Glycosyl nucleotides

Glycosyl esters ( sugar nucleotides ) are the glycosyl donors for the formation of wall polysaccharides. Some glycosyl-nucleotides can, in vivo, be synthesized directly from the corresponding monosaccharide, ATP, and the appropriate nucleoside triphosphate. In addition, some... 

Although the biochemistry of the synthesis of glycosyl-nucleotides in higher plants is well advanced, the subsequent polymerization reactions, involving their glycosyl groups, to yield the various classes of cell-wall polysaccharides is not. The enzymes catalyzing these processes have not yet been fully characterized, and the involvement of membrane systems, and the mechanism of assembly of the wall itself, are ill-understood. 

In conclusion, it may be stated that present knowledge of the enzymic mechanisms catalyzing the formation of heteroglycans from glycosyl-nucleotides as glycosyl donors, and of the relevance of such synthetic mechanisms to the formation of the intact, primary cell-wall, is very limited. In particular, no heteroglycan having properties identical to those of known, native polymers of the primary wall has yet been synthesized in vitro. 

The involvement of glycolipid and glycoprotein intermediates in the synthesis of polysaccharides from glycosyl-nucleotides in plants is considered to be a likely possibility. Such intermediates could act as specific primers, or acceptor substrates, for the formation of polysaccharides. Furthermore, subunits of complex heteropolysaccharides could be assembled on such intermediates, and later incorporated into polysaccharides, or directly cross-linked into the cell wall. Evidence of the involvement of such intermediates in the synthesis of polysaccharides in a number of organisms is presented in Sections XII,3,b and XII,3,c. 

There is at present no precise information concerning either the control mechanisms that govern wall-biogenesis or the interactions between wall biogenetic-processes and general cellular metabolism. The number of steps involved in the formation of a polysaccharide from a glycosyl-nucleotide is not known, and it is not clear how cellular control is extended beyond the plasma membrane, or how the cell wall is formed from the component polymers. Thus, it appears that the major questions posed by the problem of cell-wall biosynthesis have yet to be answered (see also, Ref. 217). 

The sugar nucleotides (an uninformative name that has been used for glycosyl nucleotides, or more strictly, glycosyl esters of nucleoside di- or mono-phosphates) were discussed in this Series12 in 1973. Since then, accumulation of new data about these derivatives has continued, and now, about 35 representatives of this class are known to participate in the biosynthesis of polysaccharide chains of bacterial polymers (for a survey, see Ref. 13). These include glycosyl esters of uridine 5 -diphosphate (UDP), thymidine 5 -diphosphate (dTDP), guanosine 5 -diphosphate (GDP), cytidine 5 -diphosphate (CDP), cytidine 5 -monophosphate (CMP), and adenosine 5 -diphosphate (ADP). 

It has been suggested12 that a distinction be made between the primary and the secondary glycosyl nucleotides. The former derivatives are produced as a result of the interaction of nucleoside 5 -triphosphates with glycosyl phosphates for nucleoside glycosyl diphosphates) or monosaccharides (for cytidine glycosyl monophosphates). The enzymes that catalyze these reactions belong to the group of nucleotidyltransferases... 

EC 2.7.7 group). Secondary glycosyl nucleotides are formed from the primary derivatives through enzymic reactions leading to modification of their monosaccharide fragments these conversions are discussed in Section III. 

In bacteria, primary glycosyl nucleotides include14 22 UDP-Glc, as well as other nucleotide derivatives of the same monosaccharide. Important representatives of this class are16"21,23"25 dTDP-Glc and17"19,21,26 29 CDP-Glc, which serve as precursors for a number of secondary glycosyl nucleotides, whereas similar enzymic reactions have not been demonstrated for GDP-Glc,1819,21,30 and ADP-Glc17 19,21-31"38 seems to participate only in the biosynthesis of bacterial glycogen, a subject that is beyond the scope of this article. 

Guanosine (D-mannosyl diphosphate) (GDP-Man, 4)39-42 and uridine (2-acetamido-2-deoxy-D-glucosyl diphosphate) (UDP-GlcNAc, 5)43,44 are other important examples of primary glycosyl nucleotides. Although formation of ADP and dTDP derivatives of the latter monosaccharide was observed,17 their functions remain obscure.45... 

Polyprenyl glycosyl monophosphates have been shown to serve as glycosyl donors in the biosynthesis of side chains of some polysaccharides. Contrary to glycosyl nucleotides, they represent membrane-bound glycosyl donors which may be preferable for some glycosyltransferases. 

Transition from the D-gluco to the D-galacto configuration occurs through enzymic epimerization at C-4 of the hexosyl group in glycosyl nucleotides. Such reactions were observed for UDP-Glc,14,102-104 UDP-GlcNAc,44,105-107... 

The symbol X in the Table means that an activated form of the monosaccharide is not known a dash shows that the monosaccharide has not been identified as a component of bacterial polysaccharides. For references, see the text. 6 See Ref. 101 for abbreviations. c Primary glycosyl nucleotide. 4 See comments in the text. 

The key intermediates in the biosynthesis of 6-deoxy sugars are the nucleoside 6-deoxyhexosyl-4-ulose diphosphates (7), formed through enzymic reactions catalyzed by NDP-sugar 4,6-dehydratases (EC 4.2.1.45-47) from primary glycosyl nucleotides. These reactions were observed... 

Reduction of dTDP-6-deoxy-D-xy/o-hexos-4-ulose (7a) at C-4 of the hexosyl group should lead to dTDP derivatives of 6-deoxy-D-glucose (d-quinovose) and 6-deoxy-D-galactose (D-fucose). Although D-quinovose and D-fucopyranose have not been found as components of bacterial polysaccharides, the corresponding glycosyl nucleotides were identified147 in extracts of Escherichia coli Y-10. 

Epimerization at C-3 and C-5 of the hexosyl group in the hexosyl-4-ulose derivatives seems to be a quite common transformation of glycosyl nucleotides. The assumptions that these two reactions may occur independently, and that the process may be completed after inversion at only one chiral center, allow suggestion of an explanation for the origin of the D-allose200... 

It is necessary to postulate the existence of a previously unknown, modification reaction of glycosyl nucleotides, that is, deoxygenation at C-4, in order to explain the formation of 4-deoxy-D-arafc//io-hexose, a monosaccharide constituent of Citrobacter lipopolysaccharide.202 4-Deoxy-L-threo-hex-4-enuronic acid, found in Klebsiella K22 polysaccharide,203 may be formed through dehydration of the D-galacturonic acid group or residue, either in the glycosyl nucleotide, or in the polysaccharide. 

In this group of monosaccharide components of bacterial polysaccharides, a primary glycosyl nucleotide is ADP-D-g/ycero-D-manno-heptose (9), identified in extracts of a mutant strain of Shigella sonnei.231... 

A modification of the monosaccharide units of polysaccharides may obviously be effected at different stages of the biosynthesis of a polymer (a) prior to formation of the activated form of a monosaccharide, (b) at the level of glycosyl nucleotides, (c) at the stage of formation of oligosaccharide intermediates, and (d) after the synthesis of a polymeric chain. 

As already mentioned, formation of glycosidic linkages between monomeric units of the carbohydrate-containing polymers of the bacterial cell-surface is catalyzed by membrane-bound glycosyltransferases, and glycosyl nucleotides are the usual glycosyl donors in the reaction. 

A series of poly prenyl diphosphates was formed when EDTA-treated cells of Acetobacter xylinum were incubated with glycosyl nucleotides. The most complicated of them is the heptasaccharide derivative344 (32), which is considered to be an intermediate in the biosynthesis of the exocellular... 

The type 3 capsular polysaccharide199 (38) was found375,376 to be efficiently formed from UDP-Glc and UDP-GlcA. Under optimal conditions, >90% of the glycosyl groups of the glycosyl nucleotides was incorporated into polymer identified by immunochemical methods. 

Formation of lipid-linked oligosaccharides that contain D-glucosyl, d-galactosyl, and L-rhamnosyl residues from the corresponding glycosyl nucleotides was demonstrated with a membrane preparation from Lactobacillus plantarum398 their function remains to be established. 

The main problem in this approach is the very low permeability of mevalonic acid to membranes, resulting in very low incorporation. Positive results have been obtained by the use of cell-free systems incubated with [14C]-mevalonic acid,26,27 [14C]isopentenyl diphosphate,28 or [32P]orthophos-phate.29 Incubation of these radioactive lipids with glycosyl nucleotides labelled in the glycosyl group with a different isotope, followed by extraction and cochromatography in different solvent systems, may indicate that both compounds are present in the same molecule. When the lipid moiety becomes labelled from mevalonic acid or isopentenyl diphosphate, chromatography on DEAE-cellulose columns should be performed, in order to avoid confusion with steryl glycosides. 

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