Ribuloses ribulose 5-phosphat

The donor of the two-carbon unit in this reaction is xylulose 5-phosphate, an epimer of ribulose 5-phosphate. A ketose is a substrate of transketolase only if its hydroxyl group at C-3 has the configuration of xylulose rather than ribulose. Ribulose 5-phosphate is converted into the appropriate epimer for the transketolase reaction by phosphopentose epimerase (see Figure 20.11) in the reverse reaction of that which occurs in the Calvin cycle. 

D-Ribulose Formed in metabolic processes. Ribulose phosphate is an intermediate in pentose phosphate pathway. ... 

Light was required for the generation of NADPH needed to reduce phosphoglyceric acid to give glyceraldehyde 3-phosphate. The NADPH was then used to give the three molecules of ATP needed to phospho-rylate ribulose phosphate and the triose phosphates, so completing the Calvin cycle. 

The product of the PNP enzyme, FDRP 9 has been purified and characterised. The evidence suggests that FDRP 9 is then isomerised to 5-fluoro-5-deoxyribulose-1-phosphate 10, acted upon by an isomerase (Scheme 7). Such ribulose phosphates are well-known products of aldolases and a reverse aldol reaction will clearly generate fluoroacetaldehyde 11. Fluoroacetaldehyde 11 is then converted after oxidation to FAc 1. We have also shown that there is a pyridoxal phosphate (PLP)-dependent enzyme which converts fluoroacetaldehyde 11 and L-threonine 12 to 4-FT 2 and acetaldehyde in a transaldol reaction as shown in Scheme 8. Thus, all of the biosynthetic steps from fluoride ion to FAc 1 and 4-FT 2 can be rationalised as illustrated in Scheme 7. 

Some lactic acid bacteria of the genus Lactobacillus, as well as Leuconostoc mesenteroides and Zymomonas mobilis, carry out the heterolactic fermentation (Eq. 17-33) which is based on the reactions of the pentose phosphate pathway. These organisms lack aldolase, the key enzyme necessary for cleavage of fructose 1,6-bisphosphate to the triose phosphates. Glucose is converted to ribulose 5-P using the oxidative reactions of the pentose phosphate pathway. The ribulose-phosphate is cleaved by phosphoketolase (Eq. 14-23) to acetyl-phosphate and glyceraldehyde 3-phosphate, which are converted to ethanol and lactate, respectively. The overall yield is only one ATP per glucose fermented. 

The 5-carbon sugar phosphates are interconverted by the action of epimerase and isomerase to yield ribulose-5-phosphate, which is phosphorylated by the enzyme ribulose phosphate kinase to make RuBP, the acceptor of C02. 

Ribulose phosphate kinase is active only when a cystine disulfide on the enzyme is reduced to two cysteines. An electron carrier, thioredoxin, carries out this reduction, and is then itself reduced by electrons from NADPH. Because the action of Photosystems I and II forms NADPH, this reduction ensures that ribulose bisphosphate is made only when enough light exists to support Photosynthesis. In other words, the light and dark reactions are coupled. 

This requires the generation of six separate molecules of ribulose bisphosphate from ribulose phosphate, at the cost of one ATP each. Furthermore, two molecules of 1,3-bisphosphoglycerate must be made from the two 3-phosphoglycerates that are the initial product of each C02 fixation reaction. Conversion of each 1,3-bisphospho-glycerate requires an NADPH as well therefore, two NADPH equivalents are consumed for each C02 fixed. Another way of saying this is that carbon is reduced from an oxidation number of -4 in C02 to an oxidation number of zero in carbohydrate (CH20). Therefore, synthesis of one mole of glucose requires the input of 18 ATPs and 12 NADPHs. 

Figure 6.4. Role of transketolase in the pentose phosphate pathway. Glucose 6-phosphate dehydrogenase, EC 1.1.1.49 phosphogluconate dehydrogenase, EC 1.1.1.44 ribulose-phosphate epimerase, EC 5.1.3.1 phosphoribose isoinerase, EC 5.3.1.6 transketolase, EC 2.2.1.1 and transaldolase, EC 2.2.1.2.

Discovered sedoheptulose and ribulose phosphates in the intermediates of the photosynthetic carbon reductive cycle. Discovered and identified... 

It is still unknown how the pyrimidine intermediate 5 is dephosphorylated (reaction VI). However, it is well established that the dephosphorylation product 6 is condensed with 3,4-dihydroxy-2-butanone 4-phosphate (8) by the catalytic action of lumazine synthase (reaction VIII). The carbohydrate substrate 8 is in turn obtained from ribulose phosphate (7) by a complex reaction sequence that is catalyzed by a single enzyme, 3,4-dihydroxy-2-butanone 4-phosphate synthase (reaction VII). As mentioned above, the lumazine 9 is converted to riboflavin (10) by the catalytic action of riboflavin synthase (reaction IX). Ultimately, riboflavin is converted to the coenzymes, riboflavin 5 -phosphate (flavin mononucleotide (FMN), 11) and flavin adenine dinucleotide (FAD, 12) by the catalytic action of riboflavin kinase (reaction X) and FAD synthase (reaction XI). These reaction steps are required in all organisms, irrespective of their acquisition of riboflavin from nutritional sources or by endogenous biosynthesis. 

Fig. 5. A simplified metabolic scheme of ethanol formation from glucose and xylose. Enzyme abbreviations GPDH Glucose 6-phosphate 1-dehydrogenase, PGDH Phosphogluconate dehydrogenase, PGI Glucose 6-phosphate-isomerase, RKI Ribose 5-phosphate isomerase, RPE Ribulose phosphate 3-epimerase, TAL Transaldolase, TKL Transketolase, XDH Xylitol dehydrogenase, XK-. Xylulokinase, XR Xylose reductase...

What really happens is that six molecules of glucose 6-phosphate are converted to 6 CO2 + 6 ribulose 5-phosphates + 12 NADPH + 12 (see Table 20.3 in the text). The ribulose phosphates are then converted back to five molecules of glucose 6-phosphate by the action of transketolase and transaldolase. By these reactions three pentoses are converted to two hexoses and one triose. Thus six pentoses can be converted to four hexoses plus two trioses, and the latter can be converted to the fifth hexose. 

Ribulose phosphate epimerase, EC 5.1.3.4. Its official name is L-ribulose-5-phosphate-4-epimerase. This is a key enzyme in the pentose phosphate pathway. An epimer is a stereoisomer variant of a sugar differing in the configuration at only one carbon atom (see Chap. 11). 

The end result of Eq. (13)-(22) is the conversion of five molecules of glyceraldehyde-3-phosphate to three molecules of ribulose-5-phosphate (see Fig. 6). Two of these molecules formed by Eq. (21) and (22) are labeled in carbon atom position 3, while the third one, from Eq. (19) and (20), is labeled in positions 1, 2, and 3. The resultant average labeling of ribulose phosphate is heavy in position 3 and lighter in positions 1 and 2. When the ribulose molecules, labeled after a few seconds photosynthesis with C 02, were degraded (Bassham et al., 1954), this pattern of... 

A Ribulose phosphate 3-epimerase (xylulose phosphate 3-epimerase) (EC 5.1.3.1). 

Scheme 11.9. A cartoon representation of the isomerization of xylulose 5-phospate to ribulose 5-phosphate through the common enol and catalyzed by ribulose-phosphate epimerase (EC 5.1.3.1).

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