The secondary and tertiary structures of ${eta}$-galactosidase from L. lactis ssp. lactis 7962 were designed using Nnpredict and Sybyl version 6.3. By using site-directed mutagenesis, the mutated enzymes, Tyr-475-phe and Glu-506-Asp, were generated based on the structural modeling of L. lactis ssp. lactis 7962. The enzymes Tyr.-475-Phe and Glu-506-Asp had <$1\%$ of the activity of the native enzyme with ONPG as substrate. The $V_{max}$ values of the mutated enzymes were greatly reduced (1,800~40,000-1314) compared with the value for the native ${eta}$-galactosidase. However, the $K_m$ values of Tyr-475-Phe and Glu-506-Asp with ONPG, PNPG, PNPF, and PNPA were not significantly different from those of the native enzyme. The results obtained support the suggestion that Tyr-475 and Glu-506 constitute very important parts of the catalytic machinery of the ${eta}$-galactosidase.