Abstract
We identified and characterized a malate dehydrogenase from Streptomyces coelicolor A3(2) (ScMDH). The molecular mass of ScMDH was 73,353.5 Da with two 36,675.0 Da subunits as analyzed by matrix-assisted laser-desorption ionization–time-of-flight mass spectrometry (MALDI-TOF-MS). The detailed kinetic parameters of recombinant ScMDH are reported here. Heat inactivation studies showed that ScMDH was more thermostable than most MDHs from other organisms, except for a few extremely thermophile bacteria. Recombinant ScMDH was highly NAD+-specific and displayed about 400-fold (kcat) and 1,050-fold (kcat⁄Km) preferences for oxaloacetate reduction over malate oxidation. Substrate inhibition studies showed that ScMDH activity was inhibited by excess oxaloacetate (Ki=5.8 mM) and excess L-malate (Ki=12.8 mM). Moreover, ScMDH activity was not affected by most metal ions, but was strongly inhibited by Fe2+ and Zn2+. Taken together, our findings indicate that ScMDH is significantly thermostable and presents a remarkably high catalytic efficiency for malate synthesis.
