Dissociation of mitochondrial malate dehydrogenase into active soluble subunits

Enzymatic and physico-chemical characteristics of recombinant cMDH and mMDH of Clonorchis sinensis

The cytosol and mitochondrial malate dehydrogenases (MDHs, EC 1.1.1.37) of Clonorchis sinensis were expressed in Escherichia coli as a fusion protein with a 6xHis and GST tag, respectively. The cytosol MDH of Clonorchis sinensis (Cs-cMDH) has higher resistibility to acid than mitochondrial MDH (Cs-mMDH). The Cs-cMDH also has higher heat resistibility and thermal stability than Cs-mMDH. Although there is only 22.8% identity between the amino acid sequences of Cs-cMDH and Cs-mMDH, they share several conserved residues. There are some differences between the circular dichroism spectra of Cs-cMDH and Cs-mMDH, but they have approximate percentages of helix. 4,4'-Bisdimethylamino diphenylcarbinol can decrease the Cs-mMDH activity but not the Cs-cMDH activity. Paraziquantel, metronidazole and albendazole did not inhibit the enzymes' activity, but adenosine 5'-monophosphate showed competitive inhibition to enzyme, with the Ki for Cs-cMDH and Cs-mMDH being 2.81 and 0.49 mM, respectively.

Differences in the oligomeric states of the LDH-like L-MalDH from the hyperthermophilic archaea Methanococcus jannaschii and Archaeoglobus fulgidus

L-Malate (MalDH) and L-lactate (LDH) dehydrogenases belong to the same family of NAD-dependent enzymes. To gain insight into molecular relationships within this family, we studied two hyperthermophilic (LDH-like) L-MalDH (proteins with LDH-like structure and MalDH enzymatic activity) from the archaea Archaeoglobus fulgidus (Af) and Methanococcus jannaschii (Mj). The structural parameters of these enzymes determined by neutron scattering and analytical centrifugation showed that the Af (LDH-like) L-MalDH is a dimer whereas the Mj (LDH-like) L-MalDH is a tetramer. The effects of high temperature, cofactor binding, and high phosphate concentration were studied. They did not modify the oligomeric state of either enzyme. The enzymatic activity of the dimeric Af (LDH-like) L-MalDH is controlled by a pH-dependent transition at pH 7 without dissociation of the subunits. The data were analyzed in the light of the crystallographic structure of the LDH-like L-MalDH from Haloarcula marismortui. This showed that a specific loop at the dimer-dimer contact regions in these enzymes controls the tetramer formation.