Amino Acid Structure and the Sulphydryl Group

Amino acids are the fundamental units that build proteins. Each amino acid consists of a central carbon atom bonded to an amino group ($-\text{NH}_2$), a carboxyl group ($-\text{COOH}$), a hydrogen atom, and a distinctive side chain, often referred to as the R-group. This R-group is what differentiates one amino acid from another and determines its unique chemical characteristics.

The question focuses on identifying the amino acid that contains a Sulphydryl functional group. This group is characterized by a sulfur atom bonded to a hydrogen atom, represented as $ \text{$-SH$} $. The presence of this group is significant in the context of protein chemistry.

Analyzing Amino Acid Side Chains for the -SH Group

To answer the question, we need to examine the structure of the side chains for each of the amino acids provided:

• Aspartic acid: This is classified as an acidic amino acid. Its side chain is $-\text{CH}_2\text{COOH}$. It contains a carboxyl group but lacks a Sulphydryl group.
• Glutamic acid: Similar to aspartic acid, glutamic acid is also acidic. Its side chain is $-\text{CH}_2\text{CH}_2\text{COOH}$, featuring a carboxyl group. It does not possess a Sulphydryl group.
• Tryptophan: This is an aromatic amino acid with a complex indole ring structure in its side chain. This structure does not include a Sulphydryl group.
• Cysteine: Cysteine is notable for its side chain, which includes a $-\text{CH}_2\text{SH}$ moiety. The terminal sulfur atom attached to a hydrogen atom constitutes the Sulphydryl (or thiol) group. This group is crucial as it can form disulfide bonds ($-\text{S-S}-$) with another cysteine residue, which helps stabilize the three-dimensional structure of proteins.

Conclusion: Cysteine's Unique Sulphydryl Feature

By reviewing the characteristic side chains, it becomes clear that Cysteine is the amino acid among the options that possesses the Sulphydryl ($-\text{SH}$) group.