Final answer:
In eukaryotic organisms, proteins begin with the amino acid methionine because it is the initiation amino acid codified by the start codon AUG. The methionine can be enzymatically removed or modified after protein synthesis, which is different from the formylated fMet found in prokaryotes.
Step-by-step explanation:
In eukaryotic organisms, all proteins initially begin with the amino acid methionine, not methylamine. This is because of specific processes involved in the synthesis of proteins, known as translation. In prokaryotes, like E. coli, a special initiator tRNA carries N-formyl-methionine (fMet), which is formylated and inserted at the beginning of every polypeptide chain. However, in eukaryotes, this initial methionine is never formylated. Furthermore, in eukaryotic proteins, the initiator methionine is often removed enzymatically after the protein is synthesized. This cleavage and sometimes additional N-terminal amino acids are removed to produce the mature protein. Approximately 50% of eukaryotic proteins have the amino group of the amino-terminal residue acetylated. These mechanisms ensure that proteins begin correctly and are processed to their final functional forms.