Question

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

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

Hint:

Shikimic acid is central to the biosynthesis of aromatic compounds, including the C6-C3 units, which are essential for the production of several biologically active molecules.

Answer 1

The Correct Option is B

C6-C3 units are the phenylpropanoid building blocks found in many plant natural products, made of an aromatic ring joined to a three-carbon chain. Tracing the pathway that produces this skeleton points to the answer.

1. Pyruvic acid: This compound is one of the two starting materials that enter the shikimate pathway (along with erythrose-4-phosphate), but the pathway needs several more steps before it reaches the intermediate that defines C6-C3 biosynthesis.
2. Shikimic acid: After pyruvic acid and erythrose-4-phosphate condense and the ring closes, shikimic acid is formed. This compound gives the pathway its name and leads onward to chorismic acid, prephenic acid, and finally phenylalanine, which is deaminated to give cinnamic acid, the parent C6-C3 unit. Because of this direct link, shikimic acid is treated as the key intermediate for C6-C3 biosynthesis.
3. Dehydroquinic acid: This intermediate forms before shikimic acid in the same pathway, so while it is on the route, it is not the compound the C6-C3 pathway is named after or characterised by.
4. Mevalonic acid: This is the starting compound of the mevalonate pathway, which makes terpenoids (C5 isoprene units), a completely separate class of natural products from phenylpropanoids.

Given that the shikimate pathway is the route to aromatic C6-C3 units and shikimic acid is its defining intermediate, the correct answer is Shikimic acid.