Question

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

Short answer

Methionine is removed by post-translational modifications, allowing other amino acids to occupy the N-terminal position.

Step-by-step solution

Understand the Question

The question is asking why amino acids other than methionine can be found at the N-terminal position of proteins in eukaryotic cells.

Protein Synthesis Basics

During protein synthesis, translation starts with the amino acid methionine, which is specified by the start codon (AUG) in the mRNA sequence.

Post-Translational Modifications

After translation, proteins often undergo post-translational modifications. One common modification is the removal of the initial methionine residue by specific enzymes, allowing other amino acids to occupy the N-terminal position.

Biological Significance

The removal of methionine can affect protein stability, function, and interaction with other molecules. This processing allows proteins to have a diverse range of functions and regulatory mechanisms.

Conclusion

Thus, amino acids other than methionine can occupy the N-terminal position due to post-translational removal of methionine and other modifications affecting protein characteristics.

Key concepts

Protein Synthesis

Protein synthesis is the process that cells use to produce proteins. This is essential for cell structure and function.
It involves two main stages: transcription and translation.
During transcription, a segment of DNA is copied into mRNA. This mRNA then travels to a ribosome, where translation occurs.
Translation begins with the start codon (AUG), which codes for the amino acid methionine. This is why methionine is typically the first amino acid in a newly formed protein.
The sequence of nucleotides in mRNA determines the order of amino acids in the protein, creating a specific sequence unique to each protein.
Similarly, the ribosome reads codons (three nucleotides) and matches them with the appropriate amino acid through tRNA. This results in the formation of a polypeptide chain.

Post-Translational Modifications

Once a polypeptide chain is synthesized, it often undergoes several modifications.
These post-translational modifications are crucial for the functionality of the protein.
One common modification is the removal of the start methionine by specific enzymes called methionine aminopeptidases.
Other modifications can include the addition of functional groups like phosphorylation or glycosylation.
These changes can affect the protein's stability, activity, and interactions with other molecules.
Thus, post-translational modifications add another layer of regulation and diversity in protein function.

Amino Acids in Eukaryotes

Amino acids are the building blocks of proteins.
There are 20 different amino acids that are used in eukaryotic protein synthesis.
Each amino acid has a central carbon atom (alpha-carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group).
The side chain determines the properties and functions of the amino acid within a protein.
In eukaryotes, the diversity of amino acids allows for the formation of a wide range of protein structures and functions.
Particularly, the N-terminal amino acid can vary due to post-translational modifications, contributing to the protein's final function and interactions.
This diversity is essential for complex cellular processes and adaptation to various biological needs.