Structural biology of carboxylases in C4 plant

磷酸烯醇式丙酮酸羧化酶(PEPC)催化C_4光合作用中大气CO_2固定的第一步。在3.0A^0分辨率下测定了C4型玉米PEPC的晶体结构，这是植物PEPC的第一个结构。该结构包括变构激活剂葡萄糖6-磷酸可能结合部位的硫酸盐离子。在2.35A^0分辨率下，还测定了E.coliPEPC与Mn2+、lt；2+和gt；、磷酸烯醇式丙酮酸类似物(3，3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate)，和变构抑制剂天冬氨酸的晶体结构。与大肠杆菌PEPC相比，玉米PEPC结构在活性中心附近的两个环附近有动态运动。基于这些分子结构，提出了PEPC的羧化反应和变构调节机制。核酮糖1，5-二磷酸羧基/加氧酶(Rubisco)是光合作用Calvin-Benson循环中的第一步酶。它催化气态CO_2与1，5-二磷酸核酮加成，生成两个分子的3-磷酸甘油酸酯。然而，该酶也催化RuBP的O2加成反应，作为光呼吸的主要反应。在1.84A^0分辨率下测定了衣藻Rubisco的晶体结构。目前的高分辨率结构揭示了新颖的翻译后修饰。在2.6A^0分辨率下测定了未活化的Galdieria Rubisco的晶体结构。电子密度图显示，硫酸盐只与活性中心的P1阴离子结合位置结合，环6是闭合的。这种相互作用可能对于理解使环6稳定在关闭状态以及对阴离子配体产生更高的亲和力是至关重要的。

Phosphoenolpyruvate carboxylase (PEPC) catalyzes the first step in the fixation of atmospheric CO_2 during C_4 photosynthesis. The crystal structure of C_4-form maize PEPC, the first structure of the plant PEPCs, has been determined at 3.0 A^^○ resolution. The structure includes a sulfate ion at the plausible binding site of an allosteric activator, glucose 6-phosphate. The crystal structure of E. coli PEPC complexed with Mn2^<2+>, phosphoenolpyruvate analogue (3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate), and an allosteric inhibitor, aspartate, has also been determined at 2.35 A^^○ resolution. Dynamic movements were found in the maize PEPC structure around two loops near the active site, compared with E. coli PEPC. Based on these molecular structures, the mechanisms for the carboxylation reaction and for the allosteric regulation of PEPC are proposed.Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the initial step enzyme in the Calvin-Benson cycle of photosynthesis. It catalyzes the addition of gaseous CO_2 to ribulose 1,5-bisphosphate and produces two molecules of 3-phosphoglycerate. However, this enzyme also catalyzes O_2 addition for RuBP as the primary reaction of photorespiration. The crystal structure of Rubisco from Chlamydomonas has also been determined at 1.84 A^^○ resolution. The present high-resolution structure has revealed novel post-translational modifications. The crystal structure of unactivated Galdieria Rubisco has also been determined at 2.6 A^^○ resolution. The electron density map reveals that a sulfate binds only to the P1 anion-binding site of the active site and the loop 6 is closed. This interaction is likely to be crucial to understanding for stabilizing the loop 6 in the close state and to making a higher affinity for anionic ligands.