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Publication Abstract from PubMed

The photosynthetic phosphoenolpyruvate carboxylase isozymes from C4 plants (PEPC-C4) play a critical role in their atmospheric CO2 assimilation and productivity. They are allosterically activated by phosphorylated trioses or hexoses, such as D-glucose-6-phosphate, and inhibited by L-malate or L-aspartate. Additionally, PEPC-C4 isozymes from grasses are activated by glycine, serine or alanine, but the allosteric site for these compounds remained unknown. Here we report a new crystal structure of the isozyme from Zea mays (ZmPEPC-C4) with glycine bound at the monomer-monomer interfaces of the two dimers of the tetramer, making interactions with residues of both monomers. This binding site is close to, but different from, the one proposed to bind glucose-6-phosphate. Docking experiments indicated that D/L-serine or D/L-alanine could also bind to this site, which does not exist in the PEPC-C4 isozyme from the eudicot plant Flaveria, mainly because of a lysyl residue at the equivalent position of Ser100 in ZmPEPC-C4. Accordingly, the ZmPEPC-C4 S100K mutant is not activated by glycine, serine or alanine. Amino acid sequence alignments showed that PEPC-C4 isozymes from the monocot family Poaceae have either serine or glycine at this position while those from Cyperaceae and eudicots families have lysine. The size and charge of the residue equivalent to Ser100 is not only crucial for the activation of PEPC-C4s by neutral amino acids but also affect their affinity for the substrate phosphoenolpyruvate and their allosteric regulation by glucose-6-phosphate and malate, accounting for the reported kinetics differences between PEPC-C4 isozymes from monocot and eudicot plants.

Identification of the allosteric site for neutral amino acids in the maize C4-isozyme of phosphoenolpyruvate carboxylase: The critical role of Ser100.,Gonzalez-Segura L, Mujica-Jimenez C, Juarez-Diaz JA, Guemez-Toro R, Martinez-Castilla LP, Munoz-Clares RA J Biol Chem. 2018 May 9. pii: RA118.002884. doi: 10.1074/jbc.RA118.002884. PMID:29743237^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.