Question

Methionine Has Only One Codon Methionine is one of two amino acids with only one codon. How does the single codon for methionine specify both the initiating residue and the interior Met residues of polypeptides synthesized by \(E\). coli?

Short answer

AUG codon serves both as an initiation signal with tRNA^fMet and as an interior Met residue marker with tRNA^Met.

Step-by-step solution

Understand Codons and Methionine

Methionine is an amino acid that is uniquely specified by only one codon, AUG, in the genetic code. This codon plays a crucial role in the initiation of protein synthesis.

Role of AUG Codon

The AUG codon serves a dual purpose: it codes for Methionine both as a starting signal for the ribosome to initiate protein synthesis, and as an indicator for the incorporation of Methionine into the interior of a protein chain. This versatility is integral to E. coli's polypeptide synthesis.

Mechanism of Initiation

In the initiation of protein synthesis in E. coli, the AUG codon is recognized by a special initiator tRNA called tRNA^fMet, which is modified to carry formyl-methionine (fMet). This distinct form allows the cell machinery to recognize the start of a protein.

Incorporation of Interior Methionine

When AUG codon appears in the middle of a mRNA sequence, it is recognized by a different tRNA, called tRNA^Met, that brings a regular Methionine to the growing peptide chain, differentiating it from the initiation role.

Key concepts

Methionine

Methionine is a special amino acid because it is one of the two amino acids that are specified by a single codon, which in this case is AUG. It holds an essential position in protein synthesis as it serves as the starting amino acid in nearly all newly synthesized polypeptide chains. Methionine is important not only because of its role as an initiator but also because it can be found within the interior of proteins.

• Methionine's unique single-codon specification gives it a distinct function in biochemical processes.
• It plays a vital role both at the initiation of protein synthesis and in various structural and functional parts of proteins once the initial synthesis is complete.

AUG Codon

The AUG codon is the genetic sequence that codes for methionine. Its significance extends beyond just coding for an amino acid. It serves a dual purpose in the genetic code.

• Firstly, AUG acts as the start codon in mRNA, signaling the ribosome where to begin translating the genetic message into a protein.
• Secondly, it is also used to code for methionine when it appears within a protein being synthesized.

This versatility ensures that proteins are synthesized correctly and that methionine is incorporated both as the initial segment and within the growing protein structure. The ability of the AUG codon to perform such distinct roles highlights the complexity and efficiency of genetic coding mechanisms.

Protein Synthesis

Protein synthesis is the process by which cells build proteins, and it begins with the role of methionine and the AUG codon. Upon encountering the AUG codon at the start of an mRNA strand, the ribosome initiates translation, commencing the assembly of a polypeptide chain.

• This process begins with the recognition of the AUG codon as the start codon, marking the spot for the ribosomal machinery to assemble and start synthesizing the protein.
• The initiation of protein synthesis is critically dependent on the presence of a specialized tRNA that aids in this process (tRNA^fMet in E. coli).
• Methionine marks the beginning, but other amino acids soon follow, added by various tRNA molecules as the ribosome travels along the mRNA sequence.

Protein synthesis is fundamental for cell function and involves intricate processes that ensure proteins are built efficiently and accurately.

tRNA

Transfer RNA (tRNA) acts as an adapter molecule in protein synthesis, linking codons in the mRNA to the appropriate amino acids. It plays an essential role in ensuring proteins are built with the correct sequence of amino acids.

• tRNA molecules have anticodons, which are complementary to codons in the mRNA. This allows them to bring the right amino acids to the ribosome for protein assembly.
• In the case of methionine, two types of tRNA are involved: tRNA^fMet, which recognizes the start codon AUG and brings formyl-methionine to initiate protein synthesis, and tRNA^Met, which incorporates regular methionine into the peptide chain interior.

tRNA's ability to interpret the genetic code and deliver specific amino acids is crucial for the accuracy and effectiveness of protein synthesis. Each tRNA is specific to one amino acid, making it a vital player in translating genetic information into functional proteins.