GDP-L-galactose phosphorylase (, VTC2, VTC5) is an enzyme with systematic name GDP:alpha-L-galactose 1-phosphate guanylyltransferase. It catalyses two related chemical reactions involving α-D-mannose 1-phosphate. In the first, this compound reacts with guanosine diphosphate galactose (GDP-β-L-galactose) to give guanosine diphosphate mannose and β-L-galactose 1-phosphate. This is an important enzyme in the Smirnoff-Wheeler pathway that allows plants to make ascorbic acid.
The plant enzyme can also use α-D-mannose 1-phosphate and GDP-α-D-glucose to yield guanosine diphosphate mannose and glucose 1-phosphate:
This reaction is also found in the nematode Caenorhabditis elegans and in mammals, using the same protein. References
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