Vitamin nomenclature: Which molecule is known as vitamin H and serves as a coenzyme for carboxylase reactions?

Introduction / Context:
Vitamins often have multiple names. Vitamin H is the historical name for a coenzyme that transfers CO2 in carboxylation reactions critical for gluconeogenesis and fatty acid metabolism.

Given Data / Assumptions:

• Biotin is covalently attached to carboxylases via a lysine (biocytin linkage).
• Key biotin-dependent enzymes: pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase.
• Alternative molecules listed have different roles.

Concept / Approach:
Vitamin H = biotin. As a swinging-arm CO2 carrier, biotin shuttles activated CO2 between active sites, enabling anaplerotic and biosynthetic reactions.

Step-by-Step Solution:
Map old name to modern: vitamin H → biotin.Confirm function: coenzyme for ATP-dependent carboxylases.Select biotin as the correct answer.

Verification / Alternative check:
Biotin deficiency (e.g., raw egg white avidin binding) impairs carboxylase activity, aligning with vitamin H identity.

Why Other Options Are Wrong:

• Carnitine transports acyl groups into mitochondria; not a vitamin in this context.
• Folic acid is vitamin B9, a one-carbon carrier of different chemistry.
• None of these: incorrect since biotin fits exactly.

Common Pitfalls:
Mixing up biotin (vitamin H) with pantothenic acid (B5) or folate (B9); each has distinct coenzymatic roles.

Final Answer:
Biotin