GlcNAc kinase

Figure 11.1 The sialic acid biosynthetic pathway. Sialic acid biosynthesis occurs within the cytosol and is governed by the bifunctional enzyme GNB, which has two domains that possess enzymatic activities UDP-GlcNAc 2-epimerase and ManNAc kinase. Synthesis starts with the epimerization of UDP-GlcNAc to ManNAc by UDP-GlcNAc 2-epimerase after the release of UDP in a two-stage process. Phosphorylation of ManNAc is catalyzed by ManNAc kinase, but in some hyposialylated cells this canbe done by GlcNAc kinase. Subsequent steps lead to formation of free...

Roseman 1958 a). In view of the poor utilization of exogeneous GlcNAc in vivo (Robinson 1968), probably due to the poor uptake of this monosaccharide into the eell, the regulatory function of the GlcNAc kinase may be related to intracellular recycling of N-acetylhexosamine. This is supported by the absence of free GleN in the cell. 

Polyphosphate kinase has been found able to phosphorylate nucleoside diphosphates to give nucleoside triphosphates, using PolyP as a phosphate donor. Therefore, the possibility of using PolyP and polyphosphate kinase instead of phosphoenol pyruvate and pyruvate kinase for enzymatic oligosaccharide synthesis was examined, because PolyP is quite cheap when compared with phosphoenol pyruvate (Noguchi and Shiba, 1998 Shiba et al, 2000). Attempts were made to synthesize N-acetyllactosamine (Gal (f> 1-4) GlcNAc) using the nucleoside diphosphate kinase-like activity of polyphosphate kinase, where UDP-Glc pyrophosphorylase and UDP-Glc 4-epimerase catalyse the synthesis of UDP-Glc from... 

To overcome the synthetic and inhibition problems, an integrated recycling system is applied. GTP and Fuc-l-P are transformed by GDP-fucosyl-pyro-phosphorylase to pyrophosphate and the activated donor (GDP-Fuc) [71]. This in turn in the presence of al-3/4-fucosyltransferase fucosylates the acceptor Gal)3l-4GlcNAc at the GlcNAc-3-position or the acceptor Gal)3l-3GlcNAc at the GlcNAc-4-position. The released GDP is phosphorylated to GTP with pyruvate kinase employing the activated phosphate donor phosphenol pyruvate, which in... 

GNE UDP-GlcNAc 2-epimerase/ManNAc 6-kinase GDP guanidine diphosphate hEBD human embryoid body-derived... 

The allyl /i-glycoside of sialyl LacNAc 240 is converted into the sLe derivative by the enzyme-catalyzed transfer of a fucose unit from GDP-fucose to position 3 of the GlcNAc unit under catalysis by -1,3-fucosyltransferase (compare Schemes 37 and 38). The glycosyl donor GDP-fucose may be regenerated by a two-step sequence of enzyme-catalyzed reactions. Under catalysis by pyruvate kinase, spent GDP accepts one phosphate group from enolpyruvate phosphate. The GTP formed reacts with fucose 1-phosphate under catalysis by GDP fucose pyrophosphorylase from porcine thyroid to form GDP-fucose and inorganic pyrophosphate. 

Figure 10 The sialoside biosynthetic pathway in eukaryotic cells A, UDP-GlcNAc-2-epimerase B, ManNAc-6-kinase C, Neu5Ac-9-P04 synthase D, NeuSAc-b-POj phosphatase E, CMP-Neu5Ac-synthetase F, sialyltransferase. Enzymes A and B function together as a bifunctional enzyme.

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