Identification of the enantiomeric nature of 2-keto-3-deoxy-galactonate in the catabolic pathway of 3,6-anhydro-l-galactose

A novel metabolic pathway of 3,6-anhydro-l-galactose (l-AHG), the main sugar component in red macroalgae, was first discovered in the marine bacterium Vibrio sp. EJY3. l-AHG is converted to 2-keto-3-deoxy-galactonate (KDGal) in two metabolic steps. Here, we identified the enantiomeric nature of KDGal in the l-AHG catabolic pathway via stereospecific enzymatic reactions accompanying the biosynthesis of enantiopure l-KDGal and d-KDGal. Enantiopure l-KDGal and d-KDGal were synthesized by enzymatic reactions derived from the fungal galacturonate and bacterial oxidative galactose pathways, respectively. KDGal, which is involved in the l-AHG pathway, was also prepared. The results obtained from the reactions with an l-KDGal aldolase, specifically acting on l-KDGal, showed that KDGal in the l-AHG pathway exists in an l-enantiomeric form. Notably, we demonstrated the utilization of l-KDGal by Escherichia coli for the first time. E. coli cannot utilize l-KDGal as the sole carbon source. However, when a mixture of l-KDGal and d-galacturonate was used, E. coli utilized both. Our study suggests a stereoselective method to determine the absolute configuration of a compound. In addition, our results can be used to explore the novel l-KDGal catabolic pathway in E. coli and to construct an engineered microbial platform that assimilates l-AHG or l-KDGal as substrates.

Automated summary

A new metabolic pathway of 3,6-anhydro-l-galactose (l-AHG) in red macroalgae was discovered, and the enantiomeric nature of 2-keto-3-deoxy-galactonate (KDGal), a product of l-AHG catabolism, was identified. The study also demonstrated the utilization of l-KDGal by Escherichia coli. These findings have implications in determining the absolute configuration of compounds and constructing engineered microbial platforms for utilizing l-AHG or l-KDGal as substrates.