Ebel C, Faou P, Kernel B, Zaccai G
Institut de Biologie Structurale, CEA-CNRS, Grenoble, France.
Biochemistry. 1999 Jul 13;38(28):9039-47. doi: 10.1021/bi9900774.
Halophilic malate dehydrogenase unfolds at low salt, and increasing the salt concentration stabilizes, first, the folded form and then, in some cases, destabilizes it. From inactivation and fluorescence measurements performed on the protein after its incubation in the presence of various salts in a large range of concentrations, the apparent effects of anions and cations were found to superimpose. A large range of ions was examined, including conditions that are in general not of physiological relevance, to explore the physical chemistry driving adaptation to extreme environments. The order of efficiency of cations and anions to maintain the folded form is, for the low-salt transition, Ca(2+) approximately Mg(2+) > Li(+) approximately NH(4)(+) approximately Na(+) > K(+) > Rb(+) > Cs(+), and SO(4)(2)(-) approximately OAc(-) approximately F(-) > Cl(-), and for the high-salt transition, NH(4)(+) approximately Na(+) approximately K(+) approximately Cs(+) > Li(+) > Mg(2+) > Ca(2+), and SO(4)(2)(-) approximately OAc(-) approximately F(-) > Cl(-) > Br(-) > I(-). If a cation or anion is very stabilizing, the effect of the salt ion of opposite charge is limited. Anions of high charge density are always the most efficient to stabilize the folded form, in accordance with the order found in the Hofmeister series, while cations of high charge density are the most efficient only at the lower salt concentrations and tend to denature the protein at higher salt concentrations. The stabilizing efficiency of cations and anions can be related in a minor way to their effect on the surface tension of the solution, but the interaction of ions with sites only present in the folded protein has also to be taken into account. Unfolding at high salt concentrations corresponds to interactions of anions of low charge density and cations of high charge density with the peptide bond, as found for nonhalophilic proteins.
嗜盐苹果酸脱氢酶在低盐条件下会发生去折叠，增加盐浓度首先会使折叠形式稳定，然后在某些情况下又会使其不稳定。通过对该蛋白质在一系列不同浓度的各种盐存在下孵育后的失活和荧光测量发现，阴离子和阳离子的表观效应相互叠加。研究了大量的离子，包括一些通常与生理无关的条件，以探索驱动适应极端环境的物理化学机制。对于低盐转变，阳离子和阴离子维持折叠形式的效率顺序为：Ca(2+)≈Mg(2+)＞Li(+)≈NH(4)(+)≈Na(+)＞K(+)＞Rb(+)＞Cs(+)，以及SO(4)(2)(-)≈OAc(-)≈F(-)＞Cl(-)；对于高盐转变，顺序为：NH(4)(+)≈Na(+)≈K(+)≈Cs(+)＞Li(+)＞Mg(2+)＞Ca(2+)，以及SO(4)(2)(-)≈OAc(-)≈F(-)＞Cl(-)＞Br(-)＞I(-)。如果一种阳离子或阴离子具有很强的稳定作用，那么相反电荷的盐离子的作用就会受到限制。根据霍夫迈斯特序列中发现的顺序，高电荷密度的阴离子总是最有效地稳定折叠形式，而高电荷密度的阳离子仅在较低盐浓度下最有效，在较高盐浓度下往往会使蛋白质变性。阳离子和阴离子的稳定效率在一定程度上与它们对溶液表面张力的影响有关，但也必须考虑离子与仅存在于折叠蛋白质中的位点的相互作用。在高盐浓度下的去折叠与低电荷密度的阴离子和高电荷密度的阳离子与肽键的相互作用有关，这与非嗜盐蛋白质的情况相同。
