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:

Question

A dipeptide, “x”, on complete hydrolysis gives “y” and “z”; “y” on treatment with aqueous HNO$_2$, produces lactic acid. On the other hand, “z” on heating gives the following cyclic molecule.

Based on the information given, the dipeptide X is:

Answer 1

Selenocysteine is structurally cysteine with selenium in place of sulfur, so the amino acid it is biosynthesized from must share that same single-carbon, chalcogen-bearing side chain.

Serine and threonine carry oxygen-based hydroxyl side chains, not the sulfur/selenium type of side chain selenocysteine needs, so neither fits. Proline is a cyclic imino acid used for backbone rigidity and has no chemical link to selenium incorporation at all.

Only cysteine's side chain matches the chemistry selenocysteine requires, so cysteine is the amino acid it is biosynthesized from.

Answer 2

Another way to reach the answer is to think about why organisms make selenocysteine at all, and which of the four candidate amino acids could plausibly be chemically upgraded into it.

Cysteine: Selenoproteins that use selenocysteine rely on selenium's greater nucleophilicity and lower pKa compared with sulfur to speed up catalysis. Swapping sulfur for selenium in cysteine's exact side-chain position produces exactly this catalytic upgrade, which is why cysteine's scaffold is the one reused.
Serine: Serine's hydroxyl group does not offer a comparable upgrade path -- converting an -OH to a selenium-containing group would require an entirely different kind of bond chemistry than is seen in this system.
Threonine: Threonine is metabolically committed to its own roles (protein synthesis, phosphorylation sites) and shows no biosynthetic connection to selenium metabolism.
Proline: Proline's ring structure exists purely for conformational reasons in proteins; it plays no part in any redox or selenium-handling chemistry.

Only cysteine's chemistry explains why replacing one atom (sulfur for selenium) is enough to generate a functional selenocysteine.

So, the correct answer is Cysteine.

Answer 3

Another way to reach the answer is to think about why organisms make selenocysteine at all, and which of the four candidate amino acids could plausibly be chemically upgraded into it.

Cysteine: Selenoproteins that use selenocysteine rely on selenium's greater nucleophilicity and lower pKa compared with sulfur to speed up catalysis. Swapping sulfur for selenium in cysteine's exact side-chain position produces exactly this catalytic upgrade, which is why cysteine's scaffold is the one reused.
Serine: Serine's hydroxyl group does not offer a comparable upgrade path -- converting an -OH to a selenium-containing group would require an entirely different kind of bond chemistry than is seen in this system.
Threonine: Threonine is metabolically committed to its own roles (protein synthesis, phosphorylation sites) and shows no biosynthetic connection to selenium metabolism.
Proline: Proline's ring structure exists purely for conformational reasons in proteins; it plays no part in any redox or selenium-handling chemistry.

Only cysteine's chemistry explains why replacing one atom (sulfur for selenium) is enough to generate a functional selenocysteine.

So, the correct answer is Cysteine.

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:

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