DTDP glucose dehydratase

Fig. 4 Reaction mechanism of dTDP-glucose 4,6-dehydratase as proposed by Allard et al. [125], The first half of the reaction involves abstraction of a proton from the 4 hydroxyl group, and hydride transfer from C-4 to NAD. In the next step, a proton is removed from the C-5 atom of the sugar, and the C-6 hydroxyl group is protonated and eliminated as water. This yields the 4-keto-5,6-ene intermediate. The product is obtained after hydride transfer from NADH to carbon atom C-6 and protonation of carbon atom C-5...

Use of the enzyme dTDP-glucose-4,6-dehydratase on the substrate dTDP-glucose has generated the corresponding 6-deoxy-4-keto product - but it partly isomerized to the 3-keto compound during workup. 3-Azido-3-deoxy- and 3-deoxy-dTDP-glucose were also substrates affording the 3-modified 6-deoxy-4-keto products. ... 

Gross, J.W., Hegeman, A.D., Vestling, M.M., Frey, RA. (2000) Characterization of Enzymatic Processes by Rapid Mix-quench Mass Spectrometry The Case of dTDP-Glucose 4,6-Dehydratase. Biochemistry 39 13633-13640. 

A. Naundorf and W. KlaHke, Substrate specificity of native dTDP-D-glucose-4,6-dehydratase chemo-enzymatic syntheses of artificial and naturally occurring deoxy sugar, Carbohydr. Res., 285 (1996) 141-150. 

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. 

Further modifications of dihydrostreptose to streptose or S -hydroxystreptose seem to occur on the final condensation products only (cf. Section V.E), rather chan during synthesis of the dTDP-activated precursor. The 5 -hydroxy-group in antibiotic produced by S. glaucescem is unlikely to stem from the original D-glucose, since then the three enzymes StrE, StrM, and StrL would need to have acquired new substrates and even a new reaction mechanism in the case of the dTDP-o-glucose 4,6-dehydratase. 

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