Coenzyme function – biotin: Biotin (vitamin B7) serves as a covalently attached prosthetic group that carries which activated species during carboxylation reactions?

Introduction / Context:
Coenzymes and prosthetic groups extend enzyme chemistry beyond amino acid side chains. Biotin (vitamin B7) is a classic example, enabling ATP-dependent carboxylation in metabolic pathways such as gluconeogenesis and fatty acid synthesis.

Given Data / Assumptions:

• Biotin is covalently attached to carboxylases via an amide linkage to a lysine residue (biocytin).
• The question asks which chemical species biotin carries.
• Focus is on mechanistic biochemistry of carboxylation.

Concept / Approach:
Biotin functions as a CO2 carrier. In a two-step mechanism, ATP and bicarbonate generate a carboxyphosphate intermediate and then carboxy-biotin; subsequently, biotin swings to the second active site (via a flexible “biotin arm”) to transfer the activated CO2 to the substrate. Typical enzymes include pyruvate carboxylase, acetyl-CoA carboxylase, and propionyl-CoA carboxylase.

Step-by-Step Solution:
Identify biotin-dependent enzyme class: biotin carboxylases/carboxyltransferases.Recall the “swinging arm” model where biotin carries CO2 between active sites.Associate the carried species as activated CO2, not methyl or sulfhydryl groups.Choose option indicating activated CO2.

Verification / Alternative check:
Biochemical textbooks consistently describe biotin as a CO2 carrier; deficiency leads to impaired carboxylation (e.g., dermatitis, alopecia) due to inadequate function of biotin-dependent enzymes.

Why Other Options Are Wrong:

• Ammonia: Carried by glutamine amidotransferases and used in transamidation; not biotin’s role.
• Methyl group: Typically carried by S-adenosylmethionine, not biotin.
• Sulfhydryl group: Characteristic of coenzyme A and lipoamide chemistry, not biotin-mediated transfer.
• Activated acetate: Carried by CoA (acetyl-CoA), not by biotin.

Common Pitfalls:
Confusing biotin (CO2 carrier) with folate or SAM (one-carbon carriers in different oxidation states). Biotin specifically handles carboxyl groups.

Final Answer:
Activated carbon dioxide (CO2).