InterPro domain: IPR003500

This entry represents the sugar isomerase enzymes ribose 5-phosphate isomerase B (RpiB), galactose isomerase subunit A (LacA) and galactose isomerase subunit B (LacB).

Galactose-6-phosphate isomerase ( 5.3.1.26 ) is a heteromultimeric protein consisting of subunits LacA and LacB, and catalyses the conversion of D-galactose 6-phosphate to D-tagatose and 6-phosphate in the tagatose 6-phosphate pathway of lactose catabolism [ 1 ]. Galactose-6-phosphate isomerase is induced by galactose or lactose.

Ribose 5-phosphate isomerase ( 5.3.1.6 ) forms a homodimer and catalyses the interconversion of D-ribose 5-phosphate and D-ribulose 5-phosphate in the non-oxidative branch of the pentose phosphate pathway. This reaction permits the synthesis of ribose from other sugars, as well as the recycling of sugars from nucleotide breakdown. Two unrelated enzymes can catalyse this reaction: RpiA (found in most organisms) and RpiB (found in some bacteria and eukaryotes). RpiB is also involved in metabolism of the rare sugar, allose, in addition to ribose sugars. The structures of RpiA and RpiB are distinct, RpiB having a Rossmann-type alpha/beta/alpha sandwich topology [ 2 ].

1. Nucleotide and deduced amino acid sequences of the lacR, lacABCD, and lacFE genes encoding the repressor, tagatose 6-phosphate gene cluster, and sugar-specific phosphotransferase system components of the lactose operon of Streptococcus mutans. J. Bacteriol. 174, 6159-70
2. The 2.2 A resolution structure of RpiB/AlsB from Escherichia coli illustrates a new approach to the ribose-5-phosphate isomerase reaction. J. Mol. Biol. 332, 1083-94