Molecular cloning and characterization of vanillin dehydrogenase from Streptomyces sp. NL15-2K

Streptomyces sp. NL15-2K, previously isolated from the forest soil, features an extensive catabolic network for lignin-derived aromatic compounds, including pathways transforming ferulic acid to vanillin, vanillic acid, and protocatechuic acid. To successfully use Streptomyces sp. NL15-2K as a biocatalyst for vanillin production, it is necessary to characterize the vanillin dehydrogenase (VDH) that degrades the produced vanillin to vanillic acid, as well as the gene encoding this enzyme. Here, we cloned the VDH-encoding gene (vdh) from strain NL15-2K and comprehensively characterized its gene product.

The vdh open reading frame contains 1488 bp and encodes a 496-amino-acid protein with a calculated molecular mass of 51,705 Da. Whereas the apparent native molecular mass of recombinant VDH was estimated to be 214 kDa by gel filtration analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a subunit molecular mass of ca. 56 kDa, indicating that VDH is a homotetramer. The recombinant enzyme showed optimal activity at 45 °C and pH 9.5. The VDH substrate specificity followed this order: vanillin (100%) > protocatechualdehyde (91%) > benzaldehyde (79%) > p-hydroxybenzaldehyde (56%) > isovanillin (49%) ≈ salicylaldehyde (48%) > anisaldehyde (15%) ≈ veratraldehyde (12%). Although peptide mass fingerprinting and BLAST searches indicated that this enzyme is a salicylaldehyde dehydrogenase (SALDH), the determined kinetic parameters clearly demonstrated that the enzyme is a vanillin dehydrogenase. Lastly, phylogenetic analysis revealed that VDH from Streptomyces sp. NL15-2K forms an independent branch in the phylogenetic tree and, hence, is evolutionarily distinct from other VDHs and SALDHs whose activities have been confirmed experimentally.

Our findings not only enhance the understanding of the enzymatic properties of VDH and the characteristics of its amino acid sequence, but also contribute to the development of Streptomyces sp. NL15-2K into a biocatalyst for the biotransformation of ferulic acid to vanillin.