Isomerization ketose-aldose isomerases

Another restriction of the aldol reaction is that it gives ketoses. While isomerization to aldose may sometimes succeed with the free sugars in the presence of glucose isomerase,34 starting with phosphates and phosphoglu-cose isomerase31 may be a safer procedure. 

Triose phosphate isomerase (TPI) catalyzes the ketose-aldose isomerism between glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) during glycolysis. The rate-determining step for the reaction is product dissociation. 

Now, finally, sedoheptulose 7-phosphate undergoes a transketolase-catalyzed (EC 2.2.1.2) process (as in Scheme 11.7) to remove two carbon atoms using the enzyme cofactor thiamine diphosphate to yield ribose 5-phosphate and a two-carbon fragment that has remained attached to the thiamine cofactor of transketo-lase (EC 2.2.1.1, sedoheptulose 7-phosphate) (Scheme 11.11). When the two-carbon fragment is added to glyceraldehyde 3-phosphate, the material of Scheme 11.8 again is applied and xylulose 5-phosphate results. The xylulose 5-phosphate isomerizes to ribulose 5-phosphate as in Scheme 11.9 (with intervention of ribulose phosphate 3-epimerase (EC 5.1.3.1). And, the ribose 5-phosphate, an aldose, isomerizes (an aldose-ketose isomerase, EC 5.3.1.6, ribose 5-phosphate isomerase) to ribulose 5-phosphate. 

The action of xylose isomerase ( glucose isomer-ase , sweet-zyme ), D-xylose-ketol-isomemse, (EC 5.3.1.5.) isomerizes the aldoses D-xylose or D-glucose into the corresponding ketoses D-xylulose or D-fructose by passing the 1,2 enediol intermediate. The thermodynamically controlled process leads to equilibria of 84% D-xylose and 16% D-xylulose or 49% D-glucose and 51% D-fructose... 

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. 

Creighton and Murthy recently reviewed the stereochemistry and related mechanistic issues associated with enzyme-catalyzed isomerizations that proceed by 1,2-hydrogen transfer or by 1,3-aUytic hydrogen transfer. In the first case, the prototypical aldose-ketose isomerase is triose-phosphate isomerase (or TPl), an enzyme that uses the carboxylate of Glu-165 as a base for abstracting a proton from the substrate during catalysis. A -3-Keto-... 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

However, the applicability of this strategy is limited by the substrate specificity of the isomerases so that only a fraction of the ketoses that can be obtained from the aldose-catalyzed reaction can be enzymatically isomerized to the corresponding aldose. Moreover, the isomerization reaction is reversible and, as a ketone is generally more stable than an isomeric aldehyde, the equilibrium produces substantial aldose isomer only if the aldose sugar can exist in a very stable aldopyra-nose form [38b]. 

Conversion of Glucose 6-Phosphate to Fructose 6-Phosphate The enzyme phosphohexose isomerase (phospho-glucose isomerase) catalyzes the reversible isomerization of glucose 6-phosphate, an aldose, to fructose 6-phosphate, a ketose ... 

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... 

Much is known about the catalytic mechanism of triose phosphate isomerase. TIM catalyzes the transfer of a hydrogen atom from carbon 1 to carbon 2 in converting dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, an intramolecular oxidation-reduction. This isomerization of a ketose into an aldose proceeds through an enediol intermediate (Figure 16.6). 

D-Xylose isomerase catalyzes the interconversion between D-xylose and D-xylulose (Fig. 17-21). Since this enzyme acts on D-glucose to produce D-fructose, it is often referred to as glucose isomerase (Fig. 17-21). The isomerization of glucose to fructose by this enzyme is a very important process for the industrial production of high fructose com syrup. This enzyme is also applicable to the synthesis of many aldoses and ketoses because of its wide substrate specificity. The enzyme gene has been cloned from various microorganisms, and the enzyme has been overexpressed, purified, and characterized. Their three dimensional structures have also been determined I203-206. ... 

The glucose-6-phosphate formed in the first reaction is rearranged to produce the structural isomer fructose-6-phosphate. The enzyme phosphoglucose isomerase catalyzes this isomerization. The result is that the C-1 carbon of the six-carbon sugar is exposed it is no longer part of the ring structure. Examination of the open-chain structures reveals that this isomerization converts an aldose into a ketose. 

The aldose-ketose isomerases constitute the best studied class of enzymes catalyzing 1,2-proton transfer reactions (Tables IV and V). Isomerization generally involves significant intramolecular hydrogen transfer with variable amounts of solvent proton exchange (16). This argues for the formation of an enediol(ate) intermediate facilitated by a single active site base partially shielded from solvent [Eq. (16)]. 

Glyceraldehyde 3-phosphate is on the direct pathway of glycolysis, whereas dihydroxyacetone phosphate is not. Unless a means exists to convert dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, a three-carbon fragment useful for generating ATP will be lost. These compounds are isomers that can be readily interconverted dihydroxyacetone phosphate is a ketose, whereas glyceraldehyde 3-phosphate is an aldose. The isomerization of these three-carbon phosphory-lated sugars is catalyzed by triose phosphate isomerase (TIM ... 

No. The reaction catalyzed by phosphoglucose isomerase is a simple isomerization between an aldose and a ketose and involves the open-chain structures of both sugars. Since glucose 6-phosphate and fructose 6-phosphate are both reducing sugars, their Haworth ring structures are in equilibrium with their open-chain forms. This equilibration is very rapid and does not require an additional enzyme. Note that this isomerization reaction is of the same type as that catalyzed by triosephosphate isomerase. 

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... 

We have developed preparative enzymatic syntheses of several unusual hexoketoses using fructose-1,6-diphosphate aldolase (FDP-aldolase, E.C.4.1.2.13) as catalyst and dihydroxyacetone phosphate (DHAP) and an aldehyde as substrates (15). The enzyme appears to be very specific for DHAP but will accept a variety of aldehydes as acceptors. The ketose-1-phosphates prepared are converted to the phosphate free ketoses after removal of the phosphate group by acid- or phosphatase-catalyzed hydrolysis. The ketoses can be isomerized stereospecifically to aldoses catalyzed by glucose isomerase (E.C.5.3.1.5.) from Flavobacteriuum arborescens. The equilibrium mixtures of aldoses and ketoses are then separated by chromatography on Dowex 50 (Ba ) or Dowex 1 (HSO "). Figure 1 illustrates the preparation of a mixture of 6-deoxy-6-fluoro-D-fructose... 

Several rare ketose 1-phosphates have been obtained by use of L-rhamnulose kinase on the corresponding ketoses which were, in turn, available by isomerization of the appropriate 2R- or 25-aldoses (D-ribose, L-lyxose, L-mannose, L-talose, D-glucose, D-allose, L-rhamnose, L-fucose) with L-rhamnose- or L-fucose-isomerase, respectively. P-L-Fucose 1-phosphate and the thermodynamically less stable p-anomer of GDP-fucose were accessible by use of enzymatic processes. A new, simple, chemical preparation of rhamnulose 1-phosphate from L-rhamnose is outlined in Scheme 5. ... 

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