Abstract：
The behavior of many plant enzymes depends on the metals and other ligands to which they are bound. A previous study demonstrated that tobacco Rubisco binds almost equally to magnesium and manganese and rapidly exchanges one metal for the other. The present study characterizes the kinetics of Rubisco and the plastidial malic enzyme when bound to either metal. When Rubisco purified from five C3 species was bound to magnesium rather than manganese, the specificity for CO2 over O2, (Sc/o) increased by 25% and the ratio of the maximum velocities of carboxylation / oxygenation (Vcmax/Vomax) increased by 39%. For the recombinant plastidial malic enzyme, the forward reaction (malate decarboxylation) was 30% slower and the reverse reaction (pyruvate carboxylation) was three times faster when bound to manganese rather than magnesium. Adding 6-phosphoglycerate and NADP+ inhibited carboxylation and oxygenation when Rubisco was bound to magnesium and stimulated oxygenation when it was bound to manganese. Conditions that favored RuBP oxygenation stimulated Rubisco to convert as much as 15% of the total RuBP consumed into pyruvate. These results are consistent with a stromal biochemical pathway in which (1) Rubisco when associated with manganese converts a substantial amount of RuBP into pyruvate, (2) malic enzyme when associated with manganese carboxylates a substantial portion of this pyruvate into malate, and (3) chloroplasts export additional malate into the cytoplasm where it generates NADH for assimilating nitrate into amino acids. Thus, plants may regulate the activities of magnesium and manganese in leaves to balance organic carbon and organic nitrogen as atmospheric CO2 fluctuates.
摘要：
许多植物酶的行为取决于它们所结合的金属和其他配体。先前的一项研究表明，烟草Rubisco与镁和锰的结合几乎相等，并迅速将一种金属交换为另一种金属。本研究表征了Rubisco和质体苹果酸酶与两种金属结合时的动力学。当从五种C3物种中纯化的Rubisco与镁而不是锰结合时，CO2对O2的特异性（Sc/o）增加了25％，羧化/氧合的最大速度之比（Vcmax/Vomax）增加了39％。对于重组质体苹果酸酶，当与锰而不是镁结合时，正向反应（苹果酸盐脱羧）慢30％，逆反应（丙酮酸盐羧化）快三倍。当Rubisco与镁结合时，添加6-磷酸甘油酸酯和NADP会抑制羧化和氧合，当与锰结合时，会刺激氧合。有利于RuBP氧合的条件刺激Rubisco将多达15%的总RuBP消耗转化为丙酮酸。这些结果与基质生化途径一致，其中（1）Rubisco与锰结合时将大量的RuBP转化为丙酮酸盐，（2）苹果酸酶与羧酸锰结合时，将该丙酮酸的大部分转化为苹果酸，和（3）叶绿体向细胞质中输出额外的苹果酸，在那里它产生NADH以将硝酸盐同化为氨基酸。因此，随着大气CO2的波动，植物可以调节叶片中镁和锰的活性，以平衡有机碳和有机氮。
