Aldose-ketose phosphate isomerase

A single base mechanism, with glutamate as the base, is common to most aldose-ketose phosphate isomerases, but the electrophilic machinery to stabilise the enolate appears to differ from enzyme to enzyme. TIM uses His95 in its... 

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-... 

Transketolases are characterized by their ability to transfer a two-carbon unit from a ketose to an aldehyde. The C3 and C7 sugar-phosphates can subsequently be converted to a C4 and a Csugar-phosphate, erythrose 4-phosphate (3.17) and fructose 6-phosphate (3.2), respectively. This reaction is catalyzed by a transaldolase, which transfers a three-carbon glyceraldehyde unit from an aldose to a ketose. Erythrose-4-phosphate (3.17) can be used in the shikimate pathway (see Section 6). A second transketolase reaction can generate a second fructose-6-phosphate (3.2) and glyceraldehyde-3-phosphate (3.4) residue from erythrose-4-phosphate (3.17) and xylulose-5-phosphate (3.15). Hexose-phosphate isomerase converts the... 

C triose phosphate isomerase none proton transfer between two 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). 

An enzymic counterpart of these complex base-catalysed rearrangements of sugars may be the reaction catalysed by 4-phospho-3,4-dihydroxy-2-butanone synthetase. The enzyme catalyses the formation of the eponymous intermediate in secondary metabolism from ribulose 5-phosphate. Labelling studies indicated that C1-C3 of the substrate became C1-C3 of the product, that H3 of the substrate derived from solvent and that C4 was lost as formate. X-ray crystal structures of the native enzyme and a partly active mutant in complex with the substrate are available. The active site of the enzyme from Met ha-nococcus jannaschii contains two metals, which can be any divalent cations of the approximate radius of Mg " or Mn ", the two usually observed. Their disposition is very reminiscent of those in the hydride transfer aldose-ketose isomerases, but also to ribulose-5-phosphate carboxylase, which works by an enolisation mechanism, so the enolisation route suggested by Steinbacher et al. is repeated in Figure 6.14, as is the Bilik-type alkyl group shift, for which an equivalent reverse aldol-aldol mechanism cannot be written. 

The substrate specificity of glucose-phosphate isomerase illuminates additional stereochemical subtleties of the isomerase reaction (JOO). In the aldose to ketose direction, the enzyme potentially operates on an equilibrium mixture of substrate forms composed of two cyclic hemiacetals (the a- and /3-anomers, of glucose 6-phosphate) and trace quantities of the acyclic aldehyde form lEq. (17)] ... 

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 ... 

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... 

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. 

P-D-2-Acetamido-2-deoxy-hexosidase Acetylcholinesterase Adenosine 3, 5 -phosphate phosphodiesterase Aldose-ketose isomerases (general)... 

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. 

Isomerases interconvert free aldoses and the corresponding 2-ketoses. Other representatives, such as glucose 6-phosphate isomerase (EC 5.3.1.9), require phosphate groups at the terminal carbon of the respective sugar (Scheme 3, compounds 11 and 12). 

Many of the mechanistic aspects of glucose isomerase catalysed aldose-ketose interconversion have been under discussion for some time and are still not fully understood. By comparison with triose phosphate isomerase (TIM, EC 5.3.1.1) and glucose 6-phosphate isomerase (EC 5.3.1.9), the base-catalysed formation of an 1,2-enediol was invoked as the key step of the epimerisation based on the work of Rose and co-workers with tritium-labelled substrates [26]. An unexplained featme of the epimerisation process was that in contrast to isomerisations with triose phosphate isomerase no proton exchange with the medium could be observed with D-xylose isomerase, a fact that was attributed to the phosphate group of the former as a mediator for the exchange process [26]. Subsequently, additional important differences between triose phosphate isomerase and xylose isomerase were recognised. For example, D-xylose isomerase is appar-endy a very slow enzyme catalysing about five molecules per second per active site with an absolute requirement for divalent cations, while TIM does not need co-factors and operates at nearly 1000-fold the speed of D-xylose isomerase at... 

Therefore, glucose, mannose, and fructose can all be in equilibrium with the same enediol. This isomerization process occurs in biochemical reactions near pH 7 for several aldoses and ketoses in enzyme-catalyzed reactions. The isomerization of glucose and fructose occurs by way of their 6-ph osphate esters and is catalyzed by glucose 6-phosphate isomerase. The equilibrium constant for the formation of fructose 6-phosphate from glucose 6-phosphate is approximately 0.3. This reaction is one of the initial steps in glycolysis. 

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... 

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 ... 

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. 

A recurring intermediate. Phosphopentose isomerase interconverts the aldose ribose 5-phosphate and the ketose ribulose 5-phosphate. Propose a mechanism. 

Isomerisation (catalysed by an isomerase) the intramolecular rearrangement of a molecule. This may be the transfer of a carboxyl group (C=0) from the end of a molecule (such as an aldose) to the middle (such as a ketose), or it may be the transfer of a phosphate group (the enzymes catalysing this latter sort of isomerisation are generally termed mutases). 

Conversion of glucose-6-phosphate to fructose-6-phosphate. During reaction 2 of glycolysis, the aldose glucose-6-phosphate is converted to the ketose fmctose-6-phosphate by phosphoglucose isomerase (PGI) in a readily reversible reaction ... 

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. 

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