Purification, properties, and kinetic studies of cytoplasmic malate dehydrogenase from Taenia solium cysticerci

Kinetic characterization and thermostability of C. elegans cytoplasmic and mitochondrial malate dehydrogenases

Malate dehydrogenase (MDH) catalyzes the conversion of NAD⁺ and malate to NADH and oxaloacetate in the citric acid cycle. Eukaryotes have one MDH isozyme that is imported into the mitochondria and one in the cytoplasm. We overexpressed and purified Caenorhabditis elegans cytoplasmic MDH-1 and mitochondrial MDH-2 in E. coli. Our goal was to compare the kinetic and structural properties of these enzymes because C. elegans can survive adverse environmental conditions, such as lack of food and elevated temperatures. In steady-state enzyme kinetics assays, we measured K_M values for oxaloacetate of 54 and 52 μM and K_M values for NADH of 61 and 107 μM for MDH-1 and MDH-2, respectively. We partially purified endogenous MDH-1 and MDH-2 from a mixed population of worms and separated them using anion exchange chromatography. Both endogenous enzymes had a K_M for oxaloacetate similar to that of the corresponding recombinant enzyme. Recombinant MDH-1 and MDH-2 had maximum activity at 40 °C and 35 °C, respectively. In a thermotolerance assay, MDH-1 was much more thermostable than MDH-2. Protein homology modeling predicted that MDH-1 had more intersubunit salt-bridges than mammalian MDH1 enzymes, and these ionic interactions may contribute to its thermostability. In contrast, the MDH-2 homology model predicted fewer intersubunit ionic interactions compared to mammalian MDH2 enzymes. These results suggest that the increased stability of MDH-1 may facilitate its ability to remain active in adverse environmental conditions. In contrast, MDH-2 may use other strategies, such as protein binding partners, to function under similar conditions.

A novel malate dehydrogenase from Ceratonia siliqua L. seeds with potential biotechnological applications

A novel malate dehydrogenase (MDH; EC 3.1.1.1.37), hereafter MDHCs, from Ceratonia siliqua seeds, commonly known as Carob tree, was purified by using ammonium sulphate precipitation, ion exchange chromatography on SteamLine SP and gel-filtration. The molecular mass of the native protein, obtained by analytical gel-filtration, was about 65 kDa, whereas, by using SDS-PAGE analysis, with and without reducing agent, was 34 kDa. The specific activity of purified MDHCs (0.25 mg/100 g seeds) was estimated to be 188 U/mg. The optimum activity of the enzyme is at pH 8.5, showing a decrease in the presence of Ca(2+), Mg(2+) and NaCl. The N-terminal sequence of the first 20 amino acids of MDHCs revealed 95 % identity with malate dehydrogenase from Medicago sativa L. Finally, the enzymatic activity of MDHCs was preserved even after absorption onto a PVDF membrane. To our knowledge, this is the first contribution to the characterization of an enzyme from Carob tree sources.

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.