Among the amino acids present (Serine, Aspartic acid, Glycine, Alanine, and Threonine), only Threonine is classified as an essential amino acid; the others are non-essential or conditionally essential. Therefore, Y = Threonine.
Examining the peptide sequence shown in the structure and cross-referencing with the provided options, the sequence is Thr-Ser-Asp-Gly-Ala from N-terminus to C-terminus.
The answer is option (4): Y is Threonine with sequence Thr-Ser-Asp-Gly-Ala.
