Biosynthesis encompasses a wide range of metabolic pathways that produce essential biomolecules such as fatty acids, amino acids, and neurotransmitters. Several statements about biosynthetic pathways are commonly discussed, but not all are accurate. This article evaluates four such statements to identify which are correct based on current biochemical understanding.

Statement 1: Carbon Source in Palmitate Biosynthesis

• Claim: In the biosynthesis of palmitate, all the carbon atoms are derived from activated malonate.

• Reality: Palmitate (a 16-carbon saturated fatty acid) is synthesized from one acetyl-CoA molecule and seven malonyl-CoA molecules.

• The acetyl-CoA contributes the initial two carbons, while the malonyl-CoA units provide the remaining 14 carbons through successive two-carbon elongations.

• Malonyl-CoA is the activated form of malonate and serves as the primary carbon donor during chain elongation.

• Therefore, not all carbons come from malonate; two carbons come from acetyl-CoA.

Statement 2: Amino Acid Precursors in Bacteria

• Claim: The amino acids methionine (Met), threonine (Thr), lysine (Lys), isoleucine (Ile), valine (Val), and leucine (Leu) are biosynthesized from oxaloacetate and pyruvate in most bacteria.

• Reality:

  • Oxaloacetate is a precursor for amino acids like aspartate, which is the starting point for Met, Thr, Lys, and Ile biosynthesis.

  • Pyruvate is a precursor for Val and Leu biosynthesis.

• This statement is correct as these amino acids are synthesized from these key metabolic intermediates in bacteria.

Statement 3: Alanine as a Precursor for Porphyrin Biosynthesis

• Claim: Alanine is a major precursor for the biosynthesis of porphyrin.

• Reality:

  • The major precursors for porphyrin biosynthesis are glycine and succinyl-CoA.

  • Glycine combines with succinyl-CoA to form δ-aminolevulinic acid (ALA), the first committed step in porphyrin synthesis.

  • Alanine is not a major precursor in this pathway.

• Therefore, this statement is incorrect.

Statement 4: Tryptophan Conversion to L-DOPA in Epinephrine Biosynthesis

• Claim: Tryptophan is converted to L-DOPA in the biosynthesis of epinephrine.

• Reality:

  • L-DOPA is synthesized from tyrosine, not tryptophan.

  • Tyrosine is hydroxylated by tyrosine hydroxylase to form L-DOPA, which is then converted to dopamine, norepinephrine, and finally epinephrine.

  • Tryptophan is a precursor for serotonin and melatonin, not catecholamines.

• Hence, this statement is incorrect.

Summary Table of Statements

Conclusion

Among the given statements about biosynthesis:

• Only statement (2) is correct: The amino acids Met, Thr, Lys, Ile, Val, and Leu are biosynthesized from oxaloacetate and pyruvate in most bacteria.

• Statements (1), (3), and (4) contain inaccuracies regarding carbon sources and precursor molecules.