Trioses to Hexoses

1 Trioses to Hexoses. - Mixtures of erythro- and t/ireo-3-pentulose have been obtained in the base-catalysed aldol condensation between unprotected DL-g ero-tetrulose and formaldehyde, the 

In the transketolase-catalysed condensation (see Vol. 27, Chapter 2, Refs. 6,16) of hydroxypyruvate with a mixture of all four 4-deoxytetrose isomers, only the 2R- isomers reacted and only products with S-stereochemistry at the new chiral centre (C-3), le., 6-deoxy-D-fhictose and 6-deoxy-L-sorbose, were formed. A preparation of D- and L-foictose by enzyimc tddol condeos on is referred to below (Ref. 23), and the enzymic conversion of D- and L- r thro-peittulose, D4i ose and D-psicose to the corre randk 3,4-rhreo-compounds is covered hi Part 4 tsi dds Chtqner (Rrf. 65). 

SelcGtive cleavage of either diastereomer of phenyl or 4 nitrophenyl P-D-ghicopyranosyl suifbxiide to e the free sugis has been achieved with P- ucosidaaes of vadous origins.  

1 Trioses to Hexoses.- A kinetic analysis of the formose reaction has led to the conclusion that the carbonyl rearrangement and retro-aldol reaction steps play a substantial role in the autocatalytic process. The condensation of D-glyceraldehyde acetonide with racemic alkyl (alkylthiomethyl) sulfoxides proceeded with high selectivity to produce diastereomeric l-alkylsulfinyl-l-alkylthio-3,4-0-isopropylidene-D-tetroses with a predominance of e/yrhro-products. An example is given in 

Polymer-supported thiazolium salts have been found to have high catalytic activity in the formose reaction, the main product being hydroxyacetone. Use of synthetic a-ketols as catalysts in the formose reaction led to selective formation of trioses, especially with o-ketols bearing electronegative substituents. Incubation of D-glucose with bakers yeast in the presence of benzyl mercaptan gave 5-benzyl thioglycerate in preparatively useful yields fermentation experiments with 

Tlie biomimetic transformation of starch to D-fractose in a reactor containing inorganic phosphate and five enzymes has been described. Transketolase-catalysed stereospecific 

Reagents i. Pseudomonas cepacia lipase, OAc ii, O3 iii, NaBH4 iv, NaI04 

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Trivial names are common in carbohydrate nomenclature. Fifteen of them form the basis of the systematic nomenclature. They are assigned to the simple aldoses (polyhydroxyaldehydes), from triose to hexoses. 

Structural family of D-carbohydrates, from triose to hexoses, with their names... 

A. R. Fernie, A. Roscher, R. G. Ratcliffe, and N. J. Kruger, Fructose 2,6 bisphosphate activates pyrophosphate fructose 6 phosphate 1 phosphotransferase and increases triose phosphate to hexose phosphate cycling in heterotrophic cells. Planta 212, 250 263 (2001). 

It remains undecided whether the formation of small amounts of glycerol reported by Oppenheimer in the case of zymase extract is due to a ph3rtochemical reduction of trioses. If hexoses are fermented in the presence of trioses with ordinary fresh yeasts which do not attack dihy-droxyacetone and glyceraldehyde, the added trioses are recovered practically unaltered after the disappearance of the hexoses. 

FIGURE 20-11 Transketolase-catalyzed reactions of the Calvin cycle, (a) General reaction catalyzed by transketolase the transfer of a two-carbon group, carried temporarily on enzyme-bound TPP, from a ketose donor to an aldose acceptor, (b) Conversion of a hexose and a triose to a four-carbon and a five-carbon sugar (step of Fig. 20-10). (c) Conversion of seven-carbon and three-carbon sugars to two pentoses (step of Fig. 20-10). 

Because of the possible alternative pathways, which probably occur simultaneously, no single set of reactions can uniquely describe the pentose phosphate pathway. One possible set of pathways is shown in figure 12.34. If the triose phosphate formed is converted to hexose phosphate, the overall pathway can be seen as regenerating five molecules of hexose phosphate for each six used initially. 

An important and common method for lengthening and shortening the carbon chains of organic molecules, both chemically and biochemically, is the aldol-type reaction. Fischer and Tafel (in 1887) first described an alkali-catalyzed condensation of two trioses to form a hexose. 

The precision obtained with periodate oxidations in following the triose reactions led to attempts to apply the procedure to hexose transformations. It is theoretically possible to follow Lobry de Bruyn-Alberda van Ekenstein isomerizations of higher sugars by periodate oxidimetry on account of the difference in periodate uptake by the aldose and ketose moieties. Thus, in... 

Like all juggling acts, this leaves one a bit dizzy, but if we look back carefully at what has happened it is really very crafty Overall we have taken three molecules of hexose phosphate. Initially, we have converted these into three molecules of pentose phosphate plus three molecules of CO2. In the process, we have achieved our initial objective by reducing six molecules of NADP+ to NADPH. Then we have used two enzymes to reshuffle the atoms of the three pentose molecules to give us back, in effect, 2j hexose phosphate molecules, back on the mainline glycolytic pathway (as we have seen in Topic 26, triose phosphates are readily reconvertible to hexose phosphates). 

The commonest monosaccharide constituents of polysaccharides are based on either 5 carbon atoms (pentoses, CjHiqOs), or 6 carbon atoms (hexoses, C6Hi20g), although known monomers range from 3 C atoms (trioses) to at least 9 C atoms (nonaoses). 

The early investigations which resulted in the elucidation of the pentose phosphate pathway have been reviewed by Dickens ( ) and will not be discussed here. The pathway and its interconnections with the glycolytic pathway are shown in Fig. 6-1. It is presented here as a cyclic pathway, in which hexose phosphate is converted to pentose phosphates via NADP -linked dehydrogenases, and in which pentose phosphates can be converted to hexose phosphate and triose phosphate. Horecker (S-5) has proposed an alternative view of the pentose phosphate pathway that of two separate pathways or branches converging on pentose phosphates. This interpretation is shown in Table 6-1. In the oxidative branch, glucose-6-P is irrever-... 

The utilization of pentose phosphate without conversion to hexose is effected by Lactobacillus pentosus. An enzyme isolated from this organism converts xylulose-5-phosphate in one step to acetyl phosphate and triose phosphate (IX).This enzyme has been named phosphopentoketolase. 

Efficient utilization of the fructose requires phosphorylation of the glyceraldehyde. Tracer experiments show that the carbon 1 of fructose appears as both carbons 1 and 6 of glucose. This is the result of triose phosphate isomerization followed by (conventional) aldolase condensation to hexose diphosphate. The conversion of fructose diphosphate to glucose-6-phosphate requires a phosphatase and an isomerase, as discussed in the pentose phosphate pathway. 

From my laboratory and the laboratories of Ef Racker, Frank Dickens and Melvin Calvin, the years that followed witnessed a series of parallel and often highly synergistic discoveries on the nature of the pentose phosphate pathway and the path of carbon in photosynthesis. Andrew Benson and others in Calvin s laboratory, had shown that phosphate esters of ribulose and sedoheptulose were early products of CO2 fixation in photosynthesis,and the immediate precursor of phosphoglyceric acid, and therefore the primary CO2 acceptor, appeared to be ribulose diphosphate. The major problems became (1) to find the enzyme or enzymes that catalyzed the formation of phosphoglyceric acid from ribulose diphosphate and (2) to define the reactions leading to the synthesis of ribulose diphosphate from triose and hexose phosphates. 

The result, then, is an augmented supply of malate and, in turn, of ox-alacetate. Part of the oxalacetate is condensed with acetyl CoA to form citrate and so keep the glyoxylate cycle going. Another portion of the oxalacetate can also be phosphorylated and decarboxylated to give phosphoenolpyruvate. This in turn can give rise to hexoses via triose phosphate. 

In outline the change involves four stages in degradation (a) glycogen to hexose, (6) hexose to two molecules of a C3 sugar, or triose, (c) oxidation of triose to pyruvate, and (d) reduction of pyruvate to lactate. The energy required for the reaction is transferred by means of phosphoric acid supplied by two carriers, creatine... 

The basic building blocks of oligosaccharides are monosaccharides with the general formula C H2 0 , where = 3 (trioses) to 9 with = 6 (hexoses) being the most abundant. The monosaccharides contain 1 hydroxy groups and one aldehyde... 

Transaldolase. While aldolase produces only trioses from hexoses (and the reverse) the analogous enzyme transaldolase transfers the dihydroxyacetone residue onto other aldoses. The enzyme is highly specific Transaldolase splits only fructose and sedoheptulose (reverse of aldol condensation) and transfers the C3 residue (a dihydroxyacetone, which remains bound to the e-amino group of a lysine residue on the enzyme surface) to a corresponding aldehyde, i.e. to glyceraldehyde phosphate or erythrose 4-phosphate, or possibly to ribose 5-phosphate ... 

At first, xylulose-5-P (xul-P) and ribose-5-P (rib-P) by a C2 transfer form sedo-heptulose-7-P and triose-P Cs Cs = C + C3. The subsequent transfer of a Cj fragment from sedoheptulose to triose forms hexose erythrose-4-P remains C7 + C3 = C4 + Ce. Finally, another C2 fragment is transferred from a third molecule of pentose (xylulose-5-P) onto erythrose-4-P the result is again a hexose (fructose-6-P) and a triose-P. 

Conversion of Trioses and Hexose-Based Sugars to Lactic Acid Under Mild Conditions... 

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