Cloning, sequencing and functional expression of a DNA encoding pig cytosolic malate dehydrogenase: purification and characterization of the recombinant enzyme

Cloning, sequencing and functional expression of cytosolic malate dehydrogenase from Taenia solium: Purification and characterization of the recombinant enzyme

We report herein the complete coding sequence of a Taenia solium cytosolic malate dehydrogenase (TscMDH). The cDNA fragment, identified from the T. solium genome project database, encodes a protein of 332 amino acid residues with an estimated molecular weight of 36517Da. For recombinant expression, the full length coding sequence was cloned into pET23a. After successful expression and enzyme purification, isoelectrofocusing gel electrophoresis allowed to confirm the calculated pI value at 8.1, as deduced from the amino acid sequence. The recombinant protein (r-TscMDH) showed MDH activity of 409U/mg in the reduction of oxaloacetate, with neither lactate dehydrogenase activity nor NADPH selectivity. Optimum pH for enzyme activity was 7.6 for oxaloacetate reduction and 9.6 for malate oxidation. K(cat) values for oxaloacetate, malate, NAD, and NADH were 665, 47, 385, and 962s(-1), respectively. Additionally, a partial characterization of TsMDH gene structure after analysis of a 1.56Kb genomic contig assembly is also reported.

Cloning, expression and biochemical characterization of mitochondrial and cytosolic malate dehydrogenase from Phytophthora infestans

The genes of the mitochondrial and cytosolic malate dehydrogenase (mMDH and cMDH) of Phytophthora infestans were cloned and overexpressed in Escherichia coli as active enzymes. The catalytic properties of these proteins were determined: both enzymes have a similar specific activity. In addition, the natural mitochondrial isoenzyme was semi-purified from mycelia and its catalytic properties determined: the recombinant mitochondrial isoform behaved as the natural enzyme. A phylogenetic analysis indicated that mMDH, present in all stramenopiles studied, can be useful to study the relationships between these organisms. MDH with the conserved domain MDH_cytoplasmic_cytosolic is absent in some stramenopiles as well as in fungi. This enzyme seems to be less related within the stramenopile group. The Phytophthora cMDHs have an insertion of six amino acids that is also present in the stramenopile cMDHs studied, with the exception of Thalassiosira pseudonana cMDH, and is absent in other known eukaryotic cMDHs.

A mathematical model of the mitochondrial NADH shuttles and anaplerosis in the pancreatic beta-cell

The pancreatic beta-cells respond to an increased glycolytic flux by secreting insulin. The signal propagation goes via mitochondrial metabolism, which relays the signal to different routes. One route is an increased ATP production that, via ATP-sensitive K(+) (K(ATP)) channels, modulates the cell membrane potential to allow calcium influx, which triggers insulin secretion. There is also at least one other "amplifying" route whose nature is debated; possible candidates are cytosolic NADPH production or malonyl-CoA production. We have used mathematical modeling to analyze this relay system. The model comprises the mitochondrial NADH shuttles and the mitochondrial metabolism. We found robust signaling toward ATP, malonyl-CoA, and NADPH production. The signal toward NADPH production was particularly strong. Furthermore, the model reproduced the experimental findings that blocking the NADH shuttles attenuates the signaling to ATP production while retaining the rate of glucose oxidation (Eto K, Tsubamoto Y, Terauchi Y, Sugiyama T, Kishimoto T, Takahashi N, Yamauchi N, Kubota N, Murayama S, Aizawa T, Akanuma Y, Aizawa S, Kasai H, Yazaki Y, Kadowaki T. Science 283: 981-985, 1999) and provides an explanation for this apparent paradox. The model also predicts that the mitochondrial malate dehydrogenase reaction may proceed backward, toward malate production, if the activity of malic enzyme is sufficiently high. An increased fatty acid oxidation rate was found to attenuate the signaling strengths. This theoretical study has implications for our understanding of both the healthy and the diabetic beta-cell.

Contribution of engineered electrostatic interactions to the stability of cytosolic malate dehydrogenase

Protein engineering is a promising tool to obtain stable proteins. Comparison between homologous thermophilic and mesophilic enzymes from a given structural family can reveal structural features responsible for the enhanced stability of thermophilic proteins. Structures from pig heart cytosolic and Thermus flavus malate dehydrogenases (cMDH, Tf MDH), two proteins showing a 55% sequence homology, were compared with the aim of increasing cMDH stability using features from the Thermus flavus enzyme. Three potential salt bridges from Tf MDH were selected on the basis of their location in the protein (surface R176-D200, inter-subunit E57-K168 and intrasubunit R149-E275) and implemented on cMDH using site-directed mutagenesis. Mutants containing E275 were not produced in any detectable amount, which shows that the energy penalty of introducing a charge imbalance in a region that was not exposed to solvent was too unfavourable to allow proper folding of the protein. The salt bridge R149-E275, if formed, would not enhance stability enough to overcome this effect. The remaining mutants were expressed and active and no differences from wild-type other than stability were found. Of the mutants assayed, Q57E/L168K led to a stability increase of 0.4 kcal/mol, as determined by either guanidinium chloride denaturalization or thermal inactivation experiments. This results in a 15 degrees C shift in the optimal temperature, thus confirming that the inter-subunit salt bridge initially present in the T.flavus enzyme was formed in the cMDH structure and that the extra energy obtained is transformed into an increase in protein stability. These results indicate that the use of structural features of thermophilic enzymes, revealed by a detailed comparison of three-dimensional structures, is a valid strategy to improve the stability of mesophilic malate dehydrogenases.

Structural basis of substrate specificity in malate dehydrogenases: crystal structure of a ternary complex of porcine cytoplasmic malate dehydrogenase, alpha-ketomalonate and tetrahydoNAD

The structural basis for the extreme discrimination achieved by malate dehydrogenases between a variety of closely related substrates encountered within the cell has been difficult to assess because of the lack of an appropriate catalytically competent structure of the enzyme. Here, we have determined the crystal structure of a ternary complex of porcine cytoplasmic malate dehydrogenase with the alternative substrate alpha-ketomalonate and the coenzyme analogue 1,4,5,6-tetrahydronicotinamide. Both subunits of the dimeric porcine heart, and from the prokaryotes Escherichia coli and Thermus flavus. However, large changes are noted around the active site, where a mobile loop now closes to bring key residues into contact with the substrate. This observation substantiates a postulated mechanism in which the enzyme achieves high levels of substrate discrimination through charge balancing in the active site. As the activated cofactor/substrate complex has a net negative charge, a positive counter-charge is provided by a conserved arginine in the active site loop. The enzyme must, however, also discriminate against smaller substrates, such as pyruvate. The structure shows in the closed (loop down) catalytically competent complex two arginine residues (91 and 97) are driven into close proximity. Without the complimentary, negative charge of the substrate side-chain of oxaloacetate or alpha-ketomalonate, charge repulsion would resist formation production of this catalytically productive conformation, hence minimising the effectiveness of pyruvate as a substrate. By this mechanism, malate dehydrogenase uses charge balancing to achieve fivefold orders of magnitude in discrimination between potential substrates.