Pentose phosphate pathway

Question 1

What is the pentose phosphate pathway?

Answer 1

• It is a group of reactions in which G6P is degraded, leading to the formation of NADPH and ribose 5-phosphate.
• More anabolic than catabolic.

Question 2

What are the two phases of the pentose phosphate pathway?

Answer 2

• Oxidative phase (NADPH production)
• Cyclical phase (5-carbon sugars produced)

Question 3

Describe the oxidative phase of the PPP?

Answer 3

• 1 G6P is converted to ribulose-5-P, CO2 & 2 NADPH
• NADPH produced by G6P dehydrogenase (G6PD) & 6 phosphogluconolactone hydrolase
• CO2 produced by: Decarboxylation generating ribulose-5P

*irreversible reaction

Question 4

Describe the cyclical phase of the PPP.

Answer 4

• Catalyse the interconversion of 3,4,5,6,7-carbon sugars
• This creates sugars that can be exchanged between the pentose phosphate pathway and glycolysis

**Reversible reaction

Question 5

What’s the functions of NADPH?

Answer 5

• It is a biochemical reductant involved in supplying electrons & hence energy to numerous pathways:
  1. Fatty acid synthesis
  2. Steroid synthesis (e.g cholesterol & steroid hormones)
  3. Antioxidant reaction –detoxification

Question 6

How is the PPP regulated?

Answer 6

• Inhibited mainly at glucose-6-P dehydrogenase reaction by NADPH (feedback inhibition/ competitive inhibitor).
• Insulin increases G6PD gene expression so pathway activity increases in well fed state.

Question 7

What is the function of the ribulose 5 phosphate?

Answer 7

1. Nucleotide biosynthesis.
2. Glycolysis intermediates = energy production.