Of D-glucose 6-phosphate

Lohmann118 detected an enzyme in muscle extracts, found later in plants and yeasts,104Co) 117,118 termed phosphoglucoisomerase, (optimum pH 9), which catalyzes the interconversion of D-glucose 6-phosphate (XVI) and D-fructose 6-phosphate (XVII). At equilibrium, which is attained rapidly, there is about 70 % of the former and 30 % of the latter. In a similar conver-... 

The oxidative pathway for the metabolism of D-glucose 6-phosphate (XLV), distinctive from the glycolytic, Embden-Meyerhof route (see p. 200) and known as the hexose monophosphate shunt, was suggested by certain experiments of Warburg,200 Gerischer,207 Lipmann,208 and Dickens209... 

A novel synthesis of D-glucose 6-phosphate (in 55% yield) has been reported it involves alcoholysis of the cyclic phosphate of catechol with 1,2-O-isopropylidene-a-D-glucofuranose, followed by acid hydrolysis of the so-formed phosphoric diester.192 The reagent... 

G9. Glaser, L., and Brown, D. H., Purification and properties of D-glucose-6-phosphate dehydrogenase. ]. Biol. Chem. 216, 67-79 (1955). 

This enzyme [EC 5.5.1.4] catalyzes the conversion of D-glucose 6-phosphate to iL-myo-inositol 1-phosphate. NAD+ is required as a cofactor. 

It had been known from at least the time of Pasteur that the presence of sodium or potassium phosphate aided the progress of a yeast fermentation. Later intensive study showed that a complex group of enzymes (phosphatases and phosphorylases) was responsible for the phosphorylation, dephosphorylation and interconversion of D-glucose 6-phosphate, D-fructose 6-phosphate, D-fructose 1,6-diphosphate and similar substances in various types of cells and muscle tissue. Detailed reviews of the field are available. - A further advance was made in 1936, when Cori and Cori noted that in certain circumstances well-washed frog muscle immersed in a sodium phosphate buffer utilized the inorganic phosphate to produce a new hexose phosphate (the Cori ester). This compound was later shown to be a-D-glucopyranose-l-phosphate and yielded crystalline dipotassium and brucine salts. The Cori ester arose because... 

J. E. G. Barnett, A. Rasheed, and D. L. Corina, Partial reactions of D-glucose 6-phosphate-1-L-myo-inositol 1-phosphate cyclase, Biochem. J. 131 21 (1973). 

When D-glucose 6-phosphate and D-fructose 6-phosphate are inter-converted by D-glucose 6-phosphate ketol isomerase in either deuterium oxide33 or water-1 (Ref. 34), isotope is incorporated at C-l of D-fructose 6-phosphate and C-2 of D-glucose 6-phosphate, indicating that the interconversion involves an enediol intermediate, which may arise from an open-chain (37) or cyclic (38) form of the sugar,... 

When the hydrogen transferred as hydride to the cofactor is retransferred to the same carbon atom in the product, the movement is far more difficult to detect. The conversion of D-glucose 6-phosphate (58) into lL-mt/o-inositol 1-phosphate (61) occurs by cyclization of the carbon skeleton, with formation of a new bond between C-l and C-6. When each carbon atom in turn was specifically labeled with tritium, there was complete retention of tritium, even in the presence of added NADH, although there was an apparent, small isotope-effect with D-glucose-5-t 6-phosphate.19 The mechanism proposed for the cyclization19 was an initial oxidation at C-5 to give NADH and xylo-hexos-5-ulose 6-phosphate (59), followed by an aldol reaction causing cyclization to lL-myo-inosose-2 1-phosphate (60), which is then... 

Switching the system to zero potential for a fixed period of time within the time window bracketing the passage of the enzyme by the detector would allow for product accumulation. When the power is switched on again, the enzyme will be separated from the product and, if the product has suitable detection properties, it will appear as a peak on the enzyme plateau. This approach is sometimes referred to as parked reaction. A practical application featuring the activity assay of D-glucose-6-phosphate NADPH oxidore-ductase is shown in Figure 8.10. 

Loewus, M.W., 1977, Hydrogen isotope effects in the cyclization of D-glucose-6-phosphate by myo-inositol-l-phosphate synthase. J. Biol. Chem. 252 7221-7223. 

Loewus, M.W., and Loewus, F.A., 1971, The isolation and characterization of D-glucose 6-phosphate cycloaldolase (NAD-dependent) from Acer pseudoplatanus L. cell cultures. Is occurrence in plants. Plant Physiol. 48 255-260. 

Byun, S.M., Jenness, R., Ridley, W.P., and Kirkwood, S., 1973, Stereospecificity of D-glucose-6-phosphate - 11-myo-inositol-l -phosphate cycloaldolase on hydrogen-atoms at C-6. Biochem. Biophys. Res. Commun. 54 961-967. 

Figure 3. Isomerization of D-glucose 6-phosphate to D-myo-inositol 3-phosphate by inositol synthase. The hypothetical intermediate is within parenthesis.

D-Glucosamine 6-phosphate is hydrolyzed by the D-glucose-6-phosphatase of rat-liver mitochondria. The rate of this hydrolysis is about 8 % of that of D-glucose 6-phosphate hydrolysis. A phosphatase which preferentially catalyzes the hydrolysis of D-glucosamine 6-phosphate has been prepared from Neurospora crassa. This enzyme is not stimulated by magnesium ions and has an optimum activity between pH 6 and 7.5. It appears to be distinct from acid, alkaline, and other specific phosphatases. 

Another central pathway by which yeasts may catabolize D-glucose is the pentose cycle (see Fig. 6), the initial stages of which are (i) the phosphorylation of D-glucose, followed by (ii) oxidation of D-glucose 6-phosphate to 6-O-phosphono-D-gluconate. The net result of the operation of this cycle is the complete oxidation of D-glucose. 

Glucose-6-phosphate dehydrogenase (E.C. 1.1.1.49) is also commonly used as a label. This enzyme has a molecular weight of 104 kDa, and pure preparations show optimum activity at pH 7.8, with 400-U/mg specific activity at 37 °C. The dehydrogenase catalyzes the oxidation of D-glucose-6-phosphate with concomitant reduction of NADP+ (Eq. 6.13). Absorbance or fluorescence of the resulting NADPH may be used for quantitation. 

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