Lipid metabolism — acyl carrier protein (ACP): In primary metabolism, ACP plays its central role in the biosynthesis of which class of biomolecules?

Introduction / Context:
Acyl carrier protein (ACP) is a small, highly conserved protein component of fatty acid synthase (FAS) systems. It carries acyl intermediates via a phosphopantetheine arm during chain elongation.

Given Data / Assumptions:

• ACP is post-translationally modified with 4′-phosphopantetheine, providing a thiol for thioester linkage.
• Fatty acid synthase cycles through condensation, reduction, dehydration, and reduction steps.

Concept / Approach:
Match ACP’s thiol-based swinging arm chemistry to iterative two-carbon elongation of fatty acyl chains, a signature of fatty acid biosynthesis.

Step-by-Step Solution:
Recognize ACP’s role as the carrier of malonyl/acyl groups in FAS.Differentiate from pathways for amino acids, sugars, and nucleotides that do not use ACP as core carriers.Select “Fatty acids.”Note: Modular polyketide synthases also employ ACP domains by analogy.

Verification / Alternative check:
Biochemistry of FAS I/FAS II systems (eukaryotes/bacteria) consistently locates ACP at the center of the enzyme complex.

Why Other Options Are Wrong:
Amino acid, sugar, and nucleotide biosyntheses use different cofactors and carriers (e.g., PLP, UDP-sugars, PRPP) rather than ACP.

Common Pitfalls:
Confusing ACP with CoA; both have phosphopantetheine, but ACP is protein-bound while CoA is soluble.

Final Answer:
Fatty acids.