ONPG (2-Nitrophenyl β-D-galactopyranoside) is a colorimetric and spectrophotometric substrate used to detect β-galactosidase activity.

β-gal:2.7±0.3 mM(Km), β-gal:464.7±7.8/s(kcat)

The enzyme displays high hydrolysis ability for ONPG (100%) and moderate activity for its natural substrate lactose (25.7%). However, the hydrolysis ability of the enzyme towards all other chromogenic nitrophenyl analogues is very weak, indicating that Gal308 is a β-galactosidase with narrow substrate specificity. To investigate the kinetic parameters of recombinant enzyme, the Michaelis-Menten constants (Km), turnover numbers (kcat), and catalytic efficiencies (kcat/Km) of Gal308 for ONPG and lactose are determined. The kcat and Km values are 464.7±7.8 s-1 and 2.7±0.3 mM for ONPG, and 264.2±2.1 s-1 and 7.1±0.8 mM for lactose, respectively. The kcat/Km value of the enzyme for ONPG (172.1 s-1mM-1) is 4.6-fold higher than that for lactose (37.2 s-1mM-1), which clearly demonstrated that the catalytic efficiency of Gal308 for ONPG is much higher than that for lactose[1].