Xylose-l-phosphate

Similar reactions have recently been found in plants to be involved in pentose metabolism.D-Xylose-l-phosphate was found to react with UTP to form UDPXy. A second enzyme, which may be different from the epimerase of glucose-galactose interconversions, epimerizes carbon 4 of the pentose to produce the UDP derivative of L-arabinose. The number of specific pyrophosphorylases for sugar phosphates in plant tissue is not known at this time. 

Phosphorylase is specific with regard to its action upon < -D-glucose-l-phosphate the / -isomer cannot serve as a substrate for this enzyme. Neither can any other sugar ester be substituted for glucose-l-phosphate. a-L-Glucose-l-phosphate, maltose-l-phosphate, a-D-xylose-l-phosphate could not be polymerized to polysaccharide by potato phosphorylase. - The a-forms of D-mannose-l-phosphate and D-galactose-l-phosphate were not acted upon by muscle phosphorylase. ... 

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. 

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. 

A simple enzymic synthesis of barium glucose 6-phosphate from starch has appeared. Ths use of 2-methylthio-4 -l,3,2-benzodioxaphosphorin-2-oxide (MTBO) (see M. Eto et al.. Tetrahedron Letters, 1971, 4263) continues to receive attention. A study of twenty amines as catalysts has shown that primary n-alkyl and alicyclic amines are the most effective, followed by secondary and tertiary amines. Increasing the branching reduced the catalytic efficacy while cyclic imines gave low yields. MTBO has been used to synthesize adenosine 2, 3 -cyclic phosphate in 50% yield from 5 -0-acetyladenosine (Scheme 5). o-Arabinose 5-phosphate and o-xylose 5-phosphate have been... 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

These enzymes vary widely in secondary and tertiary structure.1273 Mannose-6-phosphate isomerase is a 45 kDa Zn2+-containing monomer. The larger 65 kDa L-fucose isomerase, which also acts on D-arabinose, is a hexameric Mn2+-dependent enzyme.1273 L-Arabinose isomerase of E. coli, which interconverts arabinose and L-ribulose, is a hexamer of 60-kDa subunits128 while the D-xylose isomerase of Streptomyces is a tetramer of 43-kDa subunits.129 The nonenzymatic counterpart of the isomerization catalyzed by the enzyme is the base-catalyzed Lobry deBruyn-Alberda van Ekenstein transformation (Eq. 13-25).130... 

Fig. 5. A cell line selected for phosphate starvation resistance was con-stitutively induced for the excretion of APase into the medium. Three-day-old tomato cells selected for phosphate starvation resistance (PSR) and unselected cells (L. esculentum cv. VF36) were grown under Pi-sufficient conditions. Proteins excreted by the cells were separated by SDS-PAGE and immunoblotted with AP3 antiserum from which the Xylose-binding component had been removed via stem bromalin treatment (Goldstein, 1991). The selected cells showed constitutive excretion of high levels of APase protein based on the large signal obtained from the immunoblot. Measurement of enzyme activity gave a similar result (not shown).

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