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The structure of both the amino acids are shown below:

Structure of tyrosine

Structure of cysteine

The isoelectric point (pI) is the _PH at which amino acid doesn’t migrate in an electric field. Its net charge will be neutral.

The pIof amino acids that have ionizable side chain is the average of pKa values of similar ionizing groups.

For example, pIof lysine is the average of the pKa values of the two groups that are positively charged in their acidic form and uncharged in their basic form.

Amino acids like tyrosine and cysteine have ionizable side chain but don’t have similar ionizing groups. Both cysteine and tyrosine have ionizable side chain that are neutral in acidic form and negatively charged in basic form.

The pI value can be find out using this method if either positively charged ionizing to uncharged groups or uncharged groups to negatively charged groups is present in an amino acid.

So, pIvalue of these aminoacids cannot be determine using this method because there is no similar ionizing side group.