GDP-L-galactose

Plants form ascorbate from GDP-mannose via GDP-L-galactose, L-galactose and L-galactono-1,4-lactone. The latter is oxidized to L-ascorbic acid by a mitochondrial L-galactono-1,4-lactone dehydrogenase, using cytochrome c as electron acceptor. The biosyntheis of L-ascorbic acid has been reviewed.337... 

Linster CL, Gomez TA, Christensen KC, Young BD, Brenner C. Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants. J. Biol. Chem. 2007 282 18879-18885. Davidson VL. Electron transfer in quinoproteins. Arch. Biochem. Biophys. 2004 428 32-40. 

In comparison with mammals, plants contain considerably more GTs because, in addition to the reactions carried out by mammalian GTs, they are required to convert the products of photosynthesis into diverse cell carbohydrates. For example, these GTs synthesize cell wall polysaccharides as well as secondary metabolites and xenobiotics. Plant GTs differ from mammalian GTs even more by their diversity of nucleotide donors. They use not only eight of the nine mammalian nucleotide donors, but numerous others such as UDP-L-rhamnose, GDP-L-glucose, GDP-L-galactose, UDP-L-arabinose, UDP-D-galacturonic acid, UDP-D-apiose, and so on. (29). 

Similarly, GDP-L-galactose derivatives functionalized at 0-6 were prepared and studied in transfer reactions [75]. 

D-mannose and L-galactose are efficient precursors for ascorbate synthesis, that proceeds through a pathway involving GDP-D-mannose, GDP-L-galactose, L-galactose, and L-galac-tono-1,4-lactone [223],... 

A reaction catalyzing the inversions at C-3 and C-5 is postulated to occur in the formation of GDP-L-galactose from GDP-D-man-nose also, the synthesis of 6-deoxy-D-talose from GDP-D-mannose is assumed to involve a reduction at C-6 and inversion at C-4. 

An exploration of the sugar nucleotides in this alga disclosed the presence of UDP-D-galactose, GDP-L-galactose, UDP-D-glucose, GDP-D-mannose, and UDP-D-glucuronic acid. Moreover, AMP, UMP, GMP, IMP, ADP, UDP, IDP, NAD , NADP , and a new nucleotide, adenosine 3, 5 -pyrophosphate, were isolated. 

Inasmuch as GDP-L-galactose is found together with GDP-D-mannose in this red alga, it is suggested that the formation of l-galactose nucleotide probably occurs by a mechanism similar to... 

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