Ginsenosides are the principal components responsible for the pharmaceutical activities of ginseng. The minor ginsenosides, which are also pharmaceutically active, can be produced via the hydrolysis of the sugar moieties in the major ginsenosides using acid hydrolytic, heating, microbial, and enzymatic transformation techniques. The enzymatic method has a profound potential for ginsenoside transformation, owing to its high specificity, yield, and productivity, and this method is increasingly being recognized as a useful tool in structural modification and metabolism studies. In this article, the transformation methods of ginsenosides, the characterization of microbial glycosidases with ginsenoside hydrolyzing activities, and the enzymatic production of minor ginsenosides are reviewed. Moreover, the conversions of ginsenosides using cell extracts from food microorganisms and recombinant thermostable beta-D-glycosidases are proposed as feasible methods for use in industrial processes.

A putative N-acyl-D-glucosamine 2-epimerase from Caldicellulosiruptor saccharolyticus was cloned and expressed in Escherichia coli. The recombinant enzyme was identified as a cellobiose 2-epimerase by the analysis of the activity for substrates, acid-hydrolyzed products, and amino acid sequence. The cellobiose 2-epimerase was purified with a specific activity of 35 nmol min(-1) mg(-1) for D-glucose with a 47-kDa monomer. The epimerization activity for D-glucose was maximal at pH 7.5 and 75°C. The half-lives of the enzyme at 60°C, 65°C, 70°C, 75°C, and 80°C were 142, 71, 35, 18, and 4.6 h, respectively. The enzyme catalyzed the epimerization reactions of the aldoses harboring hydroxyl groups oriented in the right-hand configuration at the C2 position and the left-hand configuration at the C3 position, such as D-glucose, D-xylose, L-altrose, L-idose, and L-arabinose, to their C2 epimers, such as D-mannose, D-lyxose, L-allose, L-gulose, and L-ribose, respectively. The enzyme catalyzed also the isomerization reactions. The enzyme exhibited the highest activity for mannose among monosaccharides. Thus, mannose at 75 g l(-1) and fructose at 47.5 g l(-1) were produced from 500 g l(-1) glucose at pH 7.5 and 75°C over 3 h by the enzyme.

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The production of compound K and aglycon protopanaxadiol (APPD) from ginsenoside Rd and ginseng root extract was performed using a recombinant β-glycosidase from Pyrococcus furiosus. The activity for Rd was optimal at pH 5.5 and 95°C with a half-life of 68 h at 95°C. β-Glycosidase converted Rb(1), Rb(2), Rc, and Rd to APPD via compound K. With increases in the enzyme activity, the productivities of compound K and APPD increased. The substrate concentration was optimal at 4.0 mM Rd or 10% (w/v) ginseng root extract; 4 mM of Rd was converted to 3.3 mM compound K with a yield of 82.5% (mol/mol) and a productivity of 2,010 mg l(-1) h(-1) at 1 h and was hydrolyzed completely to APPD with 364 mg l(-1) h(-1) after 5 h. Rb(1), Rb(2), Rc, and Rd at 3.9 mM in 10% ginseng root extract were converted to 3.1 mM compound K with 79.5% and 1,610 mg l(-1) h(-1) at 1.2 h and were hydrolyzed completely to APPD with 300 mg l(-1) h(-1) after 6 h. The concentrations and productivities of compound K and APPD in the present study are the highest ever reported.

The uncharacterized gene previously proposed as a mannose-6-phosphate isomerase from Bacillus subtilis was cloned and expressed in Escherichia coli. The maximal activity of the recombinant enzyme was observed at pH 7.5 and 40°C in the presence of 0.5 mM Co 2؉ . The isomerization activity was specific for aldose substrates possessing hydroxyl groups oriented in the same direction at the C-2 and C-3 positions, such as the D and L forms of ribose, lyxose, talose, mannose, and allose. The enzyme exhibited the highest activity for L-ribulose among all pentoses and hexoses. Thus, L-ribose, as a potential starting material for many L-nucleoside-based pharmaceutical compounds, was produced at 213 g/liter from 300-g/liter L-ribulose by mannose-6-phosphate isomerase at 40°C for 3 h, with a conversion yield of 71% and a volumetric productivity of 71 g liter ؊1 h ؊1 .L-Ribose is a potential starting material for the synthesis of many L-nucleoside-based pharmaceutical compounds, and it is not abundant in nature (5, 19). L-Ribose has been produced mainly by chemical synthesis from L-arabinose, L-xylose, Dglucose, D-galactose, D-ribose, or D-mannono-1,4-lactone (2,17,23). Biological L-ribose manufacture has been investigated using ribitol or L-ribulose. Recently, L-ribose was produced from ribitol by a recombinant Escherichia coli containing an NAD-dependent mannitol-1-dehydrogenase (MDH) with a 55% conversion yield when 100 g/liter ribitol was used in a 72-h fermentation (18). However, the volumetric productivity of L-ribose in the fermentation is 28-fold lower than that of the chemical method synthesized from L-arabinose (8). L-Ribulose has been biochemically converted from L-ribose using an Lribose isomerase from an Acinetobacter sp. (9), an L-arabinose isomerase mutant from Escherichia coli (4), a D-xylose isomerase mutant from Actinoplanes missouriensis (14), and a D-lyxose isomerase from Cohnella laeviribosi (3), indicating that L-ribose can be produced from L-ribulose by these enzymes. However, the enzymatic production of L-ribulose is slow, and the enzymatic production of L-ribose from L-ribulose has been not reported.Sugar phosphate isomerases, such as ribose-5-phosphate isomerase, glucose-6-phosphate isomerase, and galactose-6-phosphate isomerase, work as general aldose-ketose isomerases and are useful tools for producing rare sugars, because they convert the substrate sugar phosphates and the substrate sugars without phosphate to have a similar configuration (11,12,21,22). L-Ribose isomerase from an Acinetobacter sp. (9) and D-lyxose isomerase from C. laeviribosi (3) had activity with L-ribose, D-lyxose, and D-mannose. Thus, we can apply mannose-6-phosphate (EC 5.3.1.8) isomerase to the production of L-ribose, because there are no sugar phosphate isomerases relating to L-ribose and D-lyxose. The production of the expensive sugar L-ribose (bulk price, $1,000/kg) from the rare sugar L-ribulose by mannose-6-phosphate isomerase may prove to be a valuable industrial process, because we have produced Lribulose from the ch...

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A putative L-rhamnose isomerase (RhaA) from Thermotoga maritima was purified with a specific activity of 55 U/mg by His-Trap affinity chromatography. The native enzyme was estimated as a 46 kDa tetramer by gel filtration chromatography. The half-lives of the enzyme at 75, 80, 85, 90 and 95°C were 773, 347, 187, 118, and 65 h, respectively, indicating that it is the most thermostable of all RhaAs. Under the optimum conditions of pH 8.0, 85°C, and 1 mM Mn(2+), RhaA with 100 U enzyme/ml converted 500 L-xylulose/l to 225 g/l L-lyxose after 3 h, and converted 500 L-fructose/l to 175 g/l L-mannose after 5 h.

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