Which of the following special amino acid is not formed by post-translational modification?
**Core Concept**
Post-translational modification (PTM) refers to the various changes that proteins undergo after translation, including amino acid modification, processing, and conjugation. These modifications can significantly alter the function, localization, and stability of proteins. One type of PTM involves the formation of special amino acids from standard amino acids.
**Why the Correct Answer is Right**
Among the special amino acids, hydroxyproline is not formed by post-translational modification. Instead, it is directly incorporated into the growing peptide chain during translation. Hydroxyproline is an amino acid found in collagen, a structural protein that provides strength and elasticity to connective tissue. It is synthesized by the enzyme prolyl hydroxylase, which acts on proline residues in the collagen molecule.
**Why Each Wrong Option is Incorrect**
* **Option A:** Hydroxylysine is a special amino acid formed by post-translational modification through the action of lysyl hydroxylase on lysine residues in collagen.
* **Option B:** 5-Hydroxytryptophan is formed by hydroxylation of tryptophan, a process that occurs through post-translational modification.
* **Option D:** Carboxylated amino acids, such as glutamate, can undergo post-translational modification through the addition of a carboxyl group.
**Clinical Pearl / High-Yield Fact**
Collagen synthesis and cross-linking are crucial for maintaining the integrity and strength of connective tissue. Disorders affecting these processes can lead to conditions such as osteogenesis imperfecta and Ehlers-Danlos syndrome.
**Correct Answer:** C. Hydroxyproline