The identity of a polypeptide is fundamentally determined by its amino acid sequence. The five amino acids you mentioned—serine, tyrosine, alanine, phenylalanine, and valine—are part of a unique order that contributes to the specific structure and function of the resulting protein.
If any of these amino acids were changed or rearranged, the entire sequence would change. This alteration can lead to the formation of a different polypeptide, which may not only carry a different name but also exhibit distinct biochemical properties. This is due to the fact that each amino acid has its own unique characteristics that influence how the polypeptide will fold and function.
For example, the hydrophobic nature of phenylalanine affects how it interacts with other molecules in the cell. If phenylalanine were replaced with aspartic acid, which is hydrophilic, this change could impact the overall stability and interactions of the polypeptide.
In summary, the arrangement and identity of amino acids in a polypeptide chain are crucial for its biological role. Therefore, even a single change in the sequence can alter the polypeptide’s identity and functionality.