Which `alpha` amino acids can cross link peptide chains

Options

A. Serine
B. Cysteine
C. Glutamine
D. Tyrosine

Answer

To determine which alpha amino acid can cross-link peptide chains, we need to analyze the options provided and understand the concept of cross-linking in proteins.

### Step-by-Step Solution:

1. **Understanding Cross-Linking in Proteins**:
- Cross-linking refers to the formation of covalent bonds between different polypeptide chains or within a single polypeptide chain. This can stabilize the structure of proteins.

2. **Identifying Common Cross-Linking Bonds**:
- The most common type of cross-linking in proteins is through the formation of disulfide bonds. Disulfide bonds are covalent bonds formed between the sulfur atoms of cysteine residues.

3. **Analyzing the Options**:
- **Serine**: This amino acid does not have a thiol (-SH) group and cannot form disulfide bonds.
- **Cysteine**: This amino acid contains a thiol group, which can oxidize to form a disulfide bond with another cysteine, thus allowing cross-linking.
- **Glutamine**: This amino acid does not have the ability to form disulfide bonds and is not involved in cross-linking.
- **Tyrosine**: Similar to serine and glutamine, tyrosine does not have a thiol group and cannot form disulfide bonds.

4. **Conclusion**:
- Among the given options, the only alpha amino acid that can cross-link peptide chains through the formation of disulfide bonds is **Cysteine**.

### Final Answer:
Cysteine is the alpha amino acid that can cross-link peptide chains.

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