Active Site Quantification of an ω-Transaminase by Performing a Half Transamination Reaction

Measurement of the active enzyme fraction in a given enzyme preparation is a requirement for accurate kinetic measurements and activity comparisons of, for example, engineered mutants. ω-Transaminases, enzymes capable of interconverting ketones and amines by use of pyridoxal-5′-phosphate (PLP), can be used for the production of pharmaceutically important chiral amines but are subject to engineering to meet the practical requirements in synthesis reactions. Therefore, an active site quantification method is needed. Such a method was developed by quantifying the amount of consumed substrate in a virtually irreversible half transamination reaction. ( S )-1-phenylethylamine was converted to acetophenone, while the holo enzyme (E-PLP) was converted to apo enzyme with bound pyridoxamine-5′-phosphate (E:PMP). Further, the mass of active enzyme was correlated to the absorbance of the holo enzyme to achieve a direct measurement method. The active Chromobacterium violaceum ω-transaminase with bound PLP can be quantified at 395 nm with an apparent extinction coefficient of 8.1 mM –1 cm –1 .