Final answer:
Methionine is the amino acid that initiates the synthesis of all proteins, as it is associated with the start codon AUG in both prokaryotic and eukaryotic organisms, and is the first amino acid incorporated during the initiation phase of translation.
Step-by-step explanation:
The amino acid which starts all protein synthesis is 1) Methionine.In the process of protein synthesis, methionine has a distinct role as the initial amino acid incorporated into all newly forming polypeptides, serving as the start signal for translation. This is due to the codon AUG, which not only codes for methionine but also functions as the initiation codon in both prokaryotic and eukaryotic organisms. When a ribosome assembles on a messenger RNA (mRNA) strand, it begins reading the sequence from the start codon, AUG, which is recognized by a unique initiator transfer RNA (tRNA) charged with methionine.
While other amino acids can be specified by multiple codons, methionine and tryptophan are unique in having only one codon each. After the protein is synthesized, the initial methionine is often enzymatically cleaved from the growing polypeptide chain. This removal of the initial methionine indicates its primary role in translation initiation rather than as a stable component of the final protein structure. Therefore, the correct answer to which amino acid starts all protein synthesis is methionine.