METABOLIC REGULATION AND STRUCTURE-FUNCTION RELATIONSHIP OF A. THALIANA NADP-MALIC ENZYME ISOFORMS

摘要

The A. thaliana genome contains four genes encoding NADP-malic enzymes (ME1-4), which are different in their expression pattern and in subcellular location of their products. Although the four proteins share a remarkably high degree of identity, recombinant ME1-4 show well-distinct kinetic properties, both in forward -malate oxidative decarboxylation- and reverse pyruvate reductive carboxylation- reactions. When analyzing the activity of each isoform in the presence of possible metabolic effectors, ME2 resulted the most highly regulated. Furthermore, several metabolites modulate both reactions, exhibiting reciprocal effects activation or inhibition- in some cases. Thus, the results obtained indicated that forward and reverse reactions catalyzed by ME1-4 may be relevant in vivo, at least in some particular tissues or metabolic situations. In order to identify residues or segments of the primary structure that could be responsible in the differential properties, ME2 mutants and deletions and several quimeras were constructed and analysed. The results obtained show that Arg 115 is involved in fumarate activation of ME2, while the amino-terminal regions is critical for aspartate and CoA activation, as well as for the reverse reaction. In conclusion, these studies show that minimal changes in the primary structure are responsible for the different kinetic behavior of each ME isoform.