Chapter 26: Pentose Phosphate Pathway

2 phases of the PPP

oxidative phase and nonoxidative phase

oxidative phase

- oxidation of glucose 6-phosphate at carbon 1 is catalyzed by glucose 6-phosphate dehydrogenase

- produces NADPH

- product (phosphogluco-δ-lactone) then hydrolyzed by lactonase

- 6-phosphogluconate then converted into ribulose 5-phosphate

- produces NADPH and releases CO2

nonoxidative phase

- ribulose 5-phosphate is converted into ribose 5-phosphate by phosphopentose isomerase

- if a cells need for NADPH is greater than its need for ribose ribose is converted and used to generate three- and six-carbon molecules

- glyceraldehyde 3-phosphate

- fructose 6-phosphate

- catalyzed by transketolase and transaldolase

transketolase and transaldolase

- transketolase transfers 2 carbon units

- transaldolase transfers 3 carbon units

interconversion of ribulose 5-phosphate

- several reactions in the NO phase depend on the interconversion of ribulose 5-phosphate and xylulose 5-phosphate

- catalyzed by phosphopentose epimerase

reactions catalyzed by transketolase

xylulose 5-phosphate + ribose 5-phosphate --> glyceraldehyde 3-phosphate + sedoheptulose 7-phosphate

erythrose 4-phosphate + xylulose 5-phosphate --> fructose 6-phosphate + glyceraldehyde 3-phosphate

reactions catalyzed by transaldolase

glyceraldehyde 3-phosphate + sedoheptulose 7-phosphate --> fructose 6-phosphate + erythrose 4-phosphate

nonoxidative phase and calvin cycle

similarities:

- both provide a mechanism for converting 3 and 6 carbon sugars that are needed for glycolysis and gluconeogenesis into 5 carbon sugars

- both use transaldolases

differences:

- all reactions in nonoxidative phase are reversible, calvin cycle reactions have a direction

- calvin cycle exclusively generates 5C sugars from 3 and 6 C

regulation of PPP

- glucose 6-phosphate dehydrogenase catalyzes the committed step in the oxidative phase

- primarily regulated by concentration of NADP+

- low concentration of substrate, NADP+, downregulates the pathway

mode 1 of balance of PPP, glycolysis, and gluconeogenesis

- nonoxidative phase primarily used

rapid growth conditions, when cells are dividing rapidly, the need for ribose is greater than the need for NADPH

- glycolysis is active

- NO phase of the PPP generates ribose 5-phosphate (isomerase)

mode 2 of balance of PPP, glycolysis, and gluconeogenesis

- oxidative phase primarily used

demands for NADPH and ribose 5-phosphate are balanced, PPP dominates

- glucose 6-phosphate is routed to the PPP

- oxidative phase of the PPP generates NADPH and ribose 5-P

mode 3 of balance of PPP, glycolysis, and gluconeogenesis

- oxidative phase and gluconeogenesis primarily used

- high demand for NADPH

demand for NADPH is greater than demand for ribose 5-phosphate

- gluconeogenesis is active

- oxidative phase of PPP generates NADPH

- NO phase used to make fructose 6-P nd GAH3P which is then converted to glucose 6-phosphate through gluconeogenesis

mode 4 of balance of PPP, glycolysis, and gluconeogenesis

- primarily uses oxidative phase and glycolysis

- simultaneous demand for ATP and NADPH

- demand for NADPH is greater than demand for ribose 5-P

- O phase of PPP generates NADPH

- NO phase used to make fructose 6-P and GAH3P which go on to generate ATP

PPP and oxidative stress

- glutathione reacts with reactive oxygen species (ROS)

- reaction reduces and inactivates ROS while oxidizing glutathione

- 2 GSH --> GSSG

- glutathione reductase regenerates glutathione, requires NADPH

- GSSG + NADPH + H+ --> GSSH + NADP+

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