The receptor X has changed shape because amino acids with charged or ionic or hydrophilic R-groups were replaced by two amino acids with uncharged or nonpolar or hydrophobic R-groups.
R-groups or side chains attached to the amino acids determine their properties. Polar/hydrophilic R-groups will render the amino acid hydrophilic. It is more likely to interact with polar environments like the extracellular matrix or the cytosol.
Whereas, non-polar/hydrophobic side chains render amino acids hydrophobic. This means that they world orient themselves towards the inside of the protein i.e. the tertiary structure of proteins.
According to the attached image, the original amino acid sequence is Threonine - Lysine - Glutamate - Valine - Glycine, whereas, Threonine - Isoleucine - Alanine - Valine - Glycine is the altered sequence.
You can observe that Lysine and Glutamate are polar amino acids, likely to be present on the surface of the protein. Whereas, isoleucine and alanine are non-polar, likely to be present on the inside. Therefore, this change in the sequence forces a conformational change in the receptor X.