Factors affecting the stability of methanol dehydrogenase from Paracoccus denitrificans

Methanol dehydrogenase from Paracoccus denitrificans was purified to homogeneity in two steps from the periplasmic fraction of methanol-grown cells. The enzyme was composed of subunits of M(r) 67,000 and 12,000, and non-covalently bound pyrroloquinoline quinone. It exhibited a pH optimum at pH values of 9.0 and above. It was not stable at pH greater than 9.0, but exhibited little loss of activity after prolonged incubation at pH values as low as 4.5. Methyl dehydrogenase was relatively stable to thermal denaturation. The thermal stability was enhanced by the presence of Ca2+ and diminished by the presence of EDTA. These data suggest a structural role for Ca2+ in this enzyme, similar to what has been observed with quinoprotein glucose and ethanol dehydrogenases.