Ketose-aldose isomerism

Aldose-ketose isomerism Fructose has the same molecular formula as glucose but differs in its structural formula, since there is a potential keto group in position 2, the anomeric carbon of fmctose (Figures 13 and 13-7), whereas there is a potential aldehyde group in position 1, the anomeric carbon of glucose (Figures 13-2 and 13-6). 

Glucose 6-phosphate is an important compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis, the pentose phosphate pathway, glycogenosis, and glycogenolysis). In glycolysis, it is converted to fructose 6-phosphate by phosphohexose-isomerase, which involves an aldose-ketose isomerization. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

B xylose isomerase Mg2+ hydride transfer between adjacent carbons during aldose/ketose isomerization... 

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... 

A number of other reactions in this general area such as the mutarotation of D-(+)-glucose and aldose-ketose isomerizations are also subject to catalysis by metal ions. 

The Lobry de Bruyn-Alberda van Ekenstein transformation has usually been considered to embrace both epimerization of aldoses and ketoses and aldose-ketose isomerization. Actually, Lobry de Bruyn and Alberda van Ekenstein observed all three reactions, so that an experimental basis for defining the transformation has existed from almost the time of its first recognition. 

Table II fists the enzyme-catalyzed reactions. These now embrace aldose-ketose isomerizations both of phosphorylated and of nonphos-...

Kinetic evidence from the DL-glycerose-1,3-dihydroxy-2-propanone isomerization has indicated that aldose-ketose isomerization and formation of a 3-deoxyosone proceed through a common intermediate. Ashmarin and coworkers and Petuely s observations, which indicate general acid and base catalysis of 2-furaldehyde formation, tend to support such a mechanism, since 2-furaldehyde and its derivatives may indeed be formed from 3-deoxyosones. 

The following scheme, in which A is a Bronsted base and HA is its conjugate acid, is in harmony with these facts for nonenzymic epimeriza-tion (shown only at C2), aldose-ketose isomerization, and the dehydration to a 3-deoxyosone. Here, aldose-ketose isomerization is shown as progressing through the hybrid anion (XXVIII), through the enediol, and, finally, through the hybrid anion (XXIX) The hybrid anion (XXVIII) is also... 

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 former base-catalyzed aldose-ketose isomerization is named the Lobry de Bruyn-van Ekenstein transformation (Scheme 6.25). Deprotonation of the a-carbonyl carbon of aldose (glucose) requires a base, and results in the form of a series of enolate intermediates. Solid bases such as cation-exchanged zeolites and Mg-Al HT catalyze glucose isomerization in water [176-178]. 

Tandem m s, and theoretical calculations have been used to study the gas-phase aldose-ketose isomerization. Results obtained with (l->3)-linked disaccharides supported a mechanism involving hydride transfer rather than a 1,3-hydrogen shift. Related theoretical studies (ab initio and semiempirical MO methods) of aldose-ketose isomerization in aqueous solution suggested that the favoured pathways in the absence and presence of metal ions are proton transfer via an enediol and hydride transfer, respectively. A strong influence of the acetonitrile mole fraction on the... 

Aldose-ketose isomerization The same 1,2-enediol can revert to either an aldose or a ketose. 

A study of the formation of oligosaccharides on thermal degradation of lactose has shown that the pyrolysis proceeds via simultaneous aldose-ketose isomerization and condensation of initially-formed monosaccharides without involving any cleavage of the carbohydrate carbon skeleton. 

Two Dutch chemists, Lobry de Bruyn and Alberda van Eckenstein, collaborated in the study of the effects of alkali on carbohydrates. The reaction with alkali produces epimerization of aldoses and ketoses and aldose-ketose isomerization [2]. At pH values of 11-13 and 20°C, alkali catalyzes the transformation of D-glucose into D-fructose and o-mannose. The transformation most probably takes place by the formation of two enediols, although the enolic forms of the sugars have never... 

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