L-
fucose and L-
rhamnose are two 6-deoxyhexoses naturally occurring in several complex
carbohydrates. In prokaryotes both of them are found in
polysaccharides of the cell wall, while in animals only L-
fucose has been described, which mainly participates to the structure of
glycoconjugates, either in the cell membrane or secreted in
biological fluids, such as ABH
blood groups and
Lewis system antigens. L-
fucose and L-
rhamnose are synthesized by two de novo biosynthetic pathways starting from
GDP-
D-mannose and dTDP-
D-glucose, respectively, which share several common features. The first step for both pathways is a
dehydration reaction catalyzed by specific
nucleotide-
sugar dehydratases. This leads to the formation of unstable 4-keto-6-deoxy intermediates, which undergo a subsequent epimerization reaction responsible for the change from D- to L-conformation, and then a
NADPH-dependent reduction of the 4-keto group, with the consequent formation of either
GDP-L-
fucose or
dTDP-L-rhamnose. These compounds are then the substrates of specific
glycosyltransferases which are responsible for insertion of either L-
fucose or L-
rhamnose in the corresponding
glycoconjugates. The
enzyme involved in the first step of
GDP-L-
fucose biosynthesis in E. coli, i.e.,
GDP-
D-mannose 4,6
dehydratase, has been recently expressed as
recombinant protein and characterized in our laboratory. We have also cloned and fully characterized a human
protein, formerly named FX, and an E. coli
protein, WcaG, which display both the
epimerase and the
reductase activities, thus indicating that only two
enzymes are required for
GDP-L-
fucose production. Fucosylated complex
glycoconjugates at the cell surface can then be recognized by specific counter-receptors in interacting cells, these mechanisms initiating important processes including
inflammation and
metastasis. The second pathway starting from dTDP-
D-glucose leads to the synthesis of
antibiotic glycosides or, alternatively, to the production of
dTDP-L-rhamnose. While several sets of data are available on the first
enzyme of the pathway, i.e., dTDP-
D-glucose dehydratase, the
enzymes involved in the following steps still need to be identified and characterized.