Structural biology of carboxylases in C4 plant

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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.