The crystal structure of the mitochondrial NAD-malic enzyme from Ascaris suum, in a quaternary complex with NADH, tartronate, and magnesium has been determined to 2.0-Å resolution. The structure closely resembles the previously determined structure of the same enzyme in binary complex with NAD. However, a significant difference is observed within the coenzyme-binding pocket of the active site with the nicotinamide ring of NADH molecule rotating by 198°over the C-1-N-1 bond into the active site without causing significant movement of the other catalytic residues. The implications of this conformational change in the nicotinamide ring to the catalytic mechanism are discussed. The structure also reveals a binding pocket for the divalent metal ion in the active site and a binding site for tartronate located in a highly positively charged environment within the subunit interface that is distinct from the active site. The tartronate binding site, presumably an allosteric site for the activator fumarate, shows striking similarities and differences with the activator site of the human NAD-malic enzyme that has been reported recently. Thus, the structure provides additional insights into the catalytic as well as the allosteric mechanisms of the enzyme.
Malic enzyme (ME)1 is an oxidative decarboxylase that catalyzes the conversion of L-malate to pyruvate and carbon dioxide, using a divalent metal ion (Mg 2ϩ or Mn 2ϩ ) and NAD ϩ or NAD(P) ϩ as cofactors (1-3). The enzyme is found in prokaryotes and eukaryotes and participates in diverse metabolic pathways such as photosynthesis, lipogenesis, and energy metabolism. Mitochondrial and cytosolic isoforms of the enzyme have been identified (1, 4). A sequence comparison of malic enzymes from different sources shows significant homology within the family but no homology to other proteins with the exception of the dinucleotide binding signature motif (5). Because of its functional importance, the enzyme has been isolated and characterized from several sources (3, 6). The mitochondrial NAD-malic enzyme (m-NAD⅐ME) from the parasitic nematode, Ascaris suum, plays a pivotal role in carbohydrate metabolism in parasitic worms (7). In the anaerobic metabolism of A. suum, malate, an intermediate in the worm's glycolytic pathway, is transported into the mitochondria where it undergoes a dismutation and is converted to pyruvate and NADH via the malic enzyme reaction and to fumarate via the fumarase reaction. Fumarate is then converted to short, branched-chain fatty acids via succinate mediated by the NADH produced in the malic enzyme reaction. The succinate dehydrogenase reaction is also involved in a site 1 oxidative phosphorylation, the main source of mitochondrial ATP (8). Since malic enzyme generates reducing equivalents (NADH) for the conversion of fumarate to succinate, it is not surprising that fumarate regulates its own utilization by activating the malic enzyme reaction (9, 10).The ascarid malic enzyme has been extensively studied in our laboratories from the standpoint of its kine...