Which enzyme acts in the pentose phosphate pathway?
Select one:
a. pyruvate kinase
b. glycogen phosphorylase
c. 6-phosphogluconate dehydrogenase
d. phosphofructokinase-1
e. aldolase
The correct answer and explanation is:
Correct Answer: c. 6-phosphogluconate dehydrogenase
The pentose phosphate pathway (PPP), also known as the hexose monophosphate shunt, is a metabolic pathway parallel to glycolysis. It serves two primary functions: generating NADPH and producing ribose-5-phosphate for nucleotide and nucleic acid synthesis.
6-phosphogluconate dehydrogenase is a key enzyme in the oxidative phase of the pentose phosphate pathway. It catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose-5-phosphate, releasing carbon dioxide and producing NADPH in the process. This enzyme plays a central role in maintaining the cell’s reducing environment, especially important in tissues engaged in biosynthetic processes or in cells exposed to oxidative stress.
The PPP begins with glucose-6-phosphate, which is oxidized by glucose-6-phosphate dehydrogenase (G6PD), forming 6-phosphoglucono-δ-lactone and producing one molecule of NADPH. The lactone is then hydrolyzed to 6-phosphogluconate. Next, 6-phosphogluconate dehydrogenase acts on 6-phosphogluconate to form ribulose-5-phosphate and another NADPH molecule.
The NADPH generated is essential for reductive biosynthesis, such as fatty acid and cholesterol synthesis, and for maintaining reduced glutathione levels that protect cells from oxidative damage. Ribulose-5-phosphate produced in the pathway can either enter the non-oxidative phase to generate sugars like ribose-5-phosphate for DNA and RNA synthesis or be recycled back into glycolytic intermediates.
The other options listed are enzymes associated with glycolysis or glycogen metabolism. Pyruvate kinase and phosphofructokinase-1 are part of glycolysis, glycogen phosphorylase breaks down glycogen, and aldolase cleaves fructose-1,6-bisphosphate in glycolysis. Therefore, 6-phosphogluconate dehydrogenase is the correct choice for its specific role in the pentose phosphate pathway.