Question

The synthesis of proteins requires 20 standard amino acids in the living systems. However, some organisms use selenocysteine as an additional amino acid for synthesizing proteins. Selenocysteine is biosynthesized from:

• A. Cysteine
• B. Serine
• C. Threonine
• D. Proline

Hint:

Selenocysteine is often called the 21st amino acid and is produced by the substitution of selenium for sulfur in cysteine.

Answer 1

The Correct Option is A

Step 1: Understanding selenocysteine.
Selenocysteine is often referred to as the 21st amino acid and is used by some organisms in protein synthesis. It is chemically similar to cysteine, but with selenium replacing sulfur.
Step 2: Analyzing the options.
(1) Cysteine: Correct — Selenocysteine is biosynthesized from cysteine by replacing sulfur with selenium.
(2) Serine: This is incorrect. Serine is involved in the synthesis of other amino acids but is not directly involved in the synthesis of selenocysteine.
(3) Threonine: This is incorrect. Threonine is a separate amino acid and not a precursor for selenocysteine.
(4) Proline: This is incorrect. Proline does not play a direct role in the biosynthesis of selenocysteine.
Step 3: Conclusion.
The correct answer is (1) Cysteine, as selenocysteine is biosynthesized from cysteine by substituting selenium for sulfur.

Answer 2

Selenocysteine is often called the twenty-first amino acid, and identifying its biosynthetic origin comes down to picking the option that shares the closest structural kinship with it. Let's weigh each candidate individually.

1. Cysteine: Cysteine carries a thiol (-SH) group on its side chain, and selenocysteine's side chain is built on exactly the same carbon skeleton with selenium substituted for sulfur. Among the four options, cysteine is the only one whose side-chain chemistry (a chalcogen-bearing -XH group attached to a single methylene carbon) mirrors selenocysteine so closely.
2. Serine: Serine has a hydroxyl (-OH) side chain, which is a poor chemical match for selenocysteine's selenol side chain -- oxygen and selenium do not behave alike here, making this a less natural fit than cysteine's route.
3. Threonine: Threonine is a branched, hydroxyl-bearing amino acid used mainly for protein structure and O-linked glycosylation sites; it has no pathway that funnels into selenocysteine production.
4. Proline: Proline's defining feature is its cyclic secondary-amine structure, used for structural rigidity in proteins. It has no side-chain chemistry compatible with selenium incorporation at all.

Ruling out threonine and proline on structural grounds, and serine on the basis of a mismatched functional group, leaves cysteine as the amino acid whose chemistry aligns with selenocysteine.

Therefore, the correct answer is Cysteine.