Substrate-specificity of benzyl alcohol dehydrogenase and benzaldehyde dehydrogenase encoded by TOL plasmid pWW0. Metabolic and mechanistic implications

The substrate-specificities of benzyl alcohol dehydrogenase and benzaldehyde dehydrogenase, encoded by TOL plasmid pWW0 of Pseudomonas putida mt-2, were determined. The rates of benzyl alcohol dehydrogenase-catalysed oxidation of substituted benzyl alcohols and reduction of substituted benzaldehydes were independent of the electronic nature of the substituents at positions 3 and 4. Substitutions at position 2 of benzyl alcohol affected the reactivity of benzyl alcohol dehydrogenase: the velocity of the benzyl alcohol dehydrogenase-catalysed oxidation was lower for compounds possessing electron-withdrawing substitutions. In the reverse reaction of benzyl alcohol dehydrogenase, none of the substitutions tested influenced the apparent kcat. values. The rates of benzaldehyde dehydrogenase-catalysed oxidation of substituted benzaldehydes were influenced by the electronic nature of the substitutions: electron-withdrawing groups at positions 3 and 4 favoured the oxidation of benzaldehydes. Substitution at position 2 of benzaldehyde greatly diminished the benzaldehyde dehydrogenase-catalysed oxidation. Substitution at position 2 with electron-donating groups essentially abolished reactivity, and only substitutions that were strongly electron-withdrawing, such as nitro and fluoro groups, permitted enzyme-catalysed oxidation. The influence of the electronic nature and the position of substitutions on the aromatic ring of the substrate on the velocity of the catalysed reactions provided some indications concerning the transition state during the oxidation of the substrates, and on the rate-limiting steps of the enzymes. Pseudomonas putida mt-2 containing TOL plasmid pWW0 cannot grow on toluene derivatives substituted at position 2, nor can it grow on 2-substituted benzyl alcohols or aldehydes. One of the reasons for this may be the substrate-specificities of the benzyl alcohol dehydrogenase and benzaldehyde dehydrogenase.