Novel modified version of nonphosphorylated sugar metabolism - An alternative l-rhamnose pathway of Sphingomonas sp.

Abstract

Several bacteria, including Azotobacter vinelandii, possess an alternative pathway of l-rhamnose metabolism, which is different from the known bacterial pathway. In a previous article, a gene cluster related to this pathway was identified, consisting of the genes encoding the four metabolic enzymesl-rhamnose-1-dehydrogenase (LRA1), l-rhamnono-γ-lactonase (LRA2), l-rhamnonate dehydratase (LRA3) and l-2-keto-3-deoxyrhamnonate (l-KDR) aldolase (LRA4), by which l-rhamnose is converted into pyruvate and l-lactaldehyde, through analogous reaction steps to the well-known Entner-Doudoroff (ED) pathway. In this study, bioinformatic analysis revealed that Sphingomonas sp. possesses a gene cluster consisting of LRA1-3 and two genes of unknown function, LRA5 and LRA6. LRA5 catalyzed the NAD+-dependent dehydrogenation of several l-2-keto-3-deoxyacid-sugars, including l-KDR. Furthermore, the reaction product was converted to pyruvate and l-lactate by LRA6; this is different from the pathway of Azotobacter vinelandii. Therefore, LRA5 and LRA6 were assigned as the novel enzymes l-KDR 4-dehydrogenase and l-2,4-diketo-3-deoxyrhamnonate hydrolase, respectively. Interestingly, both enzymes were phylogenetically similar to l-rhamnose-1-dehydrogenase and d-2-keto-3-deoxyarabinonate dehydratase, respectively, and the latter was involved in the archeal nonphosphorylative d-arabinose pathway, which is partially analogous to the ED pathway. The introduction of LRA1-4 or LRA1-3, LRA5 and LAR6 compensated for the l-rhamnose-defective phenotype of an Escherichia coli mutant. Metabolic evolution and promiscuity between the alternative l-rhamnose pathway and other sugar pathways analogous to the ED pathway are discussed. © 2009 FEBS.