D-Xylose-1-phosphate

Generally speaking, the phosphorylated deoxysugars undergo the usual reactions of carbohydrates without complication. For instance, both 2-deoxy D-ribose 5-phosphate (52, 59) and 2-deoxy D-xylose 5-phosphate (2) can be reduced to the corresponding 2-deoxy d-erythro- (48) and 2-deoxy D-threo-pentitol 5-phosphates (49). 2-deoxy ribose 5-phosphate has also been oxidized (52) to the corresponding phosphorylated acid (50). 

XYLOGLUCAN SYNTHASE d-XYLONATE DEHYDRATASE XYLOSE ISOMERASE D-Xylose 5-phosphate,... 

Scheme 5.58. (a) TK catalyzed synthesis of D-xylose-5-phosphate from D-fructose-bisphosphate... 

Of the two D-tetroses, only 4-phospho-D-erythrose is found in any quantity, as an intermediate in the photo synthetic reactions. Of the four pentoses, only D-ribose and D-xylose occur to any extent. The phospho- z/J6>-D-pentoses, D-ribose-5-phosphate and D-xylose-5-phosphate, are found in the photo synthetic reactions. Three phospho- t6>-D-pentoses are also found as intermediates in the photosynthetic reactions D-ribulose-5-phosphate, D-xylulose-5-phosphate, and D-ribulose-l,5-bis-phosphate, the latter carbohydrate being directly involved in the fixing of C02in photosynthesis. 

Another recent example arises from the scrutiny of the 2-C-methyl-D-erythritol-4-phosphate pathway for isopre-noid biosynthesis where key enzyme is 1-deoxy-D-xylose 5-phosphate reductoisomerase (DXR). DXR have no functional equivalent in humans making it an attractive target for novel antimalarial, antibacterial and herbicidal agents. ... 

Since the soluble fraction and the endoplasmic reticulum seem to be closely related, the mechanism of transfer of the product of a reaction catalyzed in one cell fraction to the other cell fraction may be a simple one. It is also possible that the endoplasmic reticulum separates pools of soluble protein, each containing its own battery of enzymes. Under such conditions, the products of the reaction catalyzed in the endoplasmic reticulum could conceivably be directed into alternative metabolic pathways. A mechanism by which the intracellular distribution of the enzyme controls cellular metabolism may be the selection of a given pathway on the basis of cell need. The transfer of the product of enzyme reactions associated with the endoplasmic reticulum to mitochondria (L-xylose) must be followed by extrusion from the mitochondria of the final product of xylose metabolism, D-xylose-5-phosphate, if the glucuronic pathway is to be complete. The mechanism that controls the passage of these metabolites through the mitochondrial membrane is most intriguing. Knowledge of this mechanism would probably provide new clues on metabolic controls. 

D-Xylulose 5-phosphate (ii-threo-2-pentulose 5-phosphate, XP) stands as an important metabolite of the pentose phosphate pathway, which plays a key fimction in the cell and provides intermediates for biosynthetic pathways. The starting compound of the pathway is glucose 6-phosphate, but XP can also be formed by direct phosphorylation of D-xylulose with li-xylulokinase. Tritsch et al. [114] developed a radiometric test system for the measurement of D-xylulose kinase (XK) activity in crude cell extracts. Aliquots were spotted onto silica plates and developed in n-propyl alcohol-ethyl acetate-water (6 1 3 (v/v) to separate o-xylose/o-xylulose from XP. Silica was scraped off and determined by liquid scintillation. The conversion rate of [ " C]o-xylose into [ " C]o-xylulose 5-phosphate was calculated. Some of the works devoted to the separation of components necessary while analyzing enzyme activity are presented in Table 9.8. 

Acetaldehyde combines with 2,3-O-isopropylidene-D-glycerose, in the presence of potassium carbonate, to give 2-deoxy-4,5-O-isopropylidene-D-ribose and -D-xylose,46 suggesting a possible biosynthetic route (see Reference 46). The suggestion that 2-deoxy-D- ribose (2-deoxy-D-erythro-pentose) arises by combination of acetaldehyde with D-glycerose 3-phosphate to give 2-deoxy-D-ribose 5-phosphate (LX) was verified by using enzyme... 

In contrast to the aldolases, 6-O-phosphono-D-gluconate dehydrogenase45 (E.C. 1.1.1.44) and UDP-D-glucuronate decarboxylase76 (E.C. 4.1.1.b) catalyze decarboxylation with inversion of configuration, giving as products, in water-t, D-eri/ffiro-pentulose-l(S)-t 5-phosphate and UDP-D-xylose-5(S)-t, respectively. 

Occurrence. D-f/treo-Pentulose 5-phosphate (d-xylulose 5-phosphate) is an intermediate in the metabolism of xylose in bacteria, and is also formed from d-erythro-pentulose 5-phosphate in the presence of an epimerase.57 D- /m o-PcnLulose has been found in a lipopolysaccharide from Pseudomonas diminuta, from which it was obtained after hydrolysis by mild acetic acid as used to release lipid A.58 It was also found in the LPS of a Yersinia enterocolitica serologic variant.59... 

Deoxy-D-/w<2 0-2-octulosonate aldolase (EC 4.1.2.23), also named 2-keto-3-deoxyoctonate (KDO) aldolase catalyzes reversible condensation of pyruvate with D-arabinose to form KDO. Preliminary investigations of the substrate specificity indicated a high specificity for KDO in the direction of the cleavage. KDO aldolase has been tested in the condensation reaction with several unnatural substrates including D-ribose, D-xylose, D-lyxose, L-arabinose, D-arabinose 5-phosphate and Af-acetylmannosamine, giving poor yield in all cases [64],... 

Xylose-1-phosphate, X-84 Xylose-3-phosphate, X-85 Xylose-5-phosphate, X-86 Xylosone, P-45 Xylostacin, X-87 Xylosyl phosphate, X-84 Xylosylamine, X-88 4-(Xylosylamino)benzoic acid, B-6 a-D-Xylosylisoprimeverose, X-59 4-p-Xylosylxylobiose, X-90 Xylotetraose, X-89 Xylothiapyranose, T-99 Xylotriose, X-90 Xylulose, P-48 Xyluronic acid, X-91... 

D-xylose is readily converted to xylose-5-phosphate by a D-xylose kinase in the presence of ATP. 

A further step in the D-xylose catabolism is believed to be phosphorylation of D-xylulose to D-xylulose-5-phosphate [2, 98, 99]. The steps after formation of xylulose phosphate appear to use a combination of pentose phosphate and EMP pathways to the key intermediate pyruvate. This process is followed by the conversion of pyruvate to ethanol by the action of pyruvate decarboxylase and alcohol dehydrogenase. D-xylulose-5-phosphate, after converting into D-glyce-raldehyde-3-phosphate via the enzymes of the pentose phosphate cycle, is also... 

The fungal oxidoreductive catabolism of these two pentoses is unique for fungi and produces ultimately D-xylulose 5-phosphate, which is an intermediate of the canonical pentose phosphate pathway (Fig. 18.3). In contrast, prokaryotes use an isomerase step to convert D-xylose to D-xylulose, and L-arabinose to L-ribulose. o-xylulose 5-phosphate is then either formed by the action of xylulokinase (in the case of D-xylose) or a sequence of L-ribulokinase and L-ribulose-5-phosphate 4-epimerase (in the case of L-arabinose) (Mishra and Singh 1993). 

M-2. Severin and Seilmeier first synthesized M-2 from pentoses and primary amine salts in 1967 (52). In the following year. Peer et al. synthesized M-2 from D-xylose or D-ribose and secondary amine salts (55), and Peer and van den Ouweland synthesized M-2 similarly from D-ribose-5-phosphate (29). In the same year, Tonsbeek et al. pubUshed identification of M-2 from beef broth (34). Subsequently, Tonsbeek et al. identified ribose-5-phosphate and pyrrolidone carboxylic acid/taurine as natural precursors of M-2 in beef (30). 

Figure I. The fungal and bacterial pathways for D-xylose and L-arabinose catabolism. All pathways have in common that D-xylulose 5-phosphate is produced. The enzymes in the bacterial pathways are xylose isomerase and xylulokinase for the D-xylose pathway and L-arabinose isomerase, ribulokinase and L-ribulosephosphate 4-epimerase for the L-arabinose pathway. The fungal D-xylose pathway has the enzymes aldose reductase, xylitol dehydrogenase and xylulokinase. The enzymes in the L-arabinose pathways ofmold and yeast are aldose reductase, L-arabinitol 4-dehydrogenase, L-xylulose reductase, xylitol dehydrogenase and xylulokinase. The differences between the mold and yeast pathway are in the cofactor requirements.

A schematic representation of glucose and xylose metabolism for PHA production is presented in Fig. 2. The first step for xylose assimilation in bacteria by the PPP is an isomerization to D-xylulose by xylose isomerase, followed by a phosphorylation by xylulokinase that produces D-xylulose 5-phosphate, yielding finally glucose 6-phosphate. This intermediate from the glucose metabolism can be produced by... 

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