Revised model of calcium and magnesium binding to the bacterial cell wall.
Revised model of calcium and magnesium binding to the bacterial cell wall.
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DOI：
10.1007/s10534-014-9797-5
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发表时间：
2014-12
期刊：
影响因子：
--
通讯作者：
Rice CV
中科院分区：
文献类型：
--
作者：
Thomas KJ 3rd;Rice CV
金属与细菌细胞壁结合，但结合机制和亲和力常数尚未完全了解。革兰氏阳性细菌的细胞壁的特征是有一层厚厚的肽聚糖和阴离子磷壁酸锚定在细胞质膜（脂磷壁酸）或共价结合到细胞壁（壁磷壁酸）。磷壁酸的多磷酸基团为钙和镁提供了一半的金属结合位点，这与之前关于钙结合 100% 依赖于磷壁酸的报道相矛盾。其余的结合位点由肽聚糖的羧基单元形成。在这项工作中，我们报告了钙和镁离子与细菌细胞壁相互作用的平衡缔合常数和总金属结合能力。金属结合力更强并且之前已报道过。结合数据的斯卡查德图的曲率和由此产生的两个结合亲和力区域表明存在负协同结合，这意味着随着更多离子与样品结合，结合亲和力降低。对于 Ca2+，区域 I 的 KA = (1.0 ± 0.2) × 106 M−1，区域 II 的 KA = (0.075 ± 0.058) × 106 M−1。对于 Mg2+，KA1 = (1.5 ± 0.1) × 106 和 KA2 = (0.17 ± 0.10) × 106。报告了两个区域的结合能力 (η)。然而，由于结合仍然发生在 II 区，因此总结合能力用 η2 表示，对于 Ca2+ 和 Mg2+ 分别为 0.70 ± 0.04 µmol/mg 和 0.67 ± 0.03 µmol/mg。这些数据与目前存在单一金属亲和力值在一定浓度范围内恒定的范例相矛盾。我们还发现平衡结合常数的测量高度依赖于样品，这表明金属通过异质细胞壁碎片扩散的作用。因此，我们能够调和许多描述二价金属阳离子的结合亲和力和结合模式的矛盾理论。
Metals bind to the bacterial cell wall yet the binding mechanisms and affinity constants are not fully understood. The cell wall of gram positive bacteria is characterized by a thick layer of peptidoglycan and anionic teichoic acids anchored in the cytoplasmic membrane (lipoteichoic acid) or covalently bound to the cell wall (wall teichoic acid). The polyphosphate groups of teichoic acid provide one-half of the metal binding sites for calcium and magnesium, contradicting previous reports that calcium binding is 100% dependent on teichoic acid. The remaining binding sites are formed with the carboxyl units of peptidoglycan. In this work we report equilibrium association constants and total metal binding capacities for the interaction of calcium and magnesium ions with the bacterial cell wall. Metal binding is much stronger and previously reported. Curvature of Scatchard plots from the binding data and the resulting two regions of binding affinity suggest the presence of negative cooperative binding, meaning that the binding affinity decreases as more ions become bound to the sample. For Ca2+, Region I has a KA = (1.0 ± 0.2) × 106 M−1 and Region II has a KA = (0.075 ± 0.058) × 106 M−1. For Mg2+, KA1 = (1.5 ± 0.1) × 106 and KA2 = (0.17 ± 0.10) × 106. A binding capacity (η) is reported for both regions. However, since binding is still occurring in Region II, the total binding capacity is denoted by η2, which are 0.70 ± 0.04 µmol/mg and 0.67 ± 0.03 µmol/mg for Ca2+ and Mg2+ respectively. These data contradict the current paradigm of there being a single metal affinity value that is constant over a range of concentrations. We also find that measurement of equilibrium binding constants is highly sample dependent, suggesting a role for diffusion of metals through heterogeneous cell wall fragments. As a result, we are able to reconcile many contradictory theories that describe binding affinity and the binding mode of divalent metal cations.