Final answer:
All proteins do begin with the amino acid methionine due to the codon AUG being the start codon; however, this initial methionine is often removed during post-translational modifications, so the final protein may not have methionine at the amino terminus.
Step-by-step explanation:
True, all proteins initiated by the start codon begins with the amino acid methionine. In both prokaryotic and eukaryotic cells, the codon AUG not only codes for methionine but also signals the start of protein synthesis.
In prokaryotes, this starting methionine is formylated to become N-formyl methionine (fMet), whereas in eukaryotes, it remains as methionine but is often removed once the polypeptide chain is complete.
Considering the entire polypeptide, methionine is not always the first amino acid in the mature protein because of post-translational modifications where the initial methionine is enzymatically cleaved off.
The process begins with an initiator tRNA carrying the methionine amino acid that recognizes the start codon AUG. For prokaryotes, the enzyme formylating methionine attached to the initiator tRNA formylates the amino group, creating fMet-tRNAfmet. In contrast, eukaryotes use a similar process without formylating the methionine.
It's important to note that not all proteins will ultimately start with methionine in their final, functional form. A post-translational modification process removes the starting methionine or modifies it, such as acetylation. Thus, the mature proteins do not usually have methionine at the N-terminus.