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Bifunctional malic/malolactic enzyme provides a novel mechanism for NADPH balancing in microbes

ABSTRACT: The redox cofactor NADPH is required as a reducing equivalent in about 100 anabolic reactions throughout metabolism. To ensure fitness in all conditions, the demand is fulfilled by a handful of dehydrogenases in central carbon metabolism that reduce NADP+ with electrons derived from catabolism of nutrients. In the case of Bacillus subtilis growing on glucose, quantitative flux analyses indicate that NADPH production largely exceed biosynthetic need, suggesting a hitherto unknown mechanism for NADPH balancing. We investigated the role of the four malic enzymes present in B. subtilis, which could bring about a metabolic cycle for transhydrogenation of NADPH into NADH. Using quantitative 13C metabolic flux analysis, we found that isoform YtsJ alone contributes to NADPH balancing in vivo and demonstrated relevant NADPH-oxidizing activity by YtsJ in vitro. To our surprise, we discovered that depending on NADPH, YtsJ switches activity from a pyruvate producing malic enzyme to a lactate generating malolactic enzyme. This switch in activity allows YtsJ to adaptively compensate cellular NADPH over- and underproduction upon demand. Finally, NADPH-dependent bifunctional activity was also detected in the YtsJ-homolog in Escherichia coli MaeB. Overall, our study extends the known redox cofactor balancing mechanisms, by providing for the first-time evidence that the type of catalyzed reaction by an enzyme depends on metabolite abundance.

ORGANISM(S): Bacillus subtilis

SUBMITTER: Hörl

PROVIDER: S-BSST566 | biostudies-other |

REPOSITORIES: biostudies-other

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Project description:RATIONALE:Metabolic remodeling in hypertrophic hearts includes inefficient glucose oxidation via increased anaplerosis fueled by pyruvate carboxylation. Pyruvate carboxylation to malate through elevated ME1 (malic enzyme 1) consumes NADPH necessary for reduction of glutathione and maintenance of intracellular redox state. OBJECTIVE:To elucidate upregulated ME1 as a potential maladaptive mechanism for inefficient glucose oxidation and compromised redox state in hypertrophied hearts. METHODS AND RESULTS:ME1 expression was selectively inhibited, in vivo, via non-native miR-ME1 (miRNA specific to ME1) in pressure-overloaded rat hearts. Rats subjected to transverse aortic constriction (TAC) or Sham surgery received either miR-ME1 or PBS. Effects of ME1 suppression on anaplerosis and reduced glutathione (GSH) content were studied in isolated hearts supplied 13C-enriched substrate: palmitate, glucose, and lactate. Human myocardium collected from failing and nonfailing hearts during surgery enabled RT-qPCR confirmation of elevated ME1 gene expression in clinical heart failure versus nonfailing human hearts (P<0.04). TAC induced elevated ME1 content, but ME1 was lowered in hearts infused with miR-ME1 versus PBS. Although Sham miR-ME1 hearts showed no further reduction of inherently low anaplerosis in normal heart, miR-ME1 reduced anaplerosis in TAC to baseline: TAC miR-ME1=0.034±0.004; TAC PBS=0.081±0.005 (P<0.01). Countering elevated anaplerosis in TAC shifted pyruvate toward oxidation in the tricarboxylic acid cycle. Importantly, via the link to NADPH consumption by pyruvate carboxylation, ME1 suppression in TAC restored GSH content, reduced lactate production, and ultimately improved contractility. CONCLUSIONS:A maladaptive increase in anaplerosis via ME1 in TAC is associated with reduced GSH content. Suppressing increased ME1 expression in hypertrophied rat hearts, which is also elevated in failing human hearts, reduced pyruvate carboxylation thereby normalizing anaplerosis, restoring GSH content, and reducing lactate accumulation. Reducing ME1 induced favorable metabolic shifts for carbohydrate oxidation, improving intracellular redox state and enhanced cardiac performance in pathological hypertrophy.

Project description:Malic enzymes have high cofactor selectivity. An isoform-specific distribution of residues 314, 346, 347 and 362 implies that they may play key roles in determining the cofactor specificity. Currently, Glu314, Ser346, Lys347 and Lys362 in human c-NADP-ME were changed to the corresponding residues of human m-NAD(P)-ME (Glu, Lys, Tyr and Gln, respectively) or Ascaris suum m-NAD-ME (Ala, Ile, Asp and His, respectively). Kinetic data demonstrated that the S346K/K347Y/K362Q c-NADP-ME was transformed into a debilitated NAD?-utilizing enzyme, as shown by a severe decrease in catalytic efficiency using NADP? as the cofactor without a significant increase in catalysis using NAD? as the cofactor. However, the S346K/K347Y/K362H enzyme displayed an enhanced value for k(cat,NAD), suggesting that His at residue 362 may be more beneficial than Gln for NAD? binding. Furthermore, the S346I/K347D/K362H mutant had a very large K(m,NADP) value compared to other mutants, suggesting that this mutant exclusively utilizes NAD? as its cofactor. Since the S346K/K347Y/K362Q, S346K/K347Y/K362H and S346I/K347D/K362H c-NADP-ME mutants did not show significant reductions in their K(m,NAD) values, the E314A mutation was then introduced into these triple mutants. Comparison of the kinetic parameters of each triple-quadruple mutant pair (for example, S346K/K347Y/K362Q versus E314A/S346K/K347Y/K362Q) revealed that all of the K(m) values for NAD? and NADP(+) of the quadruple mutants were significantly decreased, while either k(cat,NAD) or k(cat,NADP) was substantially increased. By adding the E314A mutation to these triple mutant enzymes, the E314A/S346K/K347Y/K362Q, E314A/S346K/K347Y/K362H and E314A/S346I/K347D/K362H c-NADP-ME variants are no longer debilitated but become mainly NAD?-utilizing enzymes by a considerable increase in catalysis using NAD? as the cofactor. These results suggest that abolishing the repulsive effect of Glu314 in these quadruple mutants increases the binding affinity of NAD?. Here, we demonstrate that a series of E314A-containing c-NADP-ME quadruple mutants have been changed to NAD?-utilizing enzymes by abrogating NADP? binding and increasing NAD? binding.

Project description:BACKGROUND:Anopheles stephensi mitochondrial malic enzyme (ME) emerged as having a relevant role in the provision of pyruvate for the Krebs' cycle because inhibition of this enzyme results in the complete abrogation of oxygen uptake by mitochondria. Therefore, the identification of ME in mitochondria from immortalized A. stephensi (ASE) cells and the investigation of the stereoselectivity of malate analogues are relevant in understanding the physiological role of ME in cells of this important malaria parasite vector and its potential as a possible novel target for insecticide development. METHODS:To characterize the mitochondrial ME from immortalized ASE cells (Mos. 43; ASE), mass spectrometry analyses of trypsin fragments of ME, genomic sequence analysis and biochemical assays were performed to identify the enzyme and evaluate its activity in terms of cofactor dependency and inhibitor preference. RESULTS:The encoding gene sequence and primary sequences of several peptides from mitochondrial ME were found to be highly homologous to the mitochondrial ME from Anopheles gambiae (98%) and 59% homologous to the mitochondrial NADP+-dependent ME isoform from Homo sapiens. Measurements of ME activity in mosquito mitochondria isolated from ASE cells showed that (i) Vmax with NAD+ was 3-fold higher than that with NADP+, (ii) addition of Mg2+ or Mn2+ increased the Vmax by 9- to 21-fold, with Mn2+ 2.3-fold more effective than Mg2+, (iii) succinate and fumarate increased the activity by 2- and 5-fold, respectively, at sub-saturating concentrations of malate, (iv) among the analogs of L-malate tested as inhibitors of the NAD+-dependent ME catalyzed reaction, small (2- to 3-carbons) organic diacids carrying a 2-hydroxyl/keto group behaved as the most potent inhibitors of ME activity (e.g., oxaloacetate, tartronic acid and oxalate). CONCLUSIONS:The biochemical characterization of Anopheles stephensi ME is of critical relevance given its important role in bioenergetics, suggesting that it is a suitable target for insecticide development.

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Bifunctional malic/malolactic enzyme provides a novel mechanism for NADPH balancing in microbes

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