Aldose l-phosphates

Alkenes produced from aldoses with free anomerlc groups can be converted to methylene alkenes and then cyclized to give C-glycosldes having mercurlated methyl aglycones". Prom these, phosphonates which are analogues of aldose l-phosphates can be produced. The method has been used to obtain the o-D-rlbofuranose... 

Aldoses react with ATP in a phosphotransferase-catalyzed reaction (C 1.1) either to sugar-l-phosphates, e.g., D-galactose-l-phosphate and L-arabinose-1-phosphate, or to sugar-co-phosphates, e.g., D-glucose-6-phosphate (Fig. 27), D-mannose-6-phosphate, D-galactose-6-phosphate, and D-ribose-5-phosphate. Aldose-l-phosphates may also be formed from co-phosphates by mutases (Fig. 27) with 1,co-diphosphates as intermediates. 

Important derivatives of aldose-l-phosphates are the nucleoside diphosphate sugars. In most cases they are built de novo from aldose-l-phosphates and nucleotide triphosphates (Fig. 27). The nucleotide most frequently used is uridine triphosphate. Other nucleotide triphosphates, however, may react as well... 

Aldose-l-phosphates synthesized by the reaction of aldoses with ATP or by isomerization of sugar-co-phosphates are the primary glycosides. They may act as glycosyl donors in the synthesis of other glycosides either directly or after transformation to nucleotide-diphosphate sugars (C 6). 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

Transketolase from common yeast (Saccharomyces cerevisiae) is commercially available, but it is possible to work with a partially purified enzyme, isolated with little expense from spinach leaves.54 Transketolase catalyzes the transfer of a hydroxyacetyl group, reversibly from a ketose phosphate, or irreversibly from hydroxypyruvate to an acceptor aldose, phosphorylated or not.55 It requires thiamine pyrophosphate as a coenzyme, but only in catalytic amounts. In all the cases listed in Table V, the new chiral center, C-3 of the ketose, has the l-glycero configuration. 

Monosaccharides may possess functionalities other than hydroxyls. Amino sugars are aldoses or ketoses which have a hydroxyl group replaced by an amino functionality. 2-Amino-2-deoxy-glucose is one of the most abundant amino sugars it is a constituent of the polysaccharide chitin. It also appears in mammalian glycoproteins, linking the sugar chain to the protein. Monosaccharides may also be substituted with sulfates and phosphates. Furthermore, deoxy functions can often be present, and important examples of this class of monosaccharides are L-fucose and L-rhamnose. 

In addition to serving as structural motifs, enols and enolates are involved in diverse biological processes. Several enol/enolate intermediates have been proposed to be involved in glycolysis (Section IV.A), wherein c/ -enediol 21 is proposed to be an intermediate in the catalytic mechanism of phosphohexose isomerase and an enol-containing enamine intermediate (22) has been proposed in the catalytic pathway of class I aldolase. In the case of glucose-fructose (aldose-ketose) isomerization, removal of the proton on Cl-OH produces the aldose while deprotonation of C2-OH yields the ketose, which is accompanied by protonation at the C2 and Cl positions, respectively. There are several cofactors that are involved in various biological reactions, such as NAD(H)/NADP(H) in redox reaction and coenzyme A in group transfer reactions. Pyridoxal phosphate (PLP, 23) is a widely distributed enzyme cofactor involved in the formation of a-keto acids, L/D-amino... 

In the presence of formaldehyde (0.5 mol equiv.), sugar phosphates were formed in up to 45% yield, with pentose-2,4-diphosphates dominating over hexose triphosphates by a ratio of 3 1 (Scheme 13.2, Route B). The major component was found to be D,L-ribose-2,4-diphosphate with the ratios of ribose-, arabinose-, lyxose-, and xylose-2,4-diphosphates being 52 14 23 11, respectively. The aldomerization of 2 in the presence of H2CO is a variant of the formose reaction. It avoids the formation of complex product mixtures as a consequence of the fact that aldoses, which are phosphorylated at the C(2) position, cannot undergo aldose-ketose tautomerization. The preference for ribose-2,4-diphosphate 5 and allose-2,4,6-triphosphate formation might be relevant to a discussion of the origin of ribonucleic acids. 

Non-enzymic Enolisation. The carbanion chemistry of aldoses and ketoses themselves is masked by the ring opening step and mechanistic work is limited. However, a detailed examination has been made of the enolisation of L-glyceraldehyde 3-phosphate, which cannot form monomeric rings. The use of the unnatural (l) enantiomer enabled any racemisation to the natural... 

This enzyme catalyzes the reversible transfer of the hydroxyketo group of a ketose phosphate to an aldose phosphate. The cofactor thiamine pyrophosphate (TPP) is associated with the enzyme and activates the ketose (Scheme 7). Most known donor ketoses (xylulose 5-phosphate, sedoheptulose 7-phosphate, fructose 6-phosphate, L-erythrose) have a trans arrangement of hydroxy groups at C-3 and C-4 hydroxypyruvate is an exception. A range of aldehydes (such as o-glyceraldehyde 3-phosphate, D-ribose 5-phosphate, o-erythrose 4-phosphate, glycoaldehyde) are acceptors. Transketolase has been... 

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