tetranitromethane corroborate the conformational change. Homologous monkey osteocalcin also exhibits Ca2+-dependent structure. Integration of predictive calculations from osteocalcin sequence has yielded a structural model for the protein, the dominant features of which include 2 opposing .alpha.-helical domains of 9-12 residues each, connected by a .beta. turn and stabilzed by the Cys23.sbd.Cys29 disulfide bond. Cation binding permits realization of the full .alpha.-helical potential by partial neutralization of high anionic charge in the helical domains. Periodic Gla occurrence at positions 17, 21 and 24 was strongly conserved throughout evolution and places all Gla side chains on the same face of one .alpha. helix spaced at intervals of .apprx. 5.4 .ANG., closely paralleling the interatomic separation of Ca2+ in the HA lattice. Helical osteocalcin has greatly increased affinity for HA; thus, the Ca2+-induced structural transition may perform an informational role related to bone metabolism.