Unusual metabolism of 3,6-anhydro-L-galactose in Vibrio sp EJY3 and in E-coli containing two Vibrio sp EJY3 genes

Abstract

Recently, Yun et al. [Envron. Microbiol. 17: 1677-1688 (2015)] proposed a metabolic pathway of 3,6- anhydro-L-galactose (L-AnG) in Vibrio sp. EJY3. The sequence of Yun's pathway is 3,6-anhydro-L-galactose -> 3,6-anhydro-galactonate -> 2-keto-3-deoxy-galactonate -> DeLey-Doudoroff pathway. This pathway differs significantly from one that has been established by Lee et al. [Biotechnol. Bioproc. Eng. 19: 866-878 (2014)], and which has been detected in agar-degrading bacteria such as Postechiella marina M091, Pseudoalteromonas atlantica T6c, and Streptomyces coelicolor A3(2). The sequence of Lee's pathway is 3,6-anhydro-L-galactose -> 3,6-anhydro-Lgalactonate -> 2-keto-3-deoxy-L-galactonate -> 2,5-diketo- 3-deoxy-L-galactonate -> 2-keto-3-deoxy-D-gluconate -> 2-keto-3-deoxy-6-phospho-D-gluconate -> pyruvate + Dglyceraldehyde- 3-phosphate. Because Yun's L-AnG pathway is connected to the DeLey-Doudoroff pathway and 2-keto- 3-deoxy-D-galactonate (D-KDGal) is an intermediate in the DeLey-Doudoroff pathway, the 2-keto-3-deoxy-galactonate in the Yun's pathway must be D-KDGal. On the contrary, Lee et al. showed that the product of the reaction catalyzed by the second-step enzyme is 2-keto-3-deoxy-L-galactonate (L-KDGal). The E. coli genome contains the genes that enable conversion of D-KDGal to glycolysis intermediates via the DeLey-Doudoroff pathway. However, if the reaction product of the second-step enzyme is L-KDGal, additional genes are required for the conversion of L-KDGal to glycolysis intermediates. Here I argue that the validity of the following claims by Yun et al. is very questionable: (1) the formation of D-KDGal during the metabolism of L-AnG in Vibrio sp. EJY3 and (2) the production of ethanol from L-AnG using the DeLey-Doudoroff pathway in E. coli.