Department of Biochemistry, University of California, Riverside, California 92506.
Plant Physiol. 1988 Dec;88(4):976-9. doi: 10.1104/pp.88.4.976.
Phosphoenolpyruvate carboxylase isolated from maize (Zea mays L.) leaves was assayed with varying concentrations of free phosphoenolpyruvate at several fixed-varying concentrations of free magnesium higher than required to saturate the enzyme reaction. These assays produced velocity data which were found to form a family of individual lines when plotted against free phosphoenolpyruvate or against total phosphoenolpyruvate, but not when plotted against the concentration of the complex of phosphoenolpyruvate with magnesium. In this latter case, the points from all the fixed-varying concentrations fell on the same line, which can be fitted to a modified Michaelis-Menten equation with a multiple correlation coefficient R(2) = 0.995. Similar results were obtained when the enzyme from the C(4) plant maize was assayed with manganese rather than magnesium and when phosphoenolpyruvate carboxylase from leaves of the C(3) plant wheat (Triticum vulgare Vill.) was assayed with magnesium. However, at pH 7.0 the enzyme from the Crassulacean acid metabolism plant Crassula argentea did not produce a satisfactory single line when plotted against the complex of metal ion and substrate, but did so when the assay pH was raised to 8.0. It is concluded that in general the preferred form of substrate for phosphoenolpyruvate carboxylase is the complex of phosphoenolpyruvate with the metal ion.
从玉米（Zea mays L.）叶片中分离出的磷酸烯醇丙酮酸羧激酶，在几种高于饱和酶反应所需的固定变化浓度的游离镁浓度下，用不同浓度的游离磷酸烯醇丙酮酸进行测定。这些测定产生的速度数据，当以游离磷酸烯醇丙酮酸或总磷酸烯醇丙酮酸作图时，形成了一组单独的线，但当以磷酸烯醇丙酮酸与镁的复合物浓度作图时，则不是这样。在后一种情况下，所有固定变化浓度的点都落在同一条线上，可以用一个经过修正的米氏-门坦方程来拟合，相关系数 R²=0.995。当用锰而不是镁测定 C4 植物玉米的酶时，以及当用镁测定 C3 植物小麦（Triticum vulgare Vill.）叶片中的磷酸烯醇丙酮酸羧激酶时，也得到了类似的结果。然而，在 pH 值为 7.0 时，来自景天酸代谢植物银叶菊（Crassula argentea）的酶在与金属离子和底物的复合物作图时，不能产生令人满意的单一直线，但当测定 pH 值升高到 8.0 时，就能产生。因此，可以得出结论，一般来说，磷酸烯醇丙酮酸羧激酶的首选底物形式是磷酸烯醇丙酮酸与金属离子的复合物。
