Yeasts transaldolase

The transaldolase (EC 2.2.1.2) is an ubiquitous enzyme that is involved in the pentose phosphate pathway of carbohydrate metabolism. The class I lyase, which has been cloned from human [382] and microbial sources [383], transfers a dihydroxyacetone unit between several phosphorylated metabolites. Although yeast transaldolase is commercially available and several unphosphorylated aldehydes have been shown to be able to replace the acceptor component, preparative utilization has mostly been limited to microscale studies [384,385] because of the high enzyme costs and because of the fact that the equilibria usually are close to unity. Also, the stereochemistry of transaldolase products (e.g. 38, 40) [386] matches that of the products from the FruA-type DHAP aldolase which are more effortlessly obtained. 

The first stage, involving the transfer of active glycolaldehyde, can be accomplished in the laboratory by use of spinach or rat-liver transketolase, and the products isolated and characterized as the barium salt and 2,7-anhydride, respectively. The second stage is catalyzed by liver or yeast transaldolase and is believed to involve the enzymic transfer of a 1,3-di-hydroxy-2-propanone residue sedoheptulose 7-phosphate and D-fructose... 

Similar enzyme inactivation accompanying the oxidation of an enzyme-substrate carbanion intermediate also occurs with class II fructose-1,6-bisphosphate aldolase from yeast, transaldolase, transketolase, and pyruvate decarboxylase from yeast (see the table). 

Transaldolase (EC 2.2.1.2) is an enzyme involved in the pentose phosphate pathtvay tvhere it transfers a dihydroxyacetone unit bettveen several phos-phorylated metabolites [29]. The lyase belongs to class I aldolases and has been purified from several sources, cloned, and structurally characterized [41, 42]. In preparative studies yeast transaldolase, tvhich is commercially available, has been shotvn to accept unphosphorylated aldehydes as the acceptor component [241-243]. 

Nevertheless, the activity of an isolated monomer has been shown for aldolase (Chan and Mawer, 1972 Chan et al, 1973a), and for yeast transaldolase (Chan et aU 1973b), which is a dimeric protein, by the method of matrix-bound derivative (Chan, 1970). This method consists of binding native enzyme to a Sepharose-activated matrix, followed by exhaustive washing with 6 M GuHCl which allows the dissociation and removal of... 

I, 7-diphosphate.170 1 (f> This tetrose phosphate is involved with phosphoenol pyruvate in the formation of shikimic acid via 3-deoxy-2-keto-D-ara6ino-heptonic acid 7-phosphate and, hence, of aromatic compounds.170(d) A synthesis of the tetrose phosphate has been described.170 1 Aldolase shows a high affinity for the heptulose diphosphate and, compared with that for D-fructose 1,6-diphosphate, the rate of reaction is about 60 %. The enzyme transaldolase, purified 400-fold from yeast, catalyzes the following reversible reaction by transfer of the dihydroxyacetonyl group.l70(o>... 

A molecule of dihydroxyacetone is split from sedo-heptulose-7-phosphate and is condensed with glyceraldehyde in an aldol condensation reaction. Since no free dihydroxyacetone accumulates in the medium, the enzyme has been called transaldolase rather than aldolase, and the formation of a dihydroxyacetone enzyme intermediate has been demonstrated. The enzyme was purified 700 times from yeast and was found to catalyze the reaction without the help of a prosthetic group or cofactor. 

The further conversion of E-4-P involves the enzyme transaldolase. This enzyme, which has been purified from brewers yeast (181) and Tonda (297), catalyzes the reversible transfer of the dihydroxyacetone group, with either F-6-P and S-7-P as substrate and E-4-P or Ga-3-P as acceptor (equation 18). Free dihydroxyacetone does not participate in the reaction and has been excluded as an intermediate by... 

Transaldolase does not seem to require any coenzyme. It has been found in yeast, plants, and mammals. 

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