Chapter 26 - PPP

What happens in the oxidative phase of the PPP?

The conversion of glucose 6-phosphate into ribulose 5-phosphate

What happens in the nonoxidative phase of the pentose phosphate pathway?

Conversion of ribulose 5-phosphate into ribose 5-phosphate

• if ribose is not needed it will be reshuffled into sugars

What are the reactions catalyzed in the oxidative phase of the pentose phosphate pathway?

• Glucose 6-phosphate → phosphogluco lactone

  • catalyzed by: glucose 6-phosphate dehydrogenase

    • produces NADPH

• Phosphogluco lactone → 6-phosphogluconate

  • catalyzed by: lactonase

    • produces NADPH and releases CO2

What is ribose 5-phosphate used for?

To make nucleotides

• RNA and DNA

What happens after the oxidative phase?

Ribulose 5-phosphate gets converted into ribose 5-phosphate

• this done w/ phosphopentose isomerase

What happens if a cell’s need for NADPH is greater than its need for ribose?

The ribose is converted, and is used to generated 3 and 6 carbon molecules

• Glyceraldehyde 3-phosphate

• Fructose 6-phosphate

these reactions are catalyzed by transketolase and transaldolase

What is regeneration?

The conversion of six-carbon and three carbon sugars into ribulose 5-P

What are the two essential enzymes essential in regeneration?

Transketolase

• Transfers two-carbon units

Aldolase

• Joins/cleaves sugars

• Similar to enzymes in glycolysis and gluconeogenesis

Overview of the Oxidative phase

Three reactions

• Glucose 6-phosphate + 2 NADP+ + H2O → Ribulose 5-phosphate + 2 NADPH + 2 H+ + CO2

first step gets catalyzed by glucose 6-phosphate dehydrogenase

irreversible

Overview of nonoxidative phase

Four reactions

• interconverts ribulose, ribulose, ribose and 3, 4, 6, and 7 carbon sugars

can process 5 carbon compounds for use in glycolysis

How is the PPP regulated?

Primarily by the concentration of NADP+

• Low concentration → downregulates the pathway

• Inhibition by NADPH, competes for the active site

Normally, NADPH : NADP+ ratios are high

What is the overall idea of the PPP?

It generates NADPH and enables the exchange of three and six carbon intermediates from glycolysis with five carbon sugars

• oxidative phase is irreversible and is regulated by cellular NADPH requirements

• nonoxidative phase is readily reversible, and is determined by the cells needs at the time

What happens when the demands for NADPH and ribose are balanced?

PPP dominates

• glucose 6-phosphate routed to PPP

• oxidative phase generates NADPH and ribose

What happens when theres a high demand for NADPH?

• Gluconeogenesis is active

• Oxidative phase PPP generates NADPH

• Nonoxidative phase makes fructose 6-phosphate and glyceraldehyde 3-phosphate

  • these get converted to glucose 6-phosphate

How does the PPP deal with oxidative stress?

NADPH protects cells from ROS

• Glutathione reacts with reactive oxygen species

  • Reaction reduces/inactivates ROS and simultaneously oxidizes glutathione

What happens to the oxidized glutathione/\/

Reduced Glutathione (GSH) is regenerated by glutathione reductase

• requires NADPH