Triose phosphate isomerase reaction catalyzed

Answer Problem 1 outlines the steps in glycolysis involving fructose 1,6-bisphosphate, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate. Keep in mind that the aldolase reaction is readily reversible and the triose phosphate isomerase reaction catalyzes extremely rapid interconversion of its substrates. Thus, the label at C-l of glyceraldehyde 3-phosphate would equilibrate with C-l of dihydroxyacetone phosphate (AG ° = 7.5 kJ/mol). Because the aldolase reaction has AG ° = -23.8 kJ/mol in the direction of hexose formation, fructose 1,6-bisphosphate would be readily formed, and labeled in C-3 and C-4 (see Fig. 14-6). 

Figure 7.6. The metabolic reactions involved in the conversion of glycerol to glucose, the required precursor in the formation of sophorose. Note Reaction 1 catalyzed by triose phosphate isomerase. Reaction 2 catalyzed by aldolase. Reaction 3 catalyzed by fructose 1,6-bisphosphatase. Reaction 4 catalyzed by phosphoglucose isomerase., Reaction 6 catalyzed by glucose 6-phosphatase.

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. 

Stereospecifidly of Dihydroxyacetone Phosphate Reactions. In both the aldolase and triose phosphate isomerase reactions a carbon-hydrogen bond of the hydroxymethyl group is broken. Isotope exchange experiments with tritium have shown that both enzymes catalyze an equilibration between one hydrogen of the substrate and the hydrogen of water. ° The two enzymes do not attack the same hydrogen atom each is specific for only one position. In the projection shown (XII),... 

Because dihydroxyacetone phosphate and glyceraldehyde 3-phosphate enolize to give a common intermediate, they exist in equilibrium. The enzyme triose phosphate isomerase efficiently catalyzes the isomerization. Although the enediol intermediate is chiral, the enzyme forms only the i enantiomer of glyceraldehyde 3 phosphate. In aqueous solution, an acid-catalyzed reaction would yield a racemic mixture of aldehyde 3-phosphate. 

Further steps m glycolysis use the d glyceraldehyde 3 phosphate formed m the aldolase catalyzed cleavage reaction as a substrate Its coproduct dihydroxyacetone phosphate is not wasted however The enzyme triose phosphate isomerase converts dihydroxyacetone phosphate to d glyceraldehyde 3 phosphate which enters the glycol ysis pathway for further transformations... 

Triose phosphate isomerase catalyzes the conversion of dihy-droxyacetone-P to glyceraldehyde-3-P. The standard free energy change, AG°, for this reaction is +7.6 kj/mol. However, the observed free energy change (AG) for this reaction in erythrocytes is +2.4 kj/mol. 

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

Interconversion between these three-carbon intermediates is a reversible reaction catalyzed by triose phosphate isomerase. 

The 500-residue subunits of pyruvate kinase consist of four domains,891 the largest of which contains an 8-stranded barrel similar to that present in triose phosphate isomerase (Fig. 2-28). Although these two enzymes catalyze different types of reactions, a common feature is an enolic intermediate. One could imagine that pyruvate kinase protonates its substrate phosphoenolpyruvate (PEP) synchronously with the phospho group transfer (Eq. 12-42). However, the enzyme catalyzes the rapid conversion of the enolic form of pyruvate to the oxo form (Eq. 12-43) adding the proton sterospecifically to the si face. This and other evidence favors the enol as a true intermediate... 

Eor triose phosphate isomerase Albery and Knowles obtained a value of 4 x 10 M s so close to the diffusion controlled limit that these authors regard this enzyme as a nearly perfect catalyst, one that could not have evolved further because it is already catalyzing the reaction with substrate at almost the maximum velocity that is possible. " ... 

The glycolytic pathway includes three such reactions glucose 6-phosphate isomer-ase (1,2-proton transfer), triose phosphate isomerase (1,2-proton transfer), and eno-lase (yS-elimination/dehydration). The tricarboxylic acid cycle includes four citrate synthase (Claisen condensation), aconitase (j5-elimination/dehydration followed by yS-addition/hydration), succinate dehydrogenase (hydride transfer initiated by a-proton abstraction), and fumarase (j5-elimination/dehydration). Many more reactions are found in diverse catabolic and anabolic pathways. Some enzyme-catalyzed proton abstraction reactions are facilitated by organic cofactors, e.g., pyridoxal phosphate-dependent enzymes such as amino acid racemases and transaminases and flavin cofactor-dependent enzymes such as acyl-C-A dehydrogenases others. 

Triose Phosphate Isomerase Diffusional Encounters with D-Glyceraldehyde-3-Phosphate In this section we use a real system, triose phosphate isomerase (TIM) and its substrate D-glyceraldehyde—3-phosphate (GAP) to demonstrate the capabilities of Brownian dynamics simulations with electrostatics. TIM is a glycolytic enzyme that catalyzes the interconversion of GAP and dihydroxy-acetone phosphate (DHAP). It has been described as an almost perfea catalyst because of its remarkable efficiency. Structurally, TIM is a dimeric enzyme consisting of two identical polypeptide chains of 247 amino acid residues. Each subunit consists of eight loop-p/loop-a units and contains one aaive site. Located near each aaive site is a peptide loop, which is mobile in the native enzyme and folds down to cover the active site when the substrate is bound. Kinetically, the reaction appears to be diffusion controlled and proceeds with a measured rate constant of 4.8 x 10 M s L TIM has consequently been the focus of many kinetic and struaural studies. ... 

The importance of optimal distance for proton transfer has been emphasized by work on triose-phosphate isomerase. An essential base, Glu-16S, has been replaced by Asp, effectively increasing the bond distance for proton transfer by 1 A (50). The rates of the enzyme-catalyzed enolization steps are reduced 1000-fold (50) relative to wild type. Although the mutant is impaired, its activity is still substantial considering that the wild-type enzyme accelerates the reaction 10 -fold relative to acetate ion in solution. Attempts to select for second-site revertants which restore catalytic activity have met with only modest success (51, 52), but they begin to address the important questions pertaining to the evolution of the optimal geometry of the constellation of amino acids around the active site. 

The enzyme triose phosphate isomerase catalyzes the reaction g yceraldehyde-3-phosphate dihydroxyacetone phosphate. If the equilibrium constant is 22.0 at 25 C, calculate AG .for the reaction,... 

An economically viable alternative to the synthesis of deoxyribonuclosides has been developed as a two stage process involving 2-deoxy-D-ribose 5-phosphate aldolase (DERA) (Fig. 6.5.14) (Tischer et al. 2001). The first step was the aldol addition of G3P to acetaldehyde catalyzed by DERA. G3P was generated in situ by a reverse action of EruA on L-fructose-1,6-diphosphate and triose phosphate isomerase which transformed the DHAP released into G3P. In a second stage, the action of pentose-phosphate mutase (PPM) and purine nucleoside phosphorylase (PNP), in the presence of adenine furnished the desired product. The released phosphate was consumed by sucrose phosphorylase (SP) that converts sucrose to fructose-1-phosphate, shifting the unfavorable equilibrium position of the later reaction. 

An enzyme catalyzes the interconversion of dihydroxyacetone phosphate into D-phosphoglyceraldehyde in presence of NAD. Thus, triose phosphate isomerase breaks a carbon-hydrogen bond in the hydroxymethyl group of the D-phosphoglyceraldehyde to yield dihydroxyacetone phosphate. The equilibrium of that reaction favors the formation of the dihydroxyacetone phosphate. From the description of the glycolytic pathway, it is evident that dihydroxyacetone phosphate is produced in two different enzymic reactions, catalyzed by aldolase or triose phosphate isomerase. The exact mechanism of the reaction is not known, but it has ben suggested that it involves the formation of an enolate anion that is bound to the enzyme. 

Phospho-D-glyceraldehyde is isomerized by triose phosphate isomerase to dihydroxyacetone phosphate (reaction 6). The two trioses, 3-phospho-D-glycer-aldehyde and dihydroxyacetone phosphate, are then condensed together in a aldol-type condensation, catalyzed by aldolase to give D-fructose-l,6-bisphos-phate (reaction 7). This is a key intermediate in the photosynthetic process for the formation of many other carbohydrates, including D-glucose (see Fig. 10.5). 

Isomerases catalyze the isomerization of one compound into another. There are many important isomerization reactions in the metabolism of carbohydrates. D-Glucose-6-phosphate is converted into D-fructose-6-phosphate by phosphoglu-coisomerase. Dihydroxyacetone phosphate is converted into 3-phospho-D-glyceraldehyde by the enzyme triose phosphate isomerase. In the Calvin cycle of photosynthesis, this same enzyme converts 3-phospho-D-glyceraldehyde into dihydroxyacetone phosphate. 

Helices and sheets can combine in many other ways. For example, an enzyme that catalyzes a reaction in the degradation of glucose (glycolysis) called triose phosphate isomerase, which has 248 amino acid residues, contains many strand-hehx-strand motifs p-a-P (Figure 27.16). The p strands form a parallel P-pleated sheet. Figure 27.17 shows the tertiary structure of triose phosphate... 

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

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