GDP-o.-D-mannose (GDP-Man)

GDP-Man (485) is a major sugar donor that provides flux of sugar to the synthesis of glycoproteins, polysaccharides, and ascorbic acid in plants. GDP-Man is the precursor for GDP-Fuc and GDP-Gal. Guanosine 5′-diphosphate-mannose pyrophosphorylase (GDP — Man PPase) catalyzes the conversion of a-D-Man-1-P and GTP to GDP-Man and pyrophos­phate (486). The enzyme activity requires Mg2+ and the enzyme appears to be cytosolic. The pyrophosphorylase is well studied in various organisms and the crystal structure is known. In Arabidopsis, a wall mutant cytl (487), an ozone-sensitive mutant, and ascorbic acid mutant, vtcl, identified the same gene product At2g39770 as the locus responsible for the production of GDP-Man (488, 489). The mutant likely survives since another homol­ogous gene, At4g30570, may compensate for its activity. Although the enzymatic activity

of recombinant protein was not described, work in our laboratory confirmed that the en­coded gene At2g39770 has GDP-Man PPase activity and is very specific toward Man-1-P as a substrate (Echole and Bar-Peled, unpublished).