Question

Why do amino acids other than methionine occur in the N-terminal position of proteins from eukaryotes?

Short answer

Methionine is often removed post-translation by specific enzymes, allowing other amino acids to occupy the N-terminal position.

Step-by-step solution

Understanding the Starting Point

All eukaryotic proteins initially start with methionine at the N-terminal because the start codon, AUG, codes for methionine.

Translation Process

During translation, the ribosome reads the mRNA and begins protein synthesis at the start codon, which adds methionine as the first amino acid.

Post-Translational Modifications

After translation, proteins often undergo post-translational modifications. One such modification can involve removing the initial methionine residue.

Enzyme Action

Specific enzymes, such as methionine aminopeptidase, can catalyze the removal of the initial methionine, allowing another amino acid to become the N-terminal residue.

Functional Importance

These modifications are important for the proper functioning, localization, and stability of the protein in eukaryotic cells.

Key concepts

methionine aminopeptidase

Methionine aminopeptidase is a crucial enzyme in the post-translational modification of proteins. It specifically removes the initial methionine residue from the N-terminus of newly synthesized proteins. This process is vital because it fine-tunes the protein structure for its specific function.
This enzyme works by cleaving the peptide bond between the methionine and the second amino acid.
The activity of methionine aminopeptidase is highly specific and regulated, ensuring that only the intended methionine residues are removed.

• It typically acts soon after protein synthesis is complete.
• It ensures proteins achieve their correct functional configuration.
• The removal process aids in protein stability and localization within cells.

Overall, methionine aminopeptidase plays a vital role in the life cycle of a protein, helping to ensure that it functions correctly within the cell.

N-terminal methionine removal

The removal of the N-terminal methionine is a common post-translational modification in eukaryotic proteins.
This process is essential for various reasons:

• It helps in targeting proteins to specific cellular locations.
• It impacts the protein's stability and its interactions with other molecules.
• Without this removal, some proteins might not function properly.

The initial methionine is often removed by methionine aminopeptidase. The enzyme recognises newly synthesized proteins and cleaves off the methionine.
This allows another amino acid to occupy the N-terminal position, altering the physical and chemical properties of the protein. This change can be important for the protein's role and how it interacts within the cell. Essentially, this modification is a key step in ensuring that the protein is ready to perform its designated functions after synthesis.

protein synthesis

Protein synthesis is the fundamental process by which cells build proteins. It starts with transcription, where DNA is copied into mRNA. In translation, the ribosome reads the mRNA sequence and assembles the amino acids to form a protein.
Importantly, the translation process in eukaryotes begins with the amino acid methionine, which is specified by the start codon AUG. This initiation step is crucial, as it sets the stage for the entire protein sequence.
Here's a simplified breakdown of protein synthesis:

• The ribosome binds to mRNA.
• It starts reading the mRNA codons beginning at the start codon (AUG).
• Methionine is added as the first amino acid.
• Subsequent codons are read, adding more amino acids to the growing chain.
• Once the protein synthesis is complete, the ribosome releases the newly formed protein.

After synthesis, proteins often undergo various modifications like methionine removal, ensuring they are fully functional and appropriately localized within the cell. Protein synthesis and its regulation are at the core of cellular function and life itself.