DTDP-L-rhamnose

An enzyme was purified that catalyzes epimerization at C-3 and C-5 of the hexosylulose group in dTDP-6-deoxy-D-xy/o-hexos-4-ulose (7a), resulting in formation of the 6-deoxy-L-/yxo-hexosyl-4-ulose derivative188 191 (8a). Further reduction of the latter with NADPH leads to dTDP-L-rhamnose (dTDP-Rha).23,147,190,192... 

Li, Q., Reeves, P.R. Genetic variation of dTDP-L-rhamnose pathway genes in Salmonella enterica. Microbiology 146 (2000) 2291-2307. 

On the basis of the observation that dTDP-D-mannose and dTDP-L-rhamnose occur in Streptomyces griseus37 and that cell-free preparations from this organism can convert both dTDP-D-glucose and dTDP-D-mannose into dTDP-L-rhamnose,38 it was suggested that the biosynthesis of streptose and that of L-rhamnose are related. 

It was, therefore, assumed that separation into two, or more, active protein-fractions had occurred on the Sephadex column. As the biosynthesis of dTDP-L-rhamnose and dTDP-L-dihydrostreptose are related,37,39 and as, moreover, a dTDP-L-lyxo-4-hexulose 3,5-epimerase is necessary for the formation of dTDP-L-rhamnose,1718 the Sephadex G-100 fractions were assayed for the presence of the 3,5-epimerase by... 

By analogy to results obtained in studies on the biosynthesis of dTDP-L-rhamnose,18 dTDP-6-deoxy-L-talose,17 and GDP-L-fucose,41 it seems very likely that dTDP-6-deoxy-L-/yxo-hexos-4-ulose (21), formed by the 3,5-epimerase reaction, remains enzyme-bound. 

Glucose-1-phosphate thymidylyltransferase catalyzes the first of four steps in the biosynthesis of dTDP-L-rhamnose. The reaction is between dTTP and a-D-glucose 1-phosphate to form dTDP-D-glucose and PPj. 

The keto-sugar nucleotide dTDP-L-rhamnose is synthesized from dTDP-4-keto-6-deoxy-D-glucose by dTDP-i-rhamnose synthase [104, 105). The enzyme consists of two components, a cofactor independent epimerase and an NADH-dependent reductase. The epimerase component is inactive without the reductase component. The mechanism involves epimerization of two stereocenters flanking a carbonyl group, via sequential deprotonation/reprotonation, with two enol intermediates. Complete solvent isotope incorporation into both epimerized stereocenters was observed, and primary substrate-derived KIEs have been determined [104],... 

Further progress has been made in the more complex conversions, involving the change in configuration at several carbon atoms, that occur in the process of formation of 6-deoxyhexoses, namely, the conversion of GDP-D-mannose into GDP-L-fucose, and dTDP-D-glucose into dTDP-L-rhamnose. 

Melo and Glaser have also shown that an enzyme system from Pseudomonas aeruginosa catalyzes the conversion of dTDP-6-deoxy-D-xi/lo-hexos-4-ulose into dTDP-L-rhamnose by the following reaction. 

When only dTDP-L-rhamnose and UDP-D-galactose were added... 

MK730-62F2. In vitro studies demonstrated that cpz31 glycosylated the CPZ agly-cones by using deoxythymidyldisphoshate (dTDP)-L-rhamnose as a substrate [40]. 

In this way, three enTyme modules with five enTymes were combined for in situ regeneration and transfer of dTDP-L-rhamnose by the promiscuous GT SorF [98, 104]. 

These reactions are shown in Fig. 10.7 for the conversion of dTDP-o-glucose to dTDP-L-rhamnose [17] and for the conversion of GDP-o-mannose into GDP-D-rhamnose and GDP-L-fucose [20-22]. The initial reaction to form the 4-keto sugar has a close similarity to the reaction for the conversion of UDP-Glc into UDP-Gal. 

Figure 10.7. Biosynthesis of 6-deoxyhexoses dTDP-L-rhamnose, GDP-D-rhamnose, and GDP-L-fucose.

---