L-(-)-Sorbose

L-(-)-Sorbose is a ketohexose. It has been described as a rare sugar, a monosaccharide of low natural abundance.

Oxidation to 2-keto-L-gulonic acid. Some acetic acid bacteria oxidise L-sorbose to 2-keto-L-gulonic acid (2-KLG) through L-sorbose and L-sorbosone dehydrogenases. In Gluconobacter oxydans T-100, L-sorbose dehydrogenase gives L-sorbosone, and L-sorbosone dehydrogenase converts L-sorbosone to 2-KLG. Both enzymes have been purified from this strain and their genes cloned. 2-KLG is the direct precursor of vitamin C (L-ascorbic acid) in industrial synthesis. Industrially, 2-KLG is mainly made from L-sorbose by Ketogulonicigenium vulgare aided by Bacillus megaterium. In K. vulgare WSH-001, a group of pyrroloquinoline quinone (PQQ)-dependent dehydrogenases is involved in the conversion of L-sorbose to 2-KLG. In another bacterial strain, a single quinoprotein dehydrogenase catalyses both the oxidation of L-sorbose to L-sorbosone and that of L-sorbosone to 2-KLG.

Rare sugar biocatalysis. In one study, directed evolution of a thermostable D-tagatose epimerase gave a variant with improved activity for the C-3 epimerisation of L-sorbose to L-tagatose. In a two-step enzymatic route to the rare aldohexose L-galactose, L-sorbose was first epimerised to L-tagatose with an immobilised epimerase, and an immobilised L-rhamnose isomerase then converted L-tagatose to L-galactose. An L-ribose isomerase of broad substrate specificity has been shown to catalyse the isomerisation between L-gulose and L-sorbose. Pyranose 2-oxidase oxidises L-sorbose regioselectively to 5-keto-D-fructose, a starting material for pyrrolidine iminosugars.

Iminosugar synthesis. 1-Deoxynojirimycin (1-DNJ), a glycoprocessing inhibitor, has been synthesised from an intermediate prepared from L-sorbose. In one strategy, organometallic reagents are added to an L-sorbose-derived imine, and internal reductive amination then gives nojirimycin C-glycosides with full stereocontrol. L-Sorbose is also a starting material for pyrrolidine iminosugars. Derivatives of 2,5-dideoxy-2,5-imino-D-mannitol (DMDP) have been prepared from it, and so has 2,5-dideoxy-2,5-imino-D-glucitol (DGDP), the C-2 diastereomer of DMDP. Polyhydroxylated quinolizidines made from an L-sorbose-derived ketonitrone were found to be selective α-glucosidase inhibitors.

Fungal biology. L-Sorbose induces hyperbranching of hyphae in the fungus Neurospora crassa, which results in colonial growth. In N. crassa, acid hydrolysates of cell walls from sorbose-grown hyphae contain more glucosamine and less glucose than those from normally growing hyphae. In N. crassa, sor-4, a gene encoding a glucose sensor that acts in carbon catabolite repression, has been identified as a sorbose resistance gene.