A dual L-glucose/L-galactose catabolic pathway in <i>Luteolibacter</i> species strain LG18

The L-glucose catabolic pathway of <i>Luteolibacter</i> sp. strain LG18 was determined. L-glucose dehydrogenase (LguA) and L-gluconate dehydrogenase (LguD), purified from the cell extract of strain LG18, convert L-glucose to 5-keto-L-gluconate via L-gluconate, and these recombinant enzymes also utilize L-galactose and L-galactonate, respectively. Genes encoding these enzymes are both located in the gene cluster, <i>lguABCDEF,</i> which includes other genes possibly involved in L-galactose catabolism. After oxidation of L-gluconate, 5-keto-L-gluconate is converted to D-tagaturonate by LguG, a C-4 epimerase, determined with the recombinant enzyme. The subsequent LG18 reactions are likely to proceed in the same way as <i>Escherichia coli</i> L-galactonate catabolism, wherein LguC reduces C-5 to produce D-altronate that is dehydrated by LguB to produce 2-keto-3-deoxy-D-gluconate (KDG). LguH then phosphorylates KDG C-6 to produce KDG-6-phosphate, and an aldolase reaction driven by LguE produces D-glyceraldehyde-3-phosphate and pyruvate. Both <i>lguG</i> and <i>lguH</i> lie outside the <i>lguABCDEF</i> cluster, and LguH had a novel preference in utilizing pyrophosphate as a phosphate donor rather than ATP. Gene disruption studies indicated that, with the exception of <i>lguG</i>, which is involved only in L-glucose catabolism, the identified genes are indeed responsible for both L-glucose and L-galactose catabolism, indicative of a dual L-glucose/L-galactose catabolic pathway governed by a single set of genes. All the orthologs in this pathway are conserved in several <i>Luteolibacter</i> species, which also utilize L-glucose, suggesting that the same catabolic pathway is present in this genus.IMPORTANCEL-glucose is presumably not present in natural environments, and to date, L-glucose catabolism has only been reported for a <i>Paracoccus laeviglucosivorans</i> strain 43P. The <i>Luteolibacter</i> strain LG18 differs taxonomically from 43P at the phylum level, and its L-glucose catabolic pathway differs from that of 43P at later steps from the C-4 epimerization reaction. In addition, most genes that drive LG18 L-glucose catabolism are also responsible for L-galactose catabolism, indicating the presence of a dual L-glucose/L-galactose catabolic pathway. This report contributes to a better understanding of homochirality in sugar catabolism, especially catabolism of glucose.