江浙蝮蛇毒酸性与碱性磷脂酶A_2溶液构象变化的差示扫描量热法研究

运用差示扫描量热法，在不同ｐＨ值的缓冲溶液内和各种浓度的碱土族氯化物溶液内，研究了来自江浙蝮蛇（ＡｇｋｉｓｔｒｏｄｏｎｈａｌｙｓＰａｌｌａｓ）毒的酸性与碱性磷脂酶外Ａ２（ＰＬＡ２）的热变性过程。得到表征这两种酶溶液构象变化的热力学参数。依据这些参数研究了两者的溶液构象及其变化。在ｐＨ４．５以下，分子净荷正电的这两种酶在溶液中不形成可热致伸展的有序构象；ｐＨ高于４．５时，Ａｓｐ和Ｇｌｕ的侧链羧基以负离子形式存在有利于有序构象的稳定。Ｈｉｓ是决定ＰＬＡ２活力和热稳定性的重要残基。磷酸根离子和这两种酶有结合作用而降低有序构象的热稳定性。碱土族阳离子除和这两种酶结合外，还以依赖于离子强度的方式复杂地影响酶的溶液构象，但其作用不完全是静电性的，是或多或少地随离子的不同而不同的。计算给出酸性ＰＬＡ２的△Ｈｃｄ．

Study of Conformational Transitions of Acidic and Basic Phospholipases A_2 from the Venom of Agkistrodon halys.Pallas in Solution by Differential Scanning Calorimetry (DSC)

The processes of thermal denaturation of the acidic and basic phospholipases A2 from the venom of Agkistrodon halys Pallas have been studied in buffer solutions with various pH values and in the various concentrations of alkali-earth chlorides solutions by DSC. The thermodynamic parameters characterized the conformational transitions of bothe nzymes in solution have been obtained accurately. According to these parameters and the characteristics of thermograms, the conformations and the conformational transitions of both enzymes in solution have been studied. At pH values lower than 4.5, the molecules of both enzymes in solution do not form the compact conformations with three-dimensional order which are able to unfold thermally. At PH values higher than 4.5, ordered conformations are stabilized as the carboxyl groups of the residues Asp and Glu are present in the anionic form.The phosphate ions might bind with the molecules of both enzymes to some exment and decrease the stability of ordered conformations. In addition to binding with the molecules of both enzymes, the alkali-earth cations may influence in a complicated way the conformations Of both enzymes in solution depending on the ionic strength. However,these perions are not completely electrostatic, and the effects are different from ion to ion.The contribution to denaturational enthalpy brought only with conformational changes,has been calculated for the acidic phospholipase A2.