Triose phosphate isomerase mechanism

Figure 13.6 Triose phosphate isomerase mechanism of action...

We have seen many examples of chemical reactions involving enolate anions, and should now realize just how versatile they are in chemical synthesis (see Chapter 10). We have also seen several examples of how equivalent reactions are utilized in nature. For the triose phosphate isomerase mechanism above, we did not actually invoke a distinct enolate anion intermediate in the enolization process, but proposed that there was a smooth flow of electrons. For other reactions, we shall also need to consider whether enolate anions are actually involved, or whether a more favourable alternative exists. The aldol-type reaction... 

FIGURE 19.15 A reaction mechanism for triose phosphate isomerase. 

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. 

The rather toxic methylglyoxal is formed in many organisms and within human tissues.174 It arises in part as a side reaction of triose phosphate isomerase (Eq. 13-28) and also from oxidation of acetone (Eq. 17-7) or aminoacetone, a metabolite of threonine (Chapter 24).175 In addition, yeast and some bacteria, including E. coli, have a methylglyoxal synthase that converts dihydroxyacetone to methylglyoxal, apparently using a mechanism similar to that of triose phosphate isomerase. It presumably forms enediolate 2 of Eq. 13-26, which eliminates inorganic phosphate to yield methyl-... 

Chapter 8, How Enzymes Work, starts with a description of the basic chemical mechanisms that are exploited by enzymes. The latter half of this chapter presents a detailed description of how three enzymes—chymotrypsin, RNase, and triose phosphate isomerase—exploit these basic mechanisms of enzyme catalysis. 

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). 

Figure 16.6. Catalytic Mechanism of Triose Phosphate Isomerase. Glutamate 165 transfers a proton between carbons with the assistance of histidine 95, which shuttles between the neutral and relatively rare negatively charged form. The latter is stabilized by interactions with other parts of the enzyme.

Argument by analogy. Propose a mechanism for the conversion of glucose 6-phosphate into fructose 6-phosphate by phosphoglucose isomerase based on the mechanism of triose phosphate isomerase. 

Enzyme Mechanisms.— Triose phosphate isomerase has been a popular enzyme recently, having been the chief example quoted in two reviews on perfection and efficiency in enzyme catalysis - and the subject of seven successive papers in one issue of Biochemistry including one on the evolution of enzyme function and the development of catalytic efficiency. During glycolysis in muscle, fructose 1,6-bisphos-... 

Mechanism Triose Phosphate Isomerase Salvages a Three-Carbon Fragment... 

Figure 16.5 Catalytic mechanism of triose phosphate isomerase. (1) Glutamate 165 acts as a general base by abstracting a proton (H ) from carbon 1. Histidine 95, acting as a general acid, donates a proton to the oxyger atom bonded to carbon 2, forming the enediol intermediate,...

T. Alber, D. Banner, A. Bloomer, G. Petsko, D. Phillips, P. Rivers, and I. Wilson, Phil. Trans. R. Soc. Land., 293,159 (1981). On the Three-Dimensional Structure and Catalytic Mechanism of Triose Phosphate Isomerase. 

Another interesting example of the isoracemisation mechanism is in the triose phosphate isomerase pathway. Rieder and Rose [47] indicated that exchange of the proton in the triose phosphate occurred in the presence of the isomerase not consistent with the hydride shift. Later work by Herhhy et al. [48] indicated that some 3-6% of the hydrogen at C, of the product dihydroxyacetone phosphate was transferred to Cj (Eqn. 66). 

Figure 8.49 Mechanisms of three enzymes that utilise general acid-base catalysis as part of their mechanistic paths to successful bio-catalysis, (a) triose phosphate isomerase (TIM), (b) lysozyme, (c) RNAse A. In all cases substrates are shown in red. Lone pair donor amino acid residues are general bases, lone pair acceptor amino acid residues are general acids. Note that pK (a commonly used term) is the equivalent of p/f/ or pK (as written in this text book) as appropriate for an acidic or basic functional group.

Figure 8.58 Schematic illustration of reaction coordinate diagram of Triose Phosphate Isomerase (TIM) enzyme illustrating near perfect energy landscape pathway allowing for near perfect 1 1 1 stoichiometric equilibrium between all enzyme-bound species optimal for flux through from one enzyme-bound species to another. Enzyme turnover rate kobs is at the diffusion limit, the rate determining step is the association of dihydroxy acetone phosphate (DHAP) with the TIM catalytic site, see Fig. 8.1, hence chemistry is not rate limiting. Therefore, TIM is considered a perfect enzyme For TIM enzyme assay see Fig. 8.17 for TIM enzyme mechanism see Fig. 8.49 (illustration adapted from Knowles, 1991, Fig. 2).

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. 

---