Novel modified version of nonphosphorylated sugar metabolism--an alternative L-rhamnose pathway of Sphingomonas sp.
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 enzymes L-rhamnose-1-dehydrogenase (LRA1), L-rhamnono-gamma-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.
Watanabe
Seiya
S
Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan. irab@iae.kyoto-u.ac.jp
Makino
Keisuke
K
D000595:Amino Acid Sequence; D050260:Carbohydrate Metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D008969:Molecular Sequence Data; D005810:Multigene Family; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism; D010766:Phosphorylation; D012210:Rhamnose / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D020578:Sphingomonas / Q000235:genetics / Q000378:metabolism*
D010088:Oxidoreductases; D012210:Rhamnose