Static and dynamic half-life and lifetime molecular turnover of enzymes

The static half-life of an enzyme is the half-life of a free enzyme not working without substrate and the dynamic half-life is that of an active enzyme working with plenty amount of substrate. These two half-lives were measured and compared for glucoamylase (GA) and β-galactosidase (BG). The dynamic half-life was much longer than the static half-life by one to three orders of magnitude for both enzymes. For BG, the half-life of the enzyme physically entrapped in a membrane reactor was also measured. In this case also, the half-life of BG in the membrane reactor was much longer than the free enzyme without substrate. These results suggest the large difference in stabilities between the free enzyme and the enzyme–substrate complex. This may be related to the natural enzyme metabolism. According to the difference in half-life, the lifetime molecular turnover (LMT), which is the number of product molecules produced by a single molecule of enzyme until it loses its activity completely, was much higher by one to four orders of magnitude for the active enzyme than the free enzyme. The concept of LMT, proposed here, will be important in bioreactor operations with or without immobilization. •Half-life of enzyme increased by ten-to thousand-fold with coexistence of substrate.•Enzyme-substrate complex showed much higher stability than free enzyme.•Lifetime molecular turnover was much higher for working enzyme than idle one.