Disaccharide synthase

Crowell, D.N., Anderson, M.S., Raetz, C.R.H. Molecular cloning of the genes for lipid A disaccharide synthase and UDP- V-acetylglucosamine acyltransferase in Escherichia coli. J Bacteriol 168 (1986) 152-159. 

Metzger, L.E., Raetz, C.R. Purification and characterization of the lipid A disaccharide synthase (LpxB) from Escherichia coli, a peripheral membrane protein. Biochemistry 48 (2009) 11559-11571. 

Lipid A disaccharide synthase LpxB 2.1.4.182 Haemophilus influenzae P45011 44... 

Raetz, C. R. H. (1992). Lipid A disaccharide synthase from Escherichia coli. Meth. Enzymol. 209, 455- 466,... 

After glucose synthesis in photosynthesis, the disaccharide sucrose (a-D-Glc(l —> 2)(3-D-Fru) is used as a readily transportable sugar. Sucrose synthesis successively involves the following UDP-glucose + fructose-6-phosphate —> sucrose-6-phosphate + UDP [via sucrose phosphate synthase] sucrose-6-phosphate + H20 —> sucrose + P [via sucrose-6-phosphatase]. 

Sucrose is a highly soluble disaccharide that provides a mobile energy source for all the plant cells. Sugar cane stores large amounts of sucrose in its leaves and stalk, whereas sugar beet stores it in roots. All plants make sucrose from two molecules of fructose 6-phosphate. One molecule is activated with UDP and isomerized to UDP-glucose. Sucrose 6-phosphate synthase reacts with UDP-glucose and fructose 6-phosphate to make sucrose 6-phosphate. The latter then reacts with a phosphatase to produce sucrose (Fig. 2.9). 

UDP-A-acetyl-5-thio-D-galactosamine (UDP-5.SGalNAc) was active as a donor substrate of lactose synthase, the complex of galactosyltransferase (EC 2.4.1.38) and lactalbumin. By tliis method the disaccharide /9-5SGalNAc/9(1 4)GlcNAc was prepared. UDP-5.S GalNAc was synthesized from an A -acetyIgalactosaininc... 

Tylosin contains a 16-membered lactone to which three deoxysugar moieties are attached. A disaccharide D-mycaminosyl-D-mycarose is linked at C5 and a D-mycinose at C23. Several genes have been cloned that encode deoxysugar biosynthetic enzymes most of them are involved in dTDP-D-mycaminose biosynthesis [48,49]. TylAl and TylA2 correspond to the dTDP-o-glucose synthase and dTDP-4,6-dehydratase, respectively. A putative isomerase not yet identified acts on dTDP-4-keto-6-deoxy-D-glucose, and then aminotransferase (TyiB) and C-methyltransferase (TylMl) to generate the final dTDP-D-mycaminose. 

Fig. 30.6. Lactose synthesis. Lactose is a disaccharide composed of galactose and glucose. UDP-galactose for the synthesis of lactose in the mammary gland is usually formed from the epimerization of UDP-glucose. Lactose synthase attaches the anomeric carbon of the galactose to the C4 alcohol group of glucose to form a glycosidic bond. Lactose synthase is composed of a galactosyltransferase and a-lactalbumin, which is a regulatory subunit.

Strong acids cleave C. into D- glucosamine (chitosa-mine) and acetic acid. On decomposition with alkalis, acetates and the weakly basic deacetylated, partially depolymerized, and crystallizable chitosan are formed the latter can form gels and films. The chiti-nases (EC 3.2.1.14) found in snail stomachs, some mold fungi, and bacteria can - like some lysozymes -dissolve C. The thus formed chitobiose, the disaccharide of P-1,4-linked NAG, is then cleaved to monomers by chitobiase (EC 3.2.1.30). Tlie formation of C. is catalyzed by the enzyme chitin synthase (EC 2.4.1.16). The activity of, e.g., the insecticide diflubenzurone is based on inhibition of this enzyme. 

Experiments with recombinant PmHAS demonstrated that single sugars are added to the growing chain sequentially the intrinsic fidelity of each transfer step assures the production of the GAG repeat structure. Another potential mechanism, the simultaneous addition of a disaccharide unit to the nascent chain, does hot occur. The odier synthases, PmCS, PmHSl and 2, behave in a similar fashion adding saccharides one at a time. 

The native bacterial GAG glycosyltransferase polypeptides are associated with the cell membranes this localization makes sense with respect to synthesis of polysaccharide molecules destined for the cell surface. The first Pasteurella GAG synthase to be identified was the 972-residue HA synthase from Type A strains, pmHAS (Table I). This single polypeptide transfers both sugars, GlcNAc and GlcUA, to form the HA disaccharide repeat (i). 

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