Triose phosphate isomerase reactions involving

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.

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... 

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. 

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. 

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. 

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