39. During fatty acid biosynthesis, the first intermediate malonyl-CoA is formed from
(A) Acetyl- CoA and bicarbonate
(B) Two acetyl- CoA molecules
(C) Acetyl- CoA and biotin
(D) Palmitoyl CoA and acyl-carrier protein (ACP)

Fatty acid biosynthesis builds long-chain fatty acids from simple precursors, and understanding its first committed step is crucial for biochemistry students preparing for exams like NEET, CSIR NET, or university assessments. The question—”During fatty acid biosynthesis, the first intermediate malonyl-CoA is formed from”—tests knowledge of this key reaction. Let’s break it down with the correct answer and why each option fits (or doesn’t).

Correct Answer: (A) Acetyl-CoA and Bicarbonate

Malonyl-CoA forms through the carboxylation of acetyl-CoA, catalyzed by acetyl-CoA carboxylase (ACC), the rate-limiting enzyme in fatty acid synthesis. This ATP-dependent reaction uses bicarbonate (\ceHCO3−\ceHCO3−) as the CO2 source:

\ceAcetyl−CoA+HCO3−+ATP−>Malonyl−CoA+ADP+Pi+H+\ceAcetyl−CoA+HCO3−+ATP−>Malonyl−CoA+ADP+Pi+H+

Biotin acts as a temporary CO2 carrier on the enzyme. This step commits acetyl-CoA to fatty acid production by adding a carboxyl group, making malonyl-CoA a high-energy donor for chain elongation. In plants, bacteria, and animals, this occurs in the cytosol (or chloroplasts in plants), priming the fatty acid synthase complex.

Why is this the first intermediate? Acetyl-CoA alone can’t directly extend chains; carboxylation activates it, preventing futile cycling with beta-oxidation.

Why Not the Other Options?

Here’s a clear breakdown of all choices, highlighting common misconceptions:

• (A) Acetyl-CoA and bicarbonate
  Correct. As explained, this is the precise reaction. Bicarbonate provides the carboxyl group, essential for malonyl-CoA’s role in providing two-carbon units during synthesis (with decarboxylation releasing energy).

• (B) Two acetyl-CoA molecules
  Incorrect. Two acetyl-CoAs condense later to form acetoacetyl-ACP (via beta-ketoacyl-ACP synthase), but that’s not how malonyl-CoA forms. This option confuses it with ketone body synthesis or initial chain assembly steps downstream.

• (C) Acetyl-CoA and biotin
  Incorrect but tricky. Biotin is a cofactor for ACC, binding CO2 before transfer to acetyl-CoA. However, the direct substrates are acetyl-CoA and bicarbonate—not biotin itself. Listing biotin misrepresents the reaction stoichiometry.

• (D) Palmitoyl CoA and acyl-carrier protein (ACP)
  Incorrect. Palmitoyl-CoA is the 16-carbon end product of synthesis, not a precursor. ACP (part of fatty acid synthase) tethers growing chains, but malonyl-CoA forms before ACP involvement, when malonyl is transferred to ACP as malonyl-ACP.

Key Takeaways for Fatty Acid Biosynthesis

• Malonyl-CoA prevents reverse reactions by its unstable carboxyl group.

• Regulation: ACC is activated by citrate (fed state) and inhibited by phosphorylation (fasting).

• Relevance: Dysregulation links to obesity, as malonyl-CoA also inhibits carnitine palmitoyltransferase-1 (CPT-1), blocking fatty acid oxidation.

This MCQ often appears in exams testing de novo lipogenesis. Master it alongside related topics like NADPH sources (pentose phosphate pathway) for better scores.