C-5-Epimerase

Ertesvag, H., Doseth, B., Larsen, B., Skjak-Br ek, G. and Valla, S. (1994) Cloning and expression of anAzotobacter vinelandii maimuronan C-5 epimerase gene. J. Bacteriol. 

Examination, by n.m.r. spectroscopy, of the products formed by incubation of D-mannuronan C-5-epimerase with alginic acid containing 13% of L-guluronate indicated that reaction adjacent to an existing L-gulosyluronic residue was favored. The L-guluronate content increased171 to 59%. 

Malmstrom A, Aberg L. Biosynthesis of dermatan sulfate -assay and properties of the uronosyl C-5 epimerase. Biochem. J. 1982 201(3) 489-493. 

Polysaccharides Containing Uronic Acids That Have Some of Their Carhoxyl Groups Inverted hy a C-5 Epimerase... 

Dermatan sulfate is derived from chondroitin 4-sulfate by the action of a C-5 epimerase that inverts the carboxyl group of the /3-D-glucuronic acid, giving the very rare sugar, a-L-idopy-... 

Primary alcohol groups can be exclusively oxidized to aldehyde groups with pyridinium dichromate [149,150] and to carboxyl groups with the 2,2,6,6-tetramethyl-1-piperidine oxoammonium ion (TEMPO) [151]. The aldehydes can then be reduced to primary alcohols by reaction with NaB H4 [150,152], giving radiolabeled H-starch and the carboxyl group can be inverted by the action of Azotobacter vinlandii poly- 8-D-marmuronic acid C-5-epimerase to give L-iduronic acid [153]. 

Reaction M is catalysed by GDP-mannose dehydrogenase [EC 1.1.1.132] for which the co-factor is two moles of NAD+ and reaction N by GDP-mannuronic acid mannuronan mannosyl transferase [EC 2.4.1.33]. ManA residues in mannuronan are convertible to L-GulA residues by a C-5 epimerase for which the substrate is polymeric [243, 244, 253]. At high concentrations of Ca2+ a polymer with a high L-GulA content is produced. There is evidence of a multiple attack mechanism. The epimerase is located extra-cellularly and it does not react with ManA residues in the polymer that are acetylated. In a cell free system of the brown seaweed... 

Scheme 1.—Biosynthetic Transformations of the Nonsulfated Disaccharide Units of N-Acetylheparosan (1) to the Trisulfated Disaccharide Units (5) of the Regular Regions of Heparin, Through Sequential Action of N-Deacetylase - N-Sulfotransferase (a,b), Clycosyl-uronic C-5-Epimerase (c), and O-Sulfotransferases (d.e)." (C = D-glucuronic acid I = l-iduronic acid lu = L-iduronic acid 2-suIfate A a = 2-acetamido-2-deoxy-D-glucose Ans 2 -deoxy-2-sulfoamino-D-glucose Ansds = 2-deoxy-2-suIfoamino-D-glucose 6-sulfate.)...

Isomerization of the anomeric carbon is catalyzed by mutarotase (see Chapter 3). The inversion of the configuration at C-4 or C-5 is catalyzed by C-4 epimerase and C-5 epimerase, respectively. 

Nyvall, P., Corre, E., Boisset, C., Barbeyron, T., Rousvoal, S., Scornet, D., Kloareg, B., and Boyen, C. (2003) Characterization of mannuronan C-5 epimerase genes from the brown alga Laminaria digitata. Plant Physiol, 133, 726-735. 

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