An oxidative metabolic pathway of 4-deoxy-L-erythro-5-hexoseulose uronic acid (DEHU) from alginate in an alginate-assimilating bacterium

Nishiyama et al. report the biochemical pathway for the conversion of DEHU to alpha-ketoglutarate in the alginate-assimilating marine bacterium Flavobacterium sp. strain UMI-01. They show that a parallel oxidative pathway besides the reductive route is possible, and provide evidence that this pathway might be conserved in other alginate-degrading bacteria. Alginate-assimilating bacteria degrade alginate into an unsaturated monosaccharide, which is converted into 4-deoxy-L-erythro-5-hexoseulose uronic acid (DEHU). DEHU is reduced to 2-keto-3-deoxy-D-gluconate by a DEHU-specific reductase using NAD(P)H. This is followed by pyruvate production via the Entner-Doudoroff pathway. Previously, we identified FlRed as a DEHU reductase in an alginate-assimilating bacterium, Flavobacterium sp. strain UMI-01. Here, we showed that FlRed can also catalyze the oxidation of DEHU with NAD(+), producing 2-keto-3-deoxy-D-glucarate (KDGR). FlRed showed a predilection for NADH and NAD(+) over NADPH and NADP(+), respectively, and the K-m value for NADH was approximately 2.6-fold less than that for NAD(+). Furthermore, we identified two key enzymes, FlDet and FlDeg, for KDGR catabolism. FlDet was identified as an enzyme of the ribonuclease activity regulator A family, which converts KDGR to alpha-ketoglutaric semialdehyde (alpha-KGSA). FlDeg, a type II alpha-KGSA dehydrogenase, generated alpha-ketoglutaric acid by oxidizing the aldehyde group of alpha-KGSA using NAD(P)(+). Consequently, unlike the conventional DEHU reduction pathway, DEHU can be directly converted to alpha-ketoglutaric acid without consuming NAD(P)H. Alginate upregulated the expression of not only FlRed and two enzymes of the DEHU-reduction pathway, but also FlDet and FlDeg. These results revealed dual pathways of DEHU metabolism involving reduction or oxidation by FlRed.