Identification of domains involved in tetramerization and malate inhibition of maize C4-NADP-malic enzyme

C-4 photosynthetic NADP-malic enzyme (ME) has evolved from non-C-4 isoforms and gained unique kinetic and structural properties during this process. To identify the domains responsible for the structural and kinetic differences between maize C4 and non-C-4-NADP-ME several chimeras between these isoforms were constructed and analyzed. By using this approach, we found that the region flanked by amino acid residues 102 and 247 is critical for the tetrameric state of C-4-NADP-ME. In this way, the oligomerization strategy of these NADP-ME isoforms differs markedly from the one that present non-plant NADP-ME with known crystal structures. On the other hand, the region from residue 248 to the C-terminal end of the C-4 isoform is involved in the inhibition by high malate concentrations at pH 7.0. The inhibition pattern of the C-4-NADP-ME and some of the chimeras suggested an allosteric site responsible for such behavior. This pH-dependent inhibition could be important for regulation of the C-4 isoform in vivo, with the enzyme presenting maximum activity while photosynthesis is in progress.