Biotin biochemistry Biotin occurs mainly in combined forms bound to proteins through which specific linkage (often called biocytin when released)? Choose the most accurate biochemical description.

Introduction / Context:
Biotin functions as a CO2 carrier in carboxylation reactions and is covalently attached to carboxylase enzymes. Understanding how biotin is linked to proteins is vital for biochemistry and nutrition.

Given Data / Assumptions:

• Biotin is attached via an amide bond to the ε-amino group of a lysine residue in apocarboxylase proteins.
• The biotin–lysine conjugate released during proteolysis is called biocytin (ε-N-biotinyl-L-lysine).
• Lipoic acid forms a different linkage (lipoyl-lysine) in dehydrogenase complexes.

Concept / Approach:
Identify the precise covalent linkage: an amide bond between biotin’s carboxylate and lysine’s ε-amino group, producing ε-N-biotinyl-L-lysine.

Step-by-Step Solution:
Recognize that biotinylation targets lysine side chains via an amide bond.Recall the name “biocytin” for ε-N-biotinyl-L-lysine.Select the option that explicitly states ε-N-biotinyl-L-lysine.

Verification / Alternative check:
Biotinidase deficiency causes impaired recycling of biocytin, underscoring that biotin normally exists as ε-N-biotinyl-L-lysine in proteins.

Why Other Options Are Wrong:
Generic ε-N-lysine moiety lacks the essential biotinyl group.ε-S-lysine is not a standard biochemical linkage.Lipoyl-lysine involves lipoic acid, not biotin.Disulfide to cysteine is characteristic of other modifications, not biotin.

Common Pitfalls:
Confusing biotin–lysine with lipoyl–lysine; overlooking the amide bond nature (ε-N- linkage) rather than sulfur-based linkage.

Final Answer:
ε-N-biotinyl-L-lysine.