Isomerase reaction

The triose phosphate isomerase reaction completes the first phase of glycolysis, each glucose that passes through being converted to two molecules of glyceraldehyde-3-phosphate. Although the last two steps of the pathway are... 

Pratt, J. M. Coordination Chemistry of the B12 Dependent Isomerase Reactions Wiley New York, 1982. 

Isomerase Reactions Catalyzed by Enzymes Containing 5 -Deoxyadenosylcorrinoids... 

The enzymes in the so-called "isomerase reactions contain 5 -deoxy-adenosylcorrinoids. Labeling experiments have been used to identify the C, H, and O atoms vv hich have moved in the course of the rearrangement and to show that during the reaction the hydrogen atoms of the substrate exchange with the hydrogen atoms of the C-5 atom coordinated to the cobalt ion, but not with the solvent. There is also some spectroscopic evidence that the Co—C bond is broken during the reaction. 

The CPPase substrate DMAPP (15) is formed from isopentenyl pyrophosphate (IPP) (14) via the IPP isomerase reaction. It had been assumed that IPP was generated only via mevalonic acid (12) (Fig. 2), but Rohmer discovered another route, 2-C-methyl-D-erythritol 4-phosphate (13) (MEP) pathway (Fig. 2) [22, 23]. A key step in the MEP pathway is the reaction catalyzed by 1-deoxy-D-xylulose 5-phosphate synthase (DXS), which combines hydroxyethyl thiamine pyrophosphate (hydroxyethyl TPP) generated from pyruvic acid (17) and TPP with glyceral-dehyde 3-phosphate (18) to yield 1-deoxy-D-xylulose 5-phosphate (19) containing five carbons. The mevalonate pathway operates in the cytosol of plants and animals, whereas the MEP pathway is present in the plastid of plants or in eubacteria [24-27]. 

The reaction mechanism is similar to the reaction promoted by phosphohexose isomerase in step (2) of glycolysis (Fig. 14-4). After the triose phosphate isomerase reaction, C-1, C-2, and C-3 of the starting glucose are chemically indistinguishable from C-6, C-5, and C-4, respectively (Fig. 14-6), setting up the efficient metabolism of the entire six-carbon glucose molecule. 

Fructose bisphosphate is cleaved by action of an aldolase (reaction 4) to give glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. These two triose phosphates are then equilibrated by triose phosphate isomerase (reaction 5 see also Chapter 13). As a result, both halves of the hexose can be metabolized further via glyceraldehyde 3-P to pyruvate. The oxidation of glyceraldehyde 3-P to the corresponding carboxylic acid, 3-phosphoglyceric acid (Fig. 17-7, reactions 6 and 7), is coupled to synthesis of a molecule of ATP from ADP and P . This means that two molecules of ATP are formed per hexose cleaved, and that two molecules of NAD+ are converted to NADH in the process. 

Demonstration that the enediol intermediate in aldose-ketose isomerase reactions is syn... 

Co corrinoids play central roles in the two classes of enzymic reactions, i.e. methyl transfer mediated by vitamin B,2 and mutase or isomerase reactions catalyzed by coenzyme B. 253 Though there remain many ambiguities, the former is considered to be a combination of Scheme 100, i and its reverse process, and the latter to be represented by Scheme 103. 

The substrates of triosephosphate isomerase can be labeled specifically with deuterium. The rate of the isomerase reaction with [l(R)-2H]-dihydroxyacetone phosphate (a) as substrate is slowed by about a factor of 3 relative to the rate with ordinary dihydroxyacetone phosphate. This isotope effect indicates that breaking the H—C bond to form the ene-diolate intermediate is the rate-limiting step in conversion of dihydroxyacetone phosphate to... 

Rose, I. A., Mechanism of the aldose-ketose isomerase reactions. Adv. Enz.ymol. 43 491, 1975. 

Measurements of the crude specific activity (mmoles of product synthesized per minute per mg of protein in the supernatant after a 50,000 x g centrifugation) of the two isomerases in E. coli indicated that the conversion of D-ribulose-5-phosphate to D-ri-bose-5-phosphate was approximately 20- to 30-fold greater than the conversion of D-ribulose-5-phosphate to D-arabinose-5-phosphate. This rate of reaction strongly pulls the reaction substrate to D-ribose-5-phosphate, since the isomerase reaction at equilibrium strongly favors the formation of the aldo-sugar over the key intermediate D-ribulose-5-phosphate. 

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.

Faces of molecules can also be diastereotopic an example is provided by the enediol intermediate, 44, postulated to occur in the glucose-phosphate isomerase reaction ... 

P450s catalyze a number of other oxidative reactions, on substrates that range from alkanes to complex endogenous molecules such as steroids and fatty acids. Table 10.1 lists many of the oxidative reactions catalyzed by P450s. These enzymes are also known to catalyze non-oxidative dehydrase, reductase and isomerase reactions [15],... 

Chapter 11 Lyase Reactions (Class 4), Isomerase Reactions (Class 5), and Ligase Reactions (Class 6) at 298.15 K... 

Standard Transformed Gibbs Energies of Reaction, Changes in Binding of Hydrogen Ions, and Apparent Equilibrium Constants of Isomerase Reactions... 

---