Hydroxypyruvic

A number of lyases are known which, unlike the aldolases, require thiamine pyrophosphate as a cofactor in the transfer of acyl anion equivalents, but mechanistically act via enolate-type additions. The commercially available transketolase (EC 2.2.1.1) stems from the pentose phosphate pathway where it catalyzes the transfer of a hydroxyacetyl fragment from a ketose phosphate to an aldehyde phosphate. For synthetic purposes, the donor component can be replaced by hydroxypyruvate, which forms the reactive intermediate by an irreversible, spontaneous decarboxylation. 

The high stereoselectivity of the transketolase reaction also enables the resolution of racemic a-hydroxyaldehydes23,26. Treatment of racemic 2-hydroxyaldehydes and hydroxypyruvic acid with transketolase, gave the corresponding L-2-hydroxyaldehydes that are not substrates for the enzyme and, therefore, remained unreacted. The corresponding D-enantiomers were consumed and gave the condensation products. 

Figure 10.37 Kinetic resolution by transketolase, and nonequilibrium C—C bond formation by decomposition of hydroxypyruvate.

Transketolase (TKase) [EC 2.2.1.1] essentially catalyzes the transfer of C-2 unit from D-xylulose-5-phosphate to ribose-5-phosphate to give D-sedoheptulose-7-phosphate, via a thiazolium intermediate as shown in Fig. 16. An important discovery was that hydroxypyruvate works as the donor substrate and the reaction proceeds irreversibly via a loss of carbon dioxide (Fig. 17). In this chapter, we put emphasis on the synthesis with hydroxypyruvate, as it is the typical TPP-mediated decarboxylation reaction of a-keto acid. ... 

Dihydroxyacetone is not stable under the basic conditions preferred for oxidation of the primary function to give hydroxypyruvic acid (reaction e). Under acidic conditions the rate of oxidation of a 1 mol I l aqueous solution is veiy slow (5 mol h l mol i(Pt)). On platinum the initial rate of conversion for reduced concentrations of the starting material (0.3 mol whilst retaining the same amount of catalyst, was 42 mol h mol-i(Pt), as might be expected under non-favourable acidic conditions. Hydroxypyruvic acid is evolved with a selectivity of 82% at 40% conversion (see Figure 11). 

D-erythro-Pentulose 5-phosphate (XLIV) has been formed by the action of transketolase on hydroxypyruvate (XLII) and D-glycerose 3-phosphate, the hydroxypyruvate being decarboxylated196 to active glycolaldehyde which then reacts with the triose phosphate by an acyloin reaction.28 The active glycolaldehyde is also formed from L-glycero-tetrulose, d-altro-heptulose 7-phosphate, D-fructose 6-phosphate, and D-i/ireo-pentulose 5-phosphate and it reacts with various aldehydes (acceptors) to give ketoses.198, 200 Thus, substitution of L-gfh/cero-tetrulose for hydroxypyruvate in the above experiment also resulted in formation of D-en/i/iro-pentulose... 

The Pt/C catalyst, compared with Pd/C, showed not only enhanced activity (vide supra) but also reduced selectivity for glyceric acid (only 55% at 90% conversion), favoring dihydroxyacetone formation up to 12%, compared with 8% for the Pd case [48]. The Pt/C catalyst promoted with Bi showed superior yields of dihydroxyacetone (up to 33%), at lower pHs. Glyceric and hydroxypyruvic acids, apparently, are formed as by-product and secondary product, respectively [48], The addition of Bi seems to switch the susceptibility of glycerol oxidation from the primary towards the secondary carbon atoms. 

Bi/Pt atomic ratios (changing from 0.1 to 0.4), as well as the precise preparation procedure of the BiPt/C catalysts, yielded differences in selectivity between glyceric acid, dihydroxyacetone and hydroxypyruvic acid that were not always easy to rationalize [72, 73]. The catalyst preparation procedures using acid-treated carbon (with enhanced number of surface carboxyl groups) are essentially as follows ... 

Furthermore, a base-catalyzed transformation by OH from the reaction medium between glycerate and hydroxypyruvate aldehyde (or hydroxypyruvic acid) could be excluded, while hydroxyacetone and glyceraldehyde interconversion was possible (Scheme 11.11). The existence of two major routes, of which hydroxyacetone and glyceric aldehyde are the primary oxidation products and glycolic and oxalic acid are the end-members, respectively, is now firmly established. Clearly, rapid oxidation of glyceraldehydes favors glyceric acid rather than hydroxyacetone formation. 

In a 100 mL flask, tris buffer (1.36g, 10 mmol), lithium hydroxypyruvate (154 mg, purity 60 %, 0.84 mmol), 3-thioglyceraldehyde (650 mg, 6.1 mmol), MgCl2 (21 mg, 0.105 mmol,) and 32 mg of ThDP (0.07 mmol) were added to 35 mL of distilled water. The pH was adjusted to 7.5 and the volume adjusted to 50 mL with distilled water. Then, 200 units of yeast TK were added. The reaction was stirred at 30 °C until complete disappearance of a-hydroxypyruvate. ... 

The concentration of hydroxypyruvate was determined by spectrophotometry at 340 nm. A 20 pL aliquot from the reaction mixture was introduced into 1 mL of triethanolamine buffer (0.1 m) at pH 7.6 containing 20 /rL of NADH solution in water (14 mm, 0.28 /imol) and 2 units of L-lactate dehydrogenase. The absorbance due to NADH is proportional to the concentration of hydroxypyruvate. 

This enzyme [EC 2.6.1.44] catalyzes the reversible, pyri-doxal-phosphate-dependent reaction of alanine with gly-oxylate to generate pyruvate and glycine. One component of the animal enzyme can utilize 2-oxobutanoate as a substrate instead of glyoxylate. A second component of the enzyme can also catalyze the reaction of alanine with 3-hydroxypyruvate to produce pyruvate and serine. See SerineiPyruvate Aminotransferase... 

This enzyme [EC 1.1.1.29], also known as hydroxypyru-vate reductase and hydroxypyruvate dehydrogenase, catalyzes the reaction of (R)-glycerate with NAD to produce hydroxypyruvate and NADH. 

Glyoxylate reductase [EC 1.1.1.26] catalyzes the reversible reaction of glycolate with NAD+ to produce glyoxylate and NADH. The enzyme will also catalyze the NADH-dependent interconversion of hydroxypyruvate to D-glycerate. Glyoxylate reductase (NADPH) [EC... 

I. 1.1.79] catalyzes the reversible reaction of glycolate with NADP+ to produce glyoxylate and NADPH (as well as the hydroxypyruvate to D-glycerate conversion). The enzyme can use NAD+ as a substrate, although not as effectively as NADP+. 

This thiamin pyrophosphate-dependent enzyme [EC 2.2.1.1], also known as glycolaldehyde transferase, catalyzes the reversible reaction of sedoheptulose 7-phos-phate with D-glyceraldehyde 3-phosphate to produce D-ribose 5-phosphate and o-xylulose 5-phosphate. The enzyme exhibits a wide specificity for both reactants. It also can catalyze the reaction of hydroxypyruvate with R—CHO to produce carbon dioxide and R—CH(OH)—C(=0)—CH2OH. Transketolase isolated from Alkaligenes faecalis shows high activity with D-erythrose as the acceptor substrate. 

Scheme 2.2.2.1 Principal reactions of transketolase. Ketose donor substrates include xylulose 5-phosphate (upper left) or hydroxypyruvate (lower left). Acceptor substrates are a-hydroxyaldehydes. A C2 unit ( activated glycolaldehyde ) is transferred to the acceptor substrate via a ThDP-bound a, 3-dihydroxyethyl group thereby forming a novel ketose of 3S,4R...

Stereoconfiguration, and a shortened hydroxyaldehyde (glyceraldehyde 3-phosphate in the case of xylulose 5-P as donor). If hydroxypyruvate is used, carbon dioxide is evolved and leaves the reaction, thus shifting the equilibrium of the reaction to the products side. This leads to a virtually complete reaction. 

The gram-scale preparation of rare sugars by E. coli transketolase was demonstrated successfully for (S)-erythrulose from glycolaldehyde and hydroxypyruvate in an enzyme membrane reactor which allowed the continuous production of (S)-erythrulose with high conversion and a space-time yield of 45 g L" d was reached [12]. 

Starting from commercially available fructose 1,6-bisphosphate and hydroxypyruvate, three enzymes were in use fructose 1,6-bisphosphate aldolase, triosephosphate isomerase, and transketolase see E. T. Zimmermann,... 

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