Characterization of Sphingomonas aldehyde dehydrogenase catalyzing the conversion of various aromatic aldehydes to their carboxylic acids

An aldehyde dehydrogenase gene, designated phnN, was isolated from a genome library of the 1,4-dimethylnaphthalene-utilizing soil bacterium, Sphingomonas sp. 14DN61. Escherichia coli expressing the phnN gene converted 1,4-dihydroxymethylnaphthalene to 1-hydroxymethyl-4-naphthoic acid. The putative amino acid sequence of the phnN gene product had 31–42% identity with those of NAD+-dependent short-chain aliphatic aldehyde dehydrogenases and a secondary alcohol dehydrogenase. The NAD(P)+-binding site and two consensus sequences involved in the active site for aldehyde dehydrogenase are conserved among these proteins. The PhnN enzyme purified from recombinant E. coli showed broad substrate specificity towards various aromatic aldehydes, i.e., 1- and 2-naphaldehydes, cinnamaldehyde, vanillin, syringaldehyde, benzaldehyde and benzaldehydes substituted with a hydroxyl, methyl, methoxy, chloro, fluoro, or nitro group were converted to their corresponding carboxylic acids. Interestingly, E. coli expressing phnN was able to biotransform a variety of not only aromatic aldehydes, but also aromatic alcohols to carboxylic acids.