Phosphoenolpyruvate carboxylase: Difference between revisions

The enzyme phosphoenolpyruvate carboxylase (PEPC) catalyzes the carboxylation of phosphoenolpyruvate to form oxaloacetate, with Mg2+ or Mn2+ as essential cofactors <ref name="kai2003">PMID: 12781768</ref><ref name="svensson2003">PMID: 12781769</ref>. It can be considered the key enzyme in the C4 photosynthesis process, once it’s a central part of the mechanism that makes C4 plants more efficient in carbon fixation compared to classical C3-photosynthetic pathway plants, especially in abiotic stress environments  <ref name="sage2012">PMID: 22404472</ref>. PEPC is a ubiquitous enzyme, present in the genome of all plants. However, the isoforms found in C4 metabolism plants differ in their kinetic and regulatory characteristics, when compared to C3 orthologs  <ref name="Paulus2013">PMID: 23443546</ref>. Among the Flaverina genus of the Asteraceae family, closely related C3 and C4 species are found, providing a good model to study the differences  between the two processes. The comparative analysis of F. pringlei (C3) and T. trinervia (C4) PEPC’s,  has shown that the exchange of single amino acids can be responsible for the observed differences in saturation kinetics and inhibitor tolerance between PEPC’s of C3 and C4 species <ref name="blasing2000">PMID: 10871630</ref><ref name="Paulus2013"/>.

The comparison between closely related C3 and C4 PEPC’s from F. pringlei (C3) and T. trinervia (C4) was very important in determining the changes responsible for the differences in the enzymes activity in C3 and C4 plants. Structural superposition of these two isoforms shows high levels of structural similarity, supported by the low backbone root-mean square deviation of 0.4 Å <ref name="Paulus2013"/>.  Two important site-specific differences between the two structures are  a substitution of the 884 residue located close to the feedback inhibitor-binding site, and another residue substitution at the 774 position. The first is largely  responsible for the drastic differences observed between the inhibitor tolerances of  the C3 and C4 PEPC’s, while the second is a key determinant for the different kinetic properties of F. pringlei and T. trinervia PEPC’s <ref name="blasing2000"/><ref name="Paulus2013"/>.

PEPC’s carboxylase activity is regulated by different post-translational mechanisms. In C4 and CAM plants, the phosphorylation of a serine residue near the N terminus (S15 in maize C4-PEPC) activates the enzyme by decreasing its sensitivity to allosteric inhibitors such as aspartate and malate and increasing activation by the positive allosteric regulator glucose 6-phosphate <ref name="O'Leary2011"/>. Studies on F. pringlei and T. trinervia, have positively identified the residues Arg641, Lys829, Arg888 and Asn964 as binding motif of the negative allosteric inhibitors aspartate and malate <ref name="Paulus2013"/>. Similar studies have also identified the <scene name='57/573979/Cv/5'>aspartate binding site</scene> site in maize <ref name="matsumura2002"/>. In C3 PEPC, Arg884 provides an additional hydrogen bond for inhibitor binding, whereas in C4 PEPC isoforms the substitution of this residue by a glycine, reducing the enzymes sensitivity towards both feedback inhibitors <ref>PMID: 21491491</ref><ref name="Paulus2013"/>. The positive allosteric effector glucose 6-phosphate’s binding site has also been identified in the C4-PEPC of maize. X-ray crystallography of maize’s C4-PEPC in complex with sulfate ion (a positive effector analog of  glucose 6-phosphate) revealed that the positive effector was bound to the enzyme at the dimer interface and was surrounded by four positively charged residues (R183, R184, R231, and R372 in the adjacent subunit <ref name="kai2003"/>.