Question

Ascorbic acid cannot be synthesized in humans and other primates as well as guinea pigs. Could you tell why?

• A. It happens because of absence of enzyme ascorbate synthetase
• B. It happens because of absence of enzyme UDPGlc pyrophosphorylase
• C. It happens because of absence of enzyme UDPGlc dehydrogenase
• D. It happens because of absence of enzyme L-guluconolactone oxidase

Hint:

Ascorbic acid synthesis fails at the very last step.
The GULO gene is present in the human genome, but it exists as a "pseudogene" due to evolutionary mutations.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Ascorbic acid (Vitamin C) is an essential water-soluble vitamin that acts as an antioxidant and a cofactor for collagen hydroxylation.
While most mammals can synthesize ascorbic acid from glucose via the glucuronic acid pathway, humans, other primates, bats, and guinea pigs must obtain it from their diet.

Step 2: Detailed Explanation:
In species capable of synthesizing Vitamin C, the biosynthetic pathway begins with glucose-6-phosphate.
This substrate is converted through several intermediates into UDP-glucose, UDP-glucuronic acid, and eventually L-gulononate.
L-gulononate is converted to L-gulonolactone.
The final step in this biosynthetic pathway is the oxidation of L-gulonolactone to L-ascorbic acid.
This critical reaction is catalyzed by the enzyme L-gulonolactone oxidase (spelled "L-guluconolactone oxidase" in the option).
During evolutionary history, a gene mutation occurred in the ancestor of humans, primates, and guinea pigs.
This mutation turned the gene encoding L-gulonolactone oxidase (GULO) into a non-functional pseudogene.
Because these species lack a functional copy of this enzyme, they are completely unable to complete the synthesis of ascorbic acid.
The other enzymes listed, such as UDP-glucose pyrophosphorylase and UDP-glucose dehydrogenase, remain active because they are required for other essential metabolic processes, such as glycogen synthesis and bilirubin conjugation.

Step 3: Final Answer:
The inability of humans, primates, and guinea pigs to synthesize Vitamin C is due to the genetic absence of the functional enzyme L-gulonolactone oxidase.