Answer:
It would disrupt binding to negatively charged functional groups.
Explanation:
The lysine side chain contains an amino group (-NH3) that is usually protonated under physiolgoical pH (-NH4+), making it positively charged and basic. This enables it to interact with negatively charged functional groups (e.g. deprotonated acids, -COO-). Phenylalanine contains an aromatic ring but no charged functional groups in its side chain. Therefore, a mutation from Lys to Phe would prevent interaction with negatively charged functional groups.