how is unnatural amino acid incorporated into recombinantly expressed proteins or peptides？

Unnatural amino acids can be incorporated into recombinantly expressed proteins or peptides through several methods:

1. Expanded Genetic Code: This method involves introducing an orthogonal tRNA/synthetase pair that specifically recognizes the unnatural amino acid. The orthogonal tRNA carries the unnatural amino acid, and the orthogonal synthetase charges it onto the tRNA. This allows the unnatural amino acid to be incorporated at specific codons during protein synthesis.

2. Amber Suppression: This method utilizes a stop codon (amber codon, UAG) in the genetic sequence of the protein or peptide. An orthogonal tRNA/synthetase pair is used to recognize the amber codon and insert the unnatural amino acid instead of terminating translation.

3. Chemical Modification: In this method, the recombinantly expressed protein or peptide is produced with a reactive group that can be chemically modified to introduce the unnatural amino acid. For example, a cysteine residue can be introduced into the protein sequence, and then a chemical reaction can be performed to selectively modify the cysteine with the desired unnatural amino acid.

4. Site-Directed Mutagenesis: This method involves introducing specific mutations in the genetic sequence to replace a natural amino acid with the desired unnatural amino acid. This is achieved by designing primers with the desired mutation and using PCR to amplify the gene with the mutated sequence.

These methods allow researchers to incorporate unnatural amino acids into recombinantly expressed proteins or peptides, expanding the range of functionalities and properties that can be achieved.