The plant can also use the pentose phosphate pathway to break down glucose, but the main purpose of this pathway is the generation of reducing power in the form of NADPH (nicotinamide adenine dinucleotide phosphate). In addition, this pathway provides sugar intermediates that can serve as building blocks for aromatic amino acids and nucleic acids.
OH ATP ADP
The pentose phosphate pathway can be divided in two phases: an oxidative phase during which glucose-6-phosphate is converted to ribulose-5-phosphate, and a non-oxidative phase constituting of a series of reversible reactions in which two pentose-phosphate residues are converted to a series of sugar-phosphate molecules of differing lengths (Figure 3-2).
The oxidative part of the pentose phosphate pathway starts with the oxidation of glucose-6-phosphate (3.1) to gluconolactone-6-phosphate (3.11) by glucose-6-phosphate 1-dehydrogenase with the reduction of NADP+ to NADPH. Gluconolactone-6-phosphate (3.11) is converted to gluconate-6-phosphate (3.12) by gluconate-6-phospate lactonase. Irreversible oxidative decarboxylation of (3.12) by gluconate-6-phosphate reductase results in ribulose-5-phosphate (3.13), with the generation of another NADPH molecule.
Ribulose-5-phosphate (3.13) can be converted to ribose-5-phosphate (3.14) and xylulose 5-phosphate (3.15), by the enzymes ribose-5-phosphate isomerase and ribulose 5-phosphate 3-epimerase, respectively. The two pentose-phosphate molecules, 3.14 and 3.15, are converted to a C3 and a C7 sugar-phosphate, glyceraldehyde 3-phosphate (3.4) and sedoheptulose-7-phosphate (3.16), respectively, via the action of a transketolase.
Transketolases are characterized by their ability to transfer a two-carbon unit from a ketose to an aldehyde. The C3 and C7 sugar-phosphates can subsequently be converted to a C4 and a C6 sugar-phosphate, erythrose 4-phosphate (3.17) and fructose 6-phosphate (3.2), respectively. This reaction is catalyzed by a transaldolase, which transfers a three-carbon glyceraldehyde unit from an aldose to a ketose. Erythrose-4-phosphate (3.17) can be used in the shikimate pathway (see Section 6). A second transketolase reaction can generate a second fructose-6-phosphate (3.2) and glyceraldehyde-3-phosphate (3.4) residue from erythrose-4-phosphate (3.17) and xylulose-5-phosphate (3.15). Hexose-phosphate isomerase converts the
Figure 3-2. Glycolysis. The enzymes involved in this pathway are: (a) hexose phosphate isomerase (E.C. 188.8.131.52), (b) phosphofructokinase (E.C. 184.108.40.206), (c) fructose bisphosphate aldolase (E.C. 220.127.116.11), (d) triose-phosphate isomerase (E.C. 18.104.22.168), (e) GAP-dehydrogenase (E.C. 22.214.171.124), (f) glycerate-3-phosphate kinase (E.C. 126.96.36.199), (g) glycerate phosphate mutase (E.C. 188.8.131.52), (h) enolase (phosphopyruvate hydratase; E.C. 184.108.40.206), and (i) pyruvate kinase (E.C. 220.127.116.11).
two fructose-6-phosphate molecules to glucose-6-phosphate (3.1), which can enter the pentose phosphate pathway again to generate additional NADPH.
So in summary, three glucose-6-phosphate (3.1) molecules (3 x C6) are oxidized to three ribulose-5-phosphate (3.13) residues (3 x C5) and three molecules of CO2 (3 x C1) under generation of six molecules of NADPH. The three ribulose-5-phosphate residues are then converted to one glyceraldehyde-3-phosphate (3.14) molecule (1 x C3) and two fructose-6-phosphate (3.2) molecules (2 x C6). Fructose-6-phosphate can be converted to glucose-6-phosphate and reenter the oxidative part of the pentose phosphate pathway. Fructose-6-phosphate and glyceraldehydes can also serve as intermediates in glycolysis (Section 5.1), which offers the cell considerable flexibility in terms of its metabolic flux.
The availability of the Arabidopsis genome sequence revealed that there are multiple genes encoding the different enzymes in the oxidative pentose phosphate pathway (reviewed by Kruger and von Schaewen, 2003). Studies across a range of species indicate that genes encoding individual isozymes may be differentially expressed in different tissues, at different developmental stages, and in response to different growth conditions, especially those that alter demand for NADPH or intermediates of the oxidative pentose phosphate pathway for biosynthesis. The ability to use specific isoforms that allow optimal performance under certain conditions offers the plant a greater degree of metabolic flexibility.
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