Sulphur of cysteine are not used/utilised in the body for the following process/product:
**Core Concept**
Sulphur-containing amino acids, such as methionine and cysteine, play crucial roles in various biochemical processes. While sulphur from cysteine is utilized in several pathways, there are specific processes where it is not directly involved.
**Why the Correct Answer is Right**
Cysteine contains a thiol group (-SH), which is a source of sulphur in the body. However, the introduction of sulphur in methionine is primarily facilitated by the enzyme serine acetyltransferase, which uses serine as a substrate to synthesize O-acetylserine. This O-acetylserine is then converted into homoserine, which is eventually converted into methionine through a series of reactions. In this pathway, cysteine is not directly involved as a source of sulphur for methionine synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** The conversion of cyanide to thiocyanate indeed involves sulphur-containing compounds, as the enzyme rhodanese catalyzes this reaction using thiosulfate as a sulfur donor, thereby utilizing sulphur from cysteine indirectly.
**Option B:** Thiosulfate formation can occur through the oxidation of cysteine, which involves the release of sulphur. Therefore, cysteine's sulphur is utilized in thiosulfate formation.
**Option D:** Disulphide bond formation between two adjacent peptides is a critical process that involves the oxidation of cysteine residues, resulting in the formation of a disulfide bridge. This process indeed utilizes the sulphur from cysteine.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that while cysteine's sulphur is utilized in several biochemical pathways, its role is not directly involved in methionine synthesis. This distinction highlights the importance of understanding the specific biochemical pathways and the roles of individual amino acids.
**✓ Correct Answer: C. Introduction of sulphur in methionine**