Some activated sugars such as CMP-KDO are exceptional since the free sugar, synthesized via intermediary metabolism products, is condensed directly with CTP without a prior phosphorylation of C1.

Ara-5-P + PEP ^ KDO-8-P ^ KDO ^ KDO + CTP ^ CMP-KDO

2.5.2 NDP-sugar Interconversion Pathway

The interconversion pathway ofnucleotide-sugars is a major pathway where specific enzymes convert preexisting NDP-sugars into different stereospecific NDP-sugars (Figure 5.4).

Figure 5.4 The metabolism of NDP-sugars in plants. Glucose, fructose, inositol, and sucrose are ma­jor sources of carbon that can be metabolized into NDP-sugars. A salvage pathway is defined as the ability of plant enzymes to recycle sugars that are released from glycoconjugates. The enzyme activity de­picted as numbers (italic) are UDP-glucose pyrophosphorylase (1), ADP-glucose pyrophosphorylase (2), sucrose synthase (SuSy, 3), UDP-glucose dehydrogenase (UGD, 4), UDP-apiose/xylose synthase (5), UDP — glucuronic acid decarboxylase (UXS, 6), UDP-glucuronic acid 4-epimerase (UGlcAE, 7), UDP-galactose 4-epimerase (UGE, 8), UDP-glucose 4,6 dehydratase (9), UDP-4-keto-6deoxyglucose-3′,5′-epimerase and 4′,6′-keto-reductase (NRSER, 10). Reaction 9 and reaction 10 are also carried out by a single polypep­tide (URS/MUM/RHM, 9.10), inositol oxygenase (MIOX, 11), glucuronic acid-1-P kinase (12), UDP — sugar pyrophosphorylase (Sloppy, 13), UDP 5′-diphospho-6-sulfoquinovose synthase (14), UDP-xylose epimerase (UXE, 15), UDP-arabinomutase (UAM, RGP, 38), L-arabinose-1-P kinase (Ara1, AraK, 16), D-galactose-1-P kinase (GalK, 17), D-galacturonic acid-1-P kinase (GalAk, 18), L-rhamnose-1-P kinase (RhaK, 19), UDP-rhamnose pyrophosphorylase (20), AMP sugar-1-P transferase or ADP-glucose phospho — rylase (21), glucoseamine-6-P acetyltransferase (GNA1,22), UDP-N-acetylglucoseamine pyrophosphory­lase (23), mannose-1-P pyrophosphorylase (24), GDP-Man 3′,5′ epimerase (GME, 25), GDP-L-galactose phosphorylase (39), L-galactose-1-P phosphatase (40), GDP-mannose 4,6 dehydratase (GMD, 26), GDP- 4-keto-6-deoxymannose-3,5-epimerase-4-reductase (GER1, FX, 27), fucose-1-P kinase (29), GDP-fucose pyrophosphorylase (30), mannose-6-P kinase (31), fructose-6-P kinase (32), glucose-6-P kinase (hexoki — nase, 33), phosphoglucose isomerase (PGI, 41), phosphomannose isomerase (PMI, 42), phosphomannose mutase (PMM, 43), phosphoglucose mutase (PGM, 44), KDO-8-P synthase (34), KDO-8-P phosphatase (35), CMP-KDP synthase (36).

Some of the types of modifications involved in the interconversion pathway are the iso­merization of l — and D-sugars, 4-epimerization, specific C-6 oxidation, decarboxylation, and the formation of NDP-deoxy sugar derivatives. Examples of the interconversion pathway are the conversion of UDP-GlcA to UDP-GalA and the conversion of UDP-GlcA to UDP-Xyl. The synthesis of each nucleotide-sugar is described in the following sections.