马齿苋叶片磷酸烯醇式丙酮酸羧化酶活性及调节特性的光/暗变化

在光照条件下C_4植物马齿黄金苋叶片PEPC的提取活性高于在黑暗中的。PEPC的光/暗活性比率与测定系统的pH及底物PEP浓度有关。pH升高及PEP浓度增加均可使光/暗活性比值下降。日间提取的PEPC与夜间提取的PEPC对于激活剂G6P及抑制剂Mal的敏感性有明显差异。日型PEPC的敏感性低于夜型PEPC的。G6P对PEPC的激活作用表现为增加酶对底物PEP的亲和性,Mal的抑制作用表现为既降低酶对底物PEP的亲和性,又降低酶促反应的最大速度。G6P、Mal对于日型和夜型PEPC的动力学参数的影响是不同的。

DAY/NIGHT CHANGES IN ACTIVITY AND REGULATORY PROPERTIES OF PHOSPHOENOLPYRUVATE CARBOXYLASE FROM Portulaca oleracea LEAVES

The activity of PEPC extracted in the day from Portulaca oleracea leaves ishigher than that extracted in the night, when assayed at pH 7.4 and under low PEPconcentrations. Higher values of the assay pH and the substrate PEP concentrationcause the day/night activity ratio of PEPC decreasing from about 3 to 1. This factimplies that the night form PEPC can be converted to the day form PEPC bY higherpH and PEP. Kinetic studies show that the day form and the night form of PEPChave different affinities for the substrate PEP.The day form and the night form enzymes also show very different sensitivitiesto G6P and Mal, which activates and inhibits PEPC respectively. When assayedat pH 7.4 and 1.33 m molL~(-1) PEP the night form PEPC is more sensitiveto G6P and Mal than the day form PEPC. In the presence of G6P (5 mmolL~(-1)), both forms of PEPC increase greatly the affinity for PEP and theV-S curves change from the sigmoid curve to hyperbola, but values of V_(max)remain almost unchanged. On the other hand, the two form of enzyme showdifferent kinetic properties in the presence of Mal (5 m mol L~(-1)).