Activated glycolaldehyde

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 tetrose phosphate (LVI) acts as an acceptor for active glycolaldehyde derived from n-i/ireo-pentulose 5-phosphate (LII), and thus, in the presence of transketolase, yields D-fructose 6-phosphate (LV) and D-glyc-erose 3-phosphate. The reverse of this reaction has been observed.200 The... 

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...

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... 

Transketolase (EC 2.2.1.1) an enzyme that catalyses transketolation, an important process of carbohydrate metabolism, especially in the Pentose phosphate cycle (see) and Calvin cycle (see). T. has been found in a wide variety of cells and tissues, including mammalian liver, green plants and many bacterial species. The enzyme contains divalent metal cations and the coenzyme, thiamin pyrophosphate. Transketolation involves transfer of a C2-unit (often called active glycolaldehyde or a ketol moiety) from a ketose to Cl of an aldose. Only ketoses with L-configuration at C3 and preferably irons configuration on the next carbon (i.e. Cl, 2, 3 and preferably 4 as in fructose) can serve as donors of the C2-unit. The acceptor is always an aldose. Thins-ketolation is reversible. Details of the reaction in which xylulose S-phosphate serves as the donor of... 

Transketolase requires an acceptor it cannot split a ketol to form aldehydes. Presumably an intermediate active glycolaldehyde" is formed by combination of the two-carbon unit with the enzyme. The enzyme will react with a number of aldehydes, and the reactions in many cases have been shown to be reversible. The donor requirements are not completely understood. Among the sugars only those with the hydroxyl on carbon 3 on the l side are substrates. Before the discovery of epimerase, ribulose-5-phosphate was thought to be a substrate, but it has been shown that a mixture of ribose and ribulose phosphates does not undergo the transketolase reaction until epimerase is added. Besides xylulose phosphate, fructose-6-phosphate, sedoheptulose-7-phosphate, and hydroxypyruvate are glycolaldehyde donors. 

The specificity of purified transketolase is rather broad, and several compounds have been shown (93) to act as donors of active glycolaldehyde. Included in these compounds are D-ribulose 5-phosphate, D-sedoheptulose 7-phosphate, D-fructose 6-phosphate, L-erythrulose, and hydroxypyruvic acid. A number of aldehydes have been shown to act as active glycolalde-... 

In the cleavage of pentose phosphate a 2-carbou fragment, presumably glycolaldehyde, is expected to be formed and also to react in the reverse reaction. Glycolaldehyde neither accumulates nor does it react with these enzyme preparations. It is now postulated that an active glycolaldehyde is formed in the reactions of pentose-phosphate metabolism. This may be conjugated with thiamine pyrophosphate. 

The first chiral synthetic equivalents for the overall enantioselective transfer of the active glycolaldehyde moiety were easily prepared from benzyloxy-acetaldehyde as chiral aminonitriles (scheme 23) [64], however, the subsequent metalation resulted in 3-elimination, so that the active glycolaldehyde mimicry still waits to be solved and will be part of our future projects. 

A further molecule of ribulose-5-P furnishes an active glycolaldehyde to the tetrose, forming a new molecule of F—6— P. 

The enzyme is of low specificity and it also acts on ribulose-5-P, sedo-heptulose-7-P, L-erythrulose, hydroxypyruvate and fructose-6-P. A rupture of the ketol bond occurs and the active glycolaldehyde formed is condensed with an acceptor aldehyde. When the acceptor is glyceralde-hyde 3-P, ribulose-5-P is formed. 

A most important clue to the nature of the steps between pentose phosphate and hexosemonophosphate, and thus to the role of the pentose phosphate pathway in photosynthesis, came from our discovery in 1953 of sedoheptulose 7-phosphate as the first product formed from pentose phosphate. The enzyme transketolase had been purified from rat liver and spinach in my laboratory and crystallized from yeast by Racker and his coworkers and the two laboratories simultaneously discovered that this enzyme contained thiamine pyrophosphate as its functional group, f Isotope studies in my laboratory showed that sedoheptulose was formed by the transfer of a C2 group ( active glycolaldehyde ) from one molecule of pentose phosphate to another, and that the reaction was fully reversible thus sedoheptulose 7-phosphate was also a Ca-donor. In addition, Racker s laboratory made the important finding that fructose 6-phosphate would also yield active glycolaldehyde, and Arturo Bonslgnore and his coworkers discovered that rat liver extracts catalyzed the rapid non-oxidative conversion of hexose phosphate to sedoheptulose phosphate. ... 

While formic acid accumulation in folic deficiency makes it clear that the main route for formate removal is folic-mediated, little is known in this respect about formaldehyde, which arises from the oxidation of methyl groups or the folic-mediated reduction of formate (Section II, 5). Perhaps an important connection between Ci and the principal pathways for substrates is indicated by the finding that HCHO is an excellent acceptor for the ketol group ( active glycolaldehyde ) formed from a variety of ketose phosphates and from hydroxypyruvate (Dickens and Williamson, 1958). This reaction may be formulated as follows ... 

The acceptor aldehyde can be glycolaldehyde, 3-p-glyceraldehyde, 5-p-ribose, or 5-p-desoxyribose. There are really two quite separate processes which are catalyzed by Racker s yeast enzyme first, a non-oxidative decarboxylation of hydroxypyruvic acid to active glycolaldehyde and CO2 second, a transketolation of active glycolaldehyde from the enzyme to the acceptor aldehyde. One must conceive of the enzyme as the bearer of a keto group (ECO) which reacts with hydroxy-pyruvate to form an acyloin, with CO2 being liberated in the process ... 

The analogy between the formation of active acetate from pyruvate and active glycoaldehyde from hydroxypyruvate is striking. In both instances diphosphothiamine plays an important role and the active intermediary is linked to the enzyme. The possibility that lipoic acid plays some part in the formation of active glycolaldehyde has yet to be considered experimentally. 

Transketolase catalyzes the transfer of a Cj fragment from a suitable ketol donor to an aldehyde acceptor. The enzyme has been purified from liver and spinach by Horecker et al. (183) and crystallized from yeast by de la Haba et al. (91). All three enzyme preparations require thiamine pyrophosphate as coenzyme although the precise function of this prosthetic group remains unknown. Presumably a glycolaldehyde-thiamine pyrophosphate-enzyme complex is formed this has been termed active glycolaldehyde (298). A number of phosphate esters have been shown to act as active glycolaldehyde donors, including xylulose-5-phosphate (175,324),... 

A number of aldehydes can combine with active glycolaldehyde to form the corresponding ketoses those which appear to have importance in biological systems are Ga-3-P, E-4-P and R-5-P. 

Pentose phosphate is formed directly from F-6-P by the transfer of active glycolaldehyde from this substrate to Ga-3-P (131,299) (equation 15). 

Thiamine Pyrophosphate is the coenzyme responsible for transferring active acetaldehyde and active glycolaldehyde. Simultaneously, it acts as co-decarboxylase, i.e. as the coenzyme for a lyase. The characteristic component is thiamine (vitamin Bi), one of the vitamins whose biologic role has been known longest the symptoms of thiamine deficiency (beriberi in man polyneuritis in animals) comprised one of the starting points in the study of vitamins. The name thiamine refers to its sulfur content. 

Transketolase. A C2 fragment, active glycolaldehyde, is transferred during the transketolase reaction. This is an equilibrium reaction similar to the acyloin condensation of organic chemistry, in which two moles of adlehyde yield a hydroxy-ketone ... 

Amino acids can also turn into carbohydrates by several other pathways The decarboxylation of hydroxypyruvate (arising from serine by transfl.minfl.tion) yields active glycolaldehyde, which is then available for the transketolase reaction. Thus two a-carbons of serine can enter carbohydrate. 

Sedoheptulose 7-phosphate was first detected in yeast fermentation products . It is formed by transfer of active glycolaldehyde to ribulose... 

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