6-deoxy hexosyl-4-ulose

The analogous conversion of the uridine derivative 107d has been observed in extracts of tobacco leaves136 and Chlorella cells,137 although the enzyme has not yet been purified. The enzymic reactions described here are frequently used for preparation of the 6-deoxy-hexosyl-4-ulose esters of nucleoside pyrophosphates that are inacces-... 

Reduction of dTDP-6-deoxy-D-xy/o-hexos-4-ulose (7a) at C-4 of the hexosyl group should lead to dTDP derivatives of 6-deoxy-D-glucose (d-quinovose) and 6-deoxy-D-galactose (D-fucose). Although D-quinovose and D-fucopyranose have not been found as components of bacterial polysaccharides, the corresponding glycosyl nucleotides were identified147 in extracts of Escherichia coli Y-10. 

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... 

A similar mechanism seems probable for conversion of GDP-6-deoxy-D-(yxo-hexos-4-ulose (7c) into GDP-L-fucose (GDP-Fuc),40,159 although, in this case, the enzymes that participated in the process were not separated, and intermediate formation of the 6-deoxy-L-xy/o-hexosyl-4-ulose derivative (8b) was not demonstrated. 

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