The formaldehyde dehydrogenase of Rhodococcus erythropolis, a trimeric enzyme requiring a cofactor and active with alcohols

Abstract

During growth on compounds containing methyl groups, a formaldehyde dehydrogenase is induced in the gram-positive bacteria, R. erythropolis. This formaldehyde dehydrogenase was purified to homogeneity using affinity chromatography and permeation chromatography. The isoelectric point of the enzyme was 4.7. The MW of the native enzyme was determined as 130,000 g/mol. Sodium dodecyl sulfate gel electrophoresis yielded a single subunit with a MW of 44,000 g/mol. These results, together with cross-linking experiments which yielded monomer, dimer and trimer bands, are consistent with a trimeric subunit structure of the formaldehyde dehydrogenase. A heat-stable cofactor of low MW was required for activity with formaldehyde as substrate. This cofactor was found to be oxidizable, but active only in its reduced form. Preparative electrofocusing revealed that the cofactor is a weak acid with a pK of .apprx. 6.5. The enzyme was active with the homologous series of the primary alcohols, ethanol up to octanol, without requiring the presence of the cofactor. A mutant without formaldehyde dehydrogenase activity was not impaired in its growth with ethanol as substrate. It is suggested that the alcohols mimic the true substrate of the formaldehyde dehydrogenase, which could be a hydroxymethyl derivative of the cofactor, resulting from the addition of formaldehyde.