Novel CAD-like enzymes from Escherichia coli K-12 as additional tools in chemical production

In analyzing the reductive power of Escherichia coli K-12 for metabolic engineering approaches, we identified YahK and YjgB, two medium-chain dehydrogenases/reductases subgrouped to the cinnamyl alcohol dehydrogenase family, as being important. Identification was achieved using a stepwise purification protocol starting with crude extract. For exact characterization, the genes were cloned into pET28a vector and expressed with N-terminal His tag. Substrate specificity studies revealed that a large variety of aldehydes but no ketones are converted by both enzymes. YahK and and YjgB strongly preferred NADPH as cofactor. The structure of YjgB was modeled using YahK as template for a comparison of the active center giving a first insight to the different substrate preferences. The enzyme activity for YahK, YjgB, and YqhD was determined on the basis of the temperature. YahK showed a constant increase in activity until 60 C, whereas YjgB was most active between 37 and 50 C. YqhD achieved the highest activity at 50 C. Comparing YjgB and Yahk referring to the catalytic efficiency, YjgB achieved for almost all substrates higher rates (butyraldehyde 221 s^-1 mM^-1, benzaldehyde 1,305 s ^-1 mM^-1). Exceptions are the two substrates glyceraldehydes (no activity for YjgB) and isobutyraldehyde (YjgB 0.26 s ^-1 mM^-1) which are more efficiently converted by YahK (glyceraldehyde 2.8 s^-1 mM^-1, isobutyraldehyde 14.6 s ^-1 mM^-1). YahK and even more so YjgB are good candidates for the reduction of aldehydes in metabolic engineering approaches and could replace the currently used YqhD.