Institute of Biomedical Sciences, Academia Sinica, Taipei 115, Taiwan.
Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan.
Phys Chem Chem Phys. 2022 Jul 27;24(29):17759-17769. doi: 10.1039/d2cp02072f.
Because Li and Ca differ in both charge and size, the possibility that monovalent Li could dislodge the bulkier, divalent Ca in Ca proteins had not been considered. However, our recent density functional theory/continuum dielectric calculations predicted that Li could displace the native Ca from the C2 domain of cytosolic PKCα/γ. This would reduce electrostatic interactions between the Li-bound C2 domain and the membrane, consistent with experimental studies showing that Li can inhibit the translocation of cytoplasmic PKC to membranes. Besides the trinuclear Ca-site in the PKCα/γ C2 domain, it is not known whether other Ca-sites in human proteins may be susceptible to Li substitution. Furthermore, it is unclear what factors determine the outcome of the competition between divalent Ca and monovalent Li. Here we show that the net charge of residues in the first and second coordination shell is a key determinant of the selectivity for divalent Ca over monovalent Li in proteins: neutral/anionic Ca-carboxylate sites are protected against Li attack. They are further protected by outer-shell Asp/Glu and the protein matrix rigidifying the Ca-site or limiting water entry. In contrast, buried, cationic Ca-sites surrounded by Arg/Lys, which are found in the C2 domains of PKCα/γ, as well as certain synaptotagmins, are prone to Li attack.
因为锂和钙在电荷和大小上存在差异，所以人们之前没有考虑到单价的锂可能会取代体积更大的二价钙在钙蛋白中的位置。然而，我们最近的密度泛函理论/连续介电计算预测，锂可以将天然钙从细胞质 PKCα/γ 的 C2 结构域中置换出来。这将减少锂结合的 C2 结构域与膜之间的静电相互作用，这与实验研究一致，表明锂可以抑制细胞质 PKC 向膜的易位。除了 PKCα/γ C2 结构域中的三核钙位外，尚不清楚人类蛋白中的其他钙位是否可能容易受到锂取代。此外，尚不清楚决定二价钙和单价锂竞争结果的因素是什么。在这里，我们表明，第一和第二配位壳层中残基的净电荷是决定蛋白质中二价钙对单价锂选择性的关键因素：中性/阴离子钙羧酸酯位免受锂攻击。它们进一步受到外壳天冬氨酸/谷氨酸和刚性化钙位或限制水进入的蛋白质基质的保护。相比之下，在 PKCα/γ 的 C2 结构域以及某些突触融合蛋白中发现的，被 Arg/Lys 包围的埋藏的、带正电荷的钙位容易受到锂的攻击。
