Question

The key intermediate for the biosynthesis of C6-C3 units is:

• A. Pyruvic acid
• B. Shikimic acid
• C. Dehydroquinic acid
• D. Mevalonic acid

Answer 1

The Correct Option is B

The correct option is (B): Shikimic acid.

Answer 2

The question asks which biosynthetic intermediate leads to compounds carrying a benzene ring attached to a three-carbon side chain, the C6-C3 phenylpropanoid unit. Let's check each option against its actual place in plant biosynthesis.

1. Pyruvic acid: Pyruvic acid is an early glycolytic intermediate that combines with erythrose-4-phosphate to feed into the shikimic acid pathway, but it is only a starting feeder molecule rather than the key intermediate that directly gives rise to the aromatic C6-C3 skeleton.
2. Shikimic acid: The shikimic acid pathway converts simple sugar-derived precursors into shikimic acid, which is then carried forward to chorismic acid and the aromatic amino acids phenylalanine and tyrosine. These amino acids are deaminated to give cinnamic acid derivatives, exactly the C6, aromatic ring, plus C3, three-carbon side chain, skeleton the question refers to, which is what makes shikimic acid the pivotal intermediate of this route.
3. Dehydroquinic acid: Dehydroquinic acid is an earlier intermediate formed on the way to shikimic acid within the same pathway, but it sits before shikimic acid in the sequence and is not itself recognized as the branch point feeding the C6-C3 units.
4. Mevalonic acid: Mevalonic acid belongs to an entirely separate biosynthetic route, the mevalonate pathway, which builds terpenoids and steroids from isoprene units, and it has no role in producing the aromatic C6-C3 phenylpropanoid skeleton.

Only shikimic acid sits at the branch point that leads directly toward the aromatic amino acids and their C6-C3 phenylpropanoid derivatives.

The correct answer is Shikimic acid.