Q7 — Pentose Phosphate Pathway (PPP)

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

Bioenergetics & Central Metabolism Overview

The Pentose Phosphate Pathway (PPP) is a metabolic pathway occurring in the cytoplasm that mainly generates NADPH and ribose 5-phosphate. NADPH is used for the reduction of reactive oxygen species (ROS) such as hydrogen peroxide. PPP has oxidative and non-oxidative phases, which are irreversible and reversible, respectively.

Reaction diagram. Part A (oxidative, irreversible): G6P is converted via G6PDH to C6P then C5P, releasing two NADPH and one CO2. Part B (non-oxidative, reversible): C5P interconverts with R5P and, via two transketolase (TKT) reactions, with C7P, GA3P, C4P, and F6P, ultimately yielding F6P and GA3P. Figure 1. A Oxidative. B Non-oxidative. G6P – glucose 6-phosphate, GA3P – glyceraldehyde 3-phosphate, F6P – fructose 6-phosphate, R5P – ribose 5-phosphate, G6PDH – glucose 6-phosphate dehydrogenase, TKT – transketolase, N - nucleotides. CXP - other intermediates where X is the position of phosphorylated carbon. Note that all intermediates of PPP are phosphorylated on the last carbon.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. In actively dividing cells, the reactions occurring before the first TKT-catalyzed reaction are more active than those occurring later.

B. TKT catalyzes a reaction that transfers a one-carbon fragment.

C. Deficiency of G6PDH activity will reduce the level of available NADPH in red blood cells.

D. For each molecule of G6P, there will be two molecules of F6P produced through both phases.

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