Pentose cycle

The transketolase of the pentose cycle, for instance, is a good example of such limitation. This enzyme catalyses two different reactions (see Fig. 11.1), which also operate in Calvin s cycle. 

Prinsloo, H. Effect of exposure to quartz dust in vivo on pentose cycle activity in guinea pig lung. Environ. Res. 6 68-76, 1973. 

Flow of Electrons from H20 to NADP+ Drives Proton Transport into the Thylakoid Lumen Protons Return to the Stroma through an ATP-Synthase Carbon Fixation Utilizes the Reductive Pentose Cycle Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle... 

The reductive pentose cycle, or Calvin cycle. The number of arrows drawn at each step in the diagram indicates the number of molecules proceeding through that step for every three molecules of C02 that enter the cycle. The entry of three molecules of C02 results in the formation of one molecule of glyceraldehyde-3-phosphate (box on right), and requires the oxidation of six molecules of NADPH to NADP+ and the breakdown of nine molecules of ATP to ADP. 

Carbon Fixation Utilizes the Reductive Pentose Cycle... 

Other enzymes in the stroma can convert glycerate-3-phosphate back to ribulose-1,5-bisphosphate. The reactions of this reductive pentose cycle, or Calvin cycle, are shown in figure 15.25. 3-Phosphoglycerate is first phosphorylated to glycerate-l,3-bisphosphate at the expense of ATP and then reduced to glyceraldehyde-3-phosphate by NADPH. (Note that the nucleotide specificity in the reductive step differs from that of the cytosolic glyceraldehyde-3-phos-... 

The C-4 pathway requires the cooperation of two types of cells. Mesophyll cells (left) take up C02 from the air and export malate to the bundle sheath cells (right). The bundle sheath cells return pyruvate to the mesophyll cells and fix the C02 using ribulose bisphosphate carboxylase and the reductive pentose cycle. 

Increase followed by decrease of respiratory metabolism and of pentose-cycle. 

Katz, J., Wals, P.A. 1972. Pentose cycle and reducing equivalents in rat mammary-gland slices. 

Williams JF, Arora KK, Longnecker JP. The pentose pathway a random Harvest. Impediments which oppose acceptance of the classical (F-type) pentose cycle for liver, some neoplasms and photosynthetic tissue. The case for the L-type pentose pathway. Int. J. Biochem. 1987 19 749-817. 

Casazza JP, Veech RL. The interdependence of glycolytic and pentose cycle intermediates in ad libitum fed rats. J. Biol. Chem. 1986 261 690-698. 

Wood HG, Katz J. The distribution of C in the hexose phosphates and the effects of recycling in the pentose cycle. J. Biol. Chem. 1958 233 1279-1282. 

Despite the fact that many heptoses are by far less prominent in Nature than hexoses these monosaccharides are found both as metabolic intermediates, and as structural carbohydrates of bacterial cell walls.D-Sedoheptulose 7-phosphate is an important intermediate of the pentose cycle, and D-sedoheptulose 1,7-bisphosphate is present in plants as an intermediate of the dark phase of photosynthetic reactions. L-Glycero-D-manno-heptose was isolated from the oligosaccharides obtained by partial acid hydrolysis of the lipopolysaccharide from Escherichia coli K-12 strain W3100 [153] and Haemophilus influenzae [154]. Both L-glycero-D-wtanno-heptose and D-glycero-D-ma o-heptose were isolated from the lipopolysaccharide of Vibrio parahaemolyticus [155]. 

Another central pathway by which yeasts may catabolize D-glucose is the pentose cycle (see Fig. 6), the initial stages of which are (i) the phosphorylation of D-glucose, followed by (ii) oxidation of D-glucose 6-phosphate to 6-O-phosphono-D-gluconate. The net result of the operation of this cycle is the complete oxidation of D-glucose. 

It is D-fructofuranose, not D-fructopyranose, that is utilized, at least by bakers yeast.303,304 As with D-glucose, the initial step in the intracellular utilization of either D-fructofuranose305 or D-mannose is phosphorylation by the constitutive hexokinase306 (see Ref. 307 for a review). The D-fructose 6-phosphate formed is an intermediate of both the glycolytic pathway and the pentose cycle. D-Mannose phosphate isomerase (EC 5.3.1.8) effects the conversion of D-mannose 6-phosphate into D-fructose 6-phosphate,308,309 or D-mannose 6-phosphate is epimerized to D-glucose 6-phosphate.308... 

In principle, the first step by which yeasts could convert an al-dopentose into an intermediate of the pentose cycle might be (a) an epimerization, (b) a conversion into the corresponding ketose by way of the enediol, (c) conversion by oxidation and reduction, or (d) phosphorylation. For example, D-xylose might be expected to be ca-tabolized initially, either by isomerization to D-threo-pentulose, or reduction to xylitol which would then be oxidized to D-threo-pentulose. D-f/ireo-Pentulose is phosphorylated to give u-threo-... 

Again, in principle, alditols might be oxidized to the corresponding ketoses by NAD - or NADPe-linked dehydrogenases, or to the aldoses by NADP -linked dehydrogenases. Alternatively, the aldoses might be phosphorylated. Aldoses and ketoses may be phosphorylated, and alditol phosphates oxidized to aldose or ketose phosphates. These compounds are likely to be catabolized by the reactions of the Embden-Meyerhof glycolytic pathway or of the pentose cycle. 

D-erythro-pentulose 5-phosphate - pentose-cycle reactions... 

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