Thiamine pyrophosphate Active glycolaldehyde

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. 

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... 

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. 

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. ... 

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),... 

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. 

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