Enzymes triose phosphate isomerase

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... 

The enzyme triose phosphate isomerase eatalyzes the intereonversion of D-glyeeraldehyde 3-phosphate and dihydroxyaeetone phosphate. 

Figure 5-6. Examples of tertiary structure of proteins. Top The enzyme triose phosphate isomerase. Note the elegant and symmetrical arrangement of alternating p sheets and a helices. (Courtesy of J Richardson.) Bottom Two-domain structure of the subunit of a homodimeric enzyme, a bacterial class II HMG-CoA reductase. As indicated by the numbered residues, the single polypeptide begins in the large domain, enters the small domain, and ends in the large domain. (Courtesy ofC Lawrence, V Rod well, and C Stauffacher, Purdue University.)...

As depicted in Figure 6.8 the stability screening was based on DERA activity assay, the retro-aldol reaction of 2-deoxy-D-ribose 5-phosphate to acetaldehyde and D-glyceraldehyde 3-phosphate. D-glyceraldehyde 3-phosphate is further converted by the auxiliary enzymes triose phosphate isomerase and glycerol phosphate dehydrogenase. As the latter reaction consumes NADH it can be measured spectro-pho to metrically by the decrease in absorbance at 340 nm. 

The importance of binding energy to catalysis can be readily demonstrated. For example, the glycolytic enzyme triose phosphate isomerase catalyzes the interconversion of glyceraldehyde 3-phosphate and dihy-droxyacetone phosphate ... 

Dihydroxyacetone phosphate is converted to glycer-aldehyde 3-phosphate by the glycolytic enzyme triose phosphate isomerase. Glyeeraldehyde is phosphorylated by ATP and triose kinase to glyeeraldehyde 3-phos-phate ... 

Exercise 20-20 The following interconversion is catalyzed by the enzyme triose phosphate isomerase ... 

The enzyme triose phosphate isomerase catalyzes the interconversion of D-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. 

Affinity labels are molecules that are structurally similar to the substrate for the enzyme that covalently modify active site residues. They are thus more specific for the enzyme active site than are group-specific reagents. Tosyl-l-phenylalanine chloromethyl ketone (TPCK) is a substrate analog for chymotrypsin (Figure 8.21). TPCK binds at the active site and then reacts irreversibly with a histidine residue at that site, inhibiting the enzyme. The compound 3-bromoacetol is an affinity label for the enzyme triose phosphate isomerase (TIM). It mimics the normal substrate, dihydroxyacetone phosphate, by binding at the active site then it covalently modifies the enzyme such that the enzyme is irreversibly inhibited (Figure 8.22). 

Glyceraldehyde 3-phosphate continues on in the glycolysis pathway, but dihydroxyacetone phosphate is first isomerized by the enzyme triose phosphate isomerase. As in the glucose-to-fructose conversion of step 2, the... 

BLA.ST away. Using the National Center for Biotechnology Information Web site (www.ncbi.nlm.nih.gov), find the se quence of the enzyme triose phosphate isomerase from E, coli. Use this sequence as the query for a protein—protein BLAST search. In the output, find the alignment with the sequence of triose phosphate isomerase from human beings Homo sapiens]. How many identities are observed in the alignment ... 

Another type of disorder-to-order transition is the so-called loop-cap transition in certain enzymes.279,280 This structural change is associated with a highly mobile loop of residues, from 6 to 12 amino acid residues in length, that are located near the active site. Such a structural feature has been identified in the enzyme triose phosphate isomerase (TIM). TIM makes an especially interesting case study for theoretical analysis of the loop-cap transition because there exist X-ray,279,313 kinetic,121 and thermodynamic data121 for this enzyme. 

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,... 

A detailed thermod5mamic analysis was performed with lactate dehydrogenase, in the lactate — p5mivate direction, by means of steady-state kinetics and presteady-state kinetic methods, by Laidler and Peterman (1979). A particularly detailed kinetic studies of the energetics of two multistep enzymes, triose-phosphate isomerase and prohne racemase, has been described by the research team of Albery and Knowles (Albery Knowles, 1976, 1986 Knowles, 1991). Apart from these examples, very few complete thermodynamic analyses have been performed with reactions involving more than one substrate or more than one intermediate in reaction. 

Aldolase then catalyses the reversible cleavage of the six-carbon molecule into two three-carbon molecules, triose phosphates. Yeast aldolase is inactivated by cysteine and may be reactivated by Zn +, Fe + or Co + ions. The triose phosphates are a mixture of dihydroxyacetone phosphate and D-glyceraldehyde-3-phosphate. Only the latter undergoes further change in the EMP pathway, but an equilibrium between the two is maintained by enzymic conversion of some of the dihydroxyacetone phosphate into glycer-aldehyde-3-phosphate, catalysed by the enzyme triose-phosphate isomerase. 

F. K. Brown and P. A. Kollman, ]. Mol. Biol., 198,533 (1987). Molecular Dynamics Simulations of Loop Closing in the Enzyme Triose Phosphate Isomerase. 

CO2, present in the form of HCO3—, is fixed in the acceptor, ribulose-1,5-diphosphate, by means of the enzyme carboxydismutase. An intermediate with 6 C atoms is formed, the identity of which is still unknown. This substance is unstable. It decomposes into two molecules of 3-phosphoglyceric acid. The latter is then reduced to 3-phosphoglyceraldehyde by means of the ATP and NADPH + H+ formed in the primary processes. 3-Phosphoglyceraldehyde exists in equilibrium with its isomer, dihydroxy acetone phosphate. The equilibrium is controlled by the enzyme triose phosphate isomerase. 3-Phosphoglyceralde-hyde and dihydroxy acetone phosphate ar referred to collectively as triose phosphate. 

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. 

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. 

Trioses. n-Glyceraldehyde (formula in Section 1) is the dehydrogenation product of glycerol. More important, however, is 3-phosphoglyceraldehyde, an intermediate in the degradation of carbohydrates (Chapt. XVII-6), which is in equilibrium with dihydroxyacetone phosphate the attainment of equilibrium is catalyzed by the enzyme triose phosphate isomerase (cf. Section 7). 

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