Question

The fatty acid synthase multienzyme complex contains how many identical polypeptide monomers?

• A. 2
• B. 3
• C. 4
• D. 5

Hint:

Eukaryotes use a single large, homodimeric enzyme (FAS I) for fatty acid synthesis, whereas prokaryotes use FAS II, which consists of individual, separate monofunctional enzymes.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
In eukaryotes, the de novo synthesis of fatty acids is carried out in the cytosol by a multifunctional enzyme complex known as Fatty Acid Synthase (FAS I).
This enzyme complex organizes multiple distinct catalytic activities into a single structural framework to maximize metabolic efficiency.

Step 2: Detailed Explanation:
The eukaryotic Fatty Acid Synthase (FAS I) is structurally organized as a homodimer.
This means the active enzyme complex consists of exactly 2 identical polypeptide monomers.
Each monomer has a molecular mass of approximately $270\text{ kDa}$ and contains seven distinct catalytic domains as well as an acyl carrier protein (ACP) domain.
These domains include:
Ketoacyl synthase (KS)
Malonyl/acetyl transferase (MAT)
Dehydratase (DH)
Enoyl reductase (ER)
Ketoacyl reductase (KR)
Acyl carrier protein (ACP)
Thioesterase (TE)
Although a single monomer contains all the necessary catalytic domains, the individual monomer is completely inactive.
The functional active sites are formed by the head-to-tail association of the two identical monomers.
During fatty acid synthesis, the intermediates are transferred between the catalytic sites of opposite monomers.
Thus, dimerization is structurally required for the catalytic cycle to proceed.

Step 3: Final Answer:
The fatty acid synthase multienzyme complex contains 2 identical polypeptide monomers.