Glucose enediol intermediate

Because the configuration at C-2 is lost on enolization, the enediol intermediate can revert either to D-glucose or to D-mannose. Two stereoisomers that have multiple chirality centers but differ in configuration at only one of them are refened to as... 

This enzyme interconverts ribulose-5-P and ribose-5-P via an enediol intermediate (Figure 23.30). The reaction (and mechanism) is quite similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including N/ DH, N/ DPH, F/ D, and Big), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pentose phosphate pathway is... 

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

FIGURE 9 Isomerization and elimination reactions (a) The conversion of glucose 6-phosphate to fructose 6-phosphate, a reaction of sugar metabolism catalyzed by phosphohexose isomerase. (b) This reaction proceeds through an enediol intermediate. The curved blue ar-... 

In 1895, Emil Ficher proposed an enediol intermediate for this isomerization. As would be expected, the enzyme-catalyzed isomerization of glucose-6-phosphate in 2H20 is accompanied by incorporation of deuterium into the product fructose 6-phosphate at C-l. In the reverse reaction 2H-containing fructose 6-phosphate was found to react at only 45% of the rate of the 1H-containing compound. Thus, the primary deuterium isotope effect expected for a rate-limiting cleavage of the C-H bond was observed (see Chapter 12, Section B,3). 

The isomerization of D-glucose to D-fructose by way of an enediol intermediate is an important step in glycolysis, a complex process (11 steps) by which an organism converts glucose to chemical energy. The substrate is not glucose itself but its 6-phosphate ester. The enzyme that catalyzes the isomerization is called phosphoglucose isomerase. 

The rearrangement is of interest because the corresponding enzymatic interconversion of aldoses and ketoses is an important part of the biosynthetic, photosynthetic, and metabolic pathways, as we shall see in Section 20-9. Although the biochemical rearrangement also may proceed by way of enediol intermediates, it is highly stereospecific and yields only one of two possible stereoisomeric aldoses. For example, glucose, but not mannose, can be enzymatically interconverted with fructose as the 6-phosphate ester derivative ... 

In analogy with the D-fructose D-glucose interconversion, dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate can equilibrate by way of an enediol intermediate. 

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,... 

Faces of molecules can also be diastereotopic an example is provided by the enediol intermediate, 44, postulated to occur in the glucose-phosphate isomerase reaction ... 

Isomerization reaction involves a change in molecule s functional group without changing the chemical formula. An example here is the isomerization of D-glucose to D-fructose by way of enediol intermediate during glycolysis [1]. Cephalosporin antibiotic APIs will undergo isomerization of the olefin from the A3 position to the A2 position [28]. 

Polarisation of the substrate carbonyl group appears to be achieved in yet a third way by mammalian glucose 6-phosphate isomerase, which interconverts glucose and fructose 6-phosphates, The crystal structure of the rabbit enzyme in complex with the reactive intermediate analogue o-arabinohydroxamic acid Ki = 0.2 pM) (in its hydroximic form) reveals a cluster of four water molecules hydrogen bonded to each other, to the counterparts of 01 and 02 of the enediolate intermediate and to an active site arginine. Grotthus mechanisms of proton transfer between the two tautomers of the enediolate probably occur. 

Recall that the isomerization reaction of glucose and fructose involves an enediol intermediate (Figure 7.15). This transformation makes C-l of the fructose product available for phosphorylation. 

This is an aldose-ketose isomerization that proceeds through an enediol intermediate. G6P is the aldose and ffuctose-6-phosphate (F6) is the ketose. Phosphoglucoisomerase, which catalyzes this isomerization, must not be confused with phosphoglucomutase, the enzyme that interconverts G6P and glucose-1-phosphate (GIP). The AG for the isomerization of G6P to F6P is only slightly positive, so it strongly favors neither reactants nor products in this reaction. AG = -1-1.7 kJ/mol... 

The conversion of glucose-6-phosphate to fructose-6-phosphate is analogous to the conversion of glyceraldehyde-3-phosphate to dihydroxyacetone phosphate. Both of these isomerization reactions interconvert an aldose and a ketose. Key features of the those phosphate isomerase mechanism include the hydrogen transfer between carbon 2 and carbon f (intramolecular oxidation/reduction), and the enediol intermediate (Figure... 

This interconversion of triose phosphates occurs by the same type of keto-enol tautomer-ism (Section 12.8A) and enediol intermediate we have already seen in the isomerization of D-glucose 6-phosphate to D-fructose 6-phosphate. Note that D-glyceraldehyde 3-phosphate is chiral and that two enantiomers are possible for it. The enzyme catalyzing this reaction has a high stereospecificity, and only the D enantiomer is formed. 

Conversely, when the reaction mixture contained sodium hydroxide, fructose gave glucose and mannose in addition to a mixture of unidentified carbohydrates. The aldoses must have been formed by the LdB-AvE rearrangement of fructose via an enediol intermediate. In this case, the equilibrium between the two epimers favored glucose [41b, 48]. 

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