Triose phosphate isomerase, equilibrium catalyzed

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. 

Scheme 5.—The Equilibrium Catalyzed by Triose Phosphate Isomerase.

Triose phosphate isomerase, TIM, as a nearly-diffusion-limited enzyme (Ch. 2, Section 2.2.3), catalyzes the equilibration of GAP and DHAP very efficiently. However, equilibrium concentrations of GAP were metabolized to pyruvate and further to ethanol or acetate, so the stoichiometric yield on glucose to 1,3-PPD of 42.5% was lower than the target of 50%. Thus, TIM was cloned out to prevent equilibration between the desired DHAP and the undesired side-product GAP, which successfully increased the yield beyond the minimum target... 

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. 

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). 

Glucose-6-P is then isomerized by phosphohexose isomerase to fructose-6-P (making up 30% of the equilibrium mixture). Another kinase phosphorylates the 1-position and the resulting fructose diphosphate is cleaved in an equilibrium reaction to two trioses, namely dihydroxyacetone phosphate (C-1 to C-3) and glyceral-dehyde phosphate (C-4 to C-6). The equilibrium mixture is composed of 89% hexose and 11% triose (under the conditions of Meyerhof s measurements) condensation, therefore, is the preferred (= exergonic) reaction. The reaction is analogous to the aldol condensation described in organic chemistry (Chapt. 1-2, XV-5). Catalysis of the reverse reaction by the enzyme aldolase is explained by the fact that enzymes always catalyze up to the equilibrium. ... 

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