Pentose phosphate pathway scheme

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

FIGURE 14-20 General scheme of the pentose phosphate pathway. 

The pentose phosphate pathway is the other major route by which glucose is assimilated by baker s yeast, and this pathway yields excess reducing equivalents in the form of NADPH (Scheme 2). The fate of the ribulose 5-phosphate depends on the cellular requirements for pentose sugars, but it can be converted... 

Like transketolase, transaldolase (TA, E.C. 2.2.1.2) is an enzyme in the oxidative pentose phosphate pathway. TA is a class one lyase that operates through a Schiff-base intermediate and catalyzes the transfer of the C(l)-C(3) aldol unit from D-sedoheptulose 7-phosphate to glyceraldehyde-3-phosphate (G3P) to produce D-Fru 6-P and D-erythrose 4-phosphate (Scheme 5.59). TA from human as well as microbial sources have been cloned.110 111 The crystal structure of the E. coliu and human112 transaldolases have been reported and its similarity to the aldolases is apparent, since it consists of an eight-stranded (o /(3)s or TIM barrel domain as is common to the aldolases. As well, the active site lysine residue that forms a Schiff base with the substrate was identified.14112 Thus, both structurally and mechanistically it is related to the type I class of aldolases. 

Fig. 4.2 The pentose phosphate pathway. The squares under each chemical name indicate the number of carhon atoms that each molecule contains glucose 6-phosphate, for example, is a hexose, with six carhon atoms, rihose 5-phosphate is a pentose, with five, and so on. The names shown in gray at the left of the diagram belong to molecules in the oxidative part of the pathway, which is not considered in the analysis in the text. The remainder, the nonoxidative part, can be interpreted as a scheme to allow the hexoses and pentoses to be converted into one another...

It remains to ask how living cells have solved this problem which solution is found in real metabolism If you study the scheme of the pentose phosphate pathway that appeared as Figure 4.2 in Chapter 4, you will see that the answer is that the solution that mathematical analysis proves to be the simplest is indeed the way that pentoses are transformed into hexoses in cells. 

Yes. The pathway that appears in most biochemistry textbooks is that in Fig. 11-26. It was described by Bernard Horecker in 1955. It appears to be the pathway that operates in adipocytes, red blood cells, and many other cell types hence its name the F-type (for fat-type). However, the possibility exists that other forms of group transfer reactions could take place via transketolase and aldolase, in the absence of transaldolase. It is thought that this more complicated reaction scheme operates in the liver, so it was called the L-type (for liver-type) pentose phosphate pathway by its main proponent John Williams. 

The association of the +AId with active metabolism was further confirmed by experiments in which the glucose substrate was replaced by fructose (as fructose-6-phosphate) and gluconolactone (as 6-phosphogluconolactone) which are intermediates in the glycolytic and pentose phosphate pathways respectively. Pyruvate was also incorporated into this series to determine what effect this intermediate, which precedes the formation of the terminal product of metabolism, namely lactate, would have on the Aid value. It was expected that pyruvate, because of its position in the metabolic scheme, would show either no effect or manifest an adverse effect on the metabolic behavior of the RBC. The adverse... 

Glyceraldehyde 3-phosphate, it will be recalled (Scheme 11.13), is a progenitor of glucose-6-phosphate, which, via the pentose phosphate pathway, is also (i.e., in addition to the Calvin cycle) on the path to ribulose-5-phosphate. Now, via the common enol, ribulose-5-phosphate is readily converted to ribose-5-phosphate (EC 5.3.1.6), which is then bisphosphorylated (EC 2.7.6.1) to the a-diphosphate so that a leaving group is in place that will allow replacement by the terminal amido function of glutamine (Gin, Q) on its way to glutamate (Glu, E) with the formation of 2-aminoribose-5-phosphate. 

Transketolase (TK EC 2.2.1.1), athiamine diphosphate (ThDP)-dependent enzyme, is a key enzyme in the nonoxidative branch of the pentose phosphate pathway. TK catalyzes the stereospecific formation of a C-C bond by a reversible transfer of the C1-C2 ketol unit from a ketose phosphate to an aldose phosphate. The new asymmetric center formed stereospecifically has an absolute (S) configuration (Scheme 15.1). 

Transketolase. Another group of enzymes that catalyze the stereospecific formation and cleavage of carbohydrates in vivo are the transketolases and transaldolases. Transketolase (E.C. 2.2.1.1) is a thiamin pyrophosphate (TPP) dependent enzyme that catalyzes the transfer of a hydroxyketo group from a ketose phosphate to an aldose phosphate in the pentose pathway (Scheme 12) (36). 

The routes involved in the formation of the various furan sulphides and disulphides involve the interaction of hydrogen sulphide with dicarbonyls, furanones and furfurals. Possible pathways are shown in Scheme 12.8. Furanthiols have been found in heated model systems containing hydrogen sulphide or cysteine with pentoses [56-58]. 2-Methyl-3-furanthiol has also been found as a major product in the reaction of 4-hydroxy-5-methyl-3(2H)-furanone with hydrogen sulphide or cysteine [21, 59]. This furanone is formed in the Maillard reaction of pentoses alternatively it has been suggested that it may be produced by the dephosphorylation and dehydration of ribose phosphate, and that this may be a route to its formation in cooked meat [21, 60]. 

Sugar skeletons are interconverted by way of three classes of compound, sugar phosphates, sugar nucleotides and cyclitols and the major known pathways which interconnect them are summarised in the schemes later in this chapter. The sugar nucleotides are the main intermediates in these interconversions and it is their metabolism which forms the bulk of this chapter. The cyclitols provide an important and, sometimes, dominant route from hexose to uronic acid and, thence, pentoses in plants, but their role in animals is unclear. Sugar phosphates are of importance as the entry to the sugar nucleotide pathways, but of themselves contribute little to metabolic interconversion directly for anabolic purposes. 

Under conditions of virus infection with the same CO2 production per mole of glucose, 29% of the Ci was contained in the CO2. This is equivalent to a maYinmiTTi of 29% of the glucose metabolized via the oxidative pathway or a minimum of 6%. These differences were real and reproducible and are of the same order as the shifts in the amounts of pentose and desoxypentose synthesized during glucose utilization. It was suggested as a result of these studies that the ribose of RNA was derived mainly via the oxidative pathway, whereas the desoxyribose of DNA arose from triose phosphate generated from the Embden-Meyerhof scheme. 

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