Pentose Phosphate Pathway: Practice Questions and Key Concepts

Pentose Phosphate Pathway

Overview

The Pentose Phosphate Pathway (PPP) is a metabolic pathway parallel to glycolysis. It primarily serves to generate NADPH and ribose-5-phosphate for nucleotide synthesis, as well as to interconvert various sugars for biosynthetic processes.

Key Concepts and Practice Questions

• Transketolase and transaldolase are enzymes that transfer carbon units between sugar molecules in the PPP.

• Transketolase transfers a 2-carbon segment, while transaldolase transfers a 3-carbon segment.

Transketolase and Transaldolase Reactions

• Transketolase: Transfers a 2-carbon unit from a ketose donor to an aldose acceptor.

• Transaldolase: Transfers a 3-carbon unit from a ketose donor to an aldose acceptor.

Key Intermediates

• Erythrose-4-phosphate and xylulose-5-phosphate are important intermediates in the non-oxidative phase of the PPP.

• Ribose-5-phosphate is a precursor for nucleotide synthesis.

Pathway Functions

• Generates NADPH for reductive biosynthesis and antioxidant defense.

• Produces ribose-5-phosphate for nucleotide and nucleic acid synthesis.

• Interconverts sugars for glycolysis and biosynthetic pathways.

Sample Questions and Answers

1. Transketolase transfers how many carbons? Answer: 2 carbons

2. Transaldolase transfers how many carbons? Answer: 3 carbons

3. Which of the following is an intermediate in the PPP? Answer: erythrose-4-phosphate

4. Which of the following is NOT a product of the PPP? Answer: glucose-1-phosphate

5. Which carbon of glucose ends up as carbon 1 of ribulose 5-phosphate in the PPP? Answer: C-1

Important Equations

• Overall oxidative phase reaction: $\text{Glucose-6-phosphate} + 2\ NADP^+ + H_2O \rightarrow \text{Ribulose-5-phosphate} + 2\ NADPH + 2\ H^+ + CO_2$

Table: Key Enzymes and Their Functions in the PPP

Enzyme	Function	Carbons Transferred
Transketolase	Transfers 2-carbon units	2
Transaldolase	Transfers 3-carbon units	3

Additional info: The PPP is especially important in tissues with high rates of fatty acid or nucleotide synthesis, such as the liver, adipose tissue, and rapidly dividing cells.