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 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 . Galactose-6-phosphate isomerase is induced by galactose or lactose. This entry represents the LacB subunit.
Ribose 5-phosphate isomerase 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 .