Glyceraldehyde 3-phosphate, formation

Figure 5.4 Outline of the newly discovered glyceraldehyde phosphate/pyruvate pathway for the formation of C5 isoprenoid units. None of the intermediates after 2-C-methyl-D-erythritol 4-phosphate is known. P indicates a phosphate moiety. TPP, thiamine pyrophosphate NADP, nicotinamide adenine dinucleotide phosphate.

Figure 16.9. Free-Energy Profiles for Glyceraldehyde Oxidation Followed by Acyl-Phosphate Formation. (A) A...

Figure 16.7 Free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation. (A) A hypothetical case with no coupling between the two processes. The second step must have a large activation barrier, making the reaction very slow. (B) The actual case with the two reactions coupled through a thioester intermediate.

Aldolase (see Fig. 1-7) is the enzyme that catalyzes the splitting of hexose diphosphate to yield triose phosphates [49-51]. This reaction is reversible, and, in fact, the equilibrium favors hexose diphosphate formation. Thus, aldolase condenses dihydroxyacetone phosphate and D-glyceraldehyde phosphate in a typical aldol condensation this type of reaction explains the origin of... 

Phosphofructokinase, the enzyme that phosphory-lates fructose-1-phosphate to yield the diphosphate, the precursor of the triose phosphates, has a fate similar to that of hexokinase, except that its prenatal activity is only three times greater than that of adult liver, and the prenatal activity drops to adult values within 9 days after birth. Fructose-1,6-diphosphate, triose-P-isomerase, and glyceraldehyde phosphate dehydrogenase all have high fetal activities that slightly increase at the adult levels in the newborn. Thus, in the fetal liver the activity of these enzymes seems to favor the formation rather than the use of lactic acid. 

The enzyme has also been employed in a multi-enzymatic scheme for conversion of starch into o-fructose in this scheme transaldolase vas used to formally dephosphorylate fructose 6-phosphate by transferring a dihydroxyacetone moiety to o-glyceraldehyde vith formation of o-fructose and 34 [244]. 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

FIGURE 16.10 Formation of a covalent intermediate in the glyceraldehyde-3-phos-phate dehydrogenase reaction. Nucleophilic attack by a cysteine —SH group forms a covalent acylcysteine intermediate. Following hydride transfer to NAD, nucleophilic attack by phosphate yields the product, 1,3-bisphosphoglycerate. 

The transaldolase functions primarily to make a useful glycolytic substrate from the sedoheptulose-7-phosphate produced by the first transketolase reaction. This reaction (Figure 23.35) is quite similar to the aldolase reaction of glycolysis, involving formation of a Schiff base intermediate between the sedohep-tulose-7-phosphate and an active-site lysine residue (Figure 23.36). Elimination of the erythrose-4-phosphate product leaves an enamine of dihydroxyacetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. 

Bloxham, D.P., and Cooper, C.K. (1982) Formation of a polymethylene bis(disulfide) inter-subunit crosslink between cys-281 residues in rabbit muscle glyceraldehyde-3-phosphate dehydrogenase using octamethylene bzs-(methane[35]thiosulfonate). Biochemistry 21, 1807. 

Boyer, P.D. Segal, H.L. (1954). Sulfhydryl groups and glyceraldehyde-3-phosphate dehydrogenase and acyl-enzyme formation. In Metabolic Pathways. (Greenberg, D.M., Ed.), 2nd ed., Vol. 1, pp. 520-532. Academic Press, New York. 

A practical, inexpensive one-step procedure was developed for the RhaD-catalyzed gram-scale synthesis of L-fructose. The requirement for DHAP as the donor substrate was circumvented by use of borate buffer, presumably by in situ formation of borate esters as a phosphate ester mimic. Racemic glyceraldehyde was also used, as the enzyme preferentially accepted the L-enantiomer as a substrate. The method can also be apphed to other products, including L-rhamnulose, and towards a two-step synthesis of L-iminocychtols. ... 

The reverse aldol reaction results in the formation of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Dihydroxyacetone phosphate... 

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