Heat and photochemical inactivations of Taka-amylase A and mechanisms of protection by substrate.

The heat, ultraviolet and rihoflavin-sensitized visible inactivations of taka-amylase A are strongly inhibited by the presence of its substrate. The stabilization of the secondary structure of enzyme protein by the conformation change due to the formation of enzyme-substrate and -product complexes is responsible for the protection of enzyme from the heat inactivation. The photoinactivations are brought about by the combined effects of heat and photochemical processes. The observed protections from photoinactivations are due to the inhibition effects of substrate and its decomposition products on both processes. The thermodynamic quantities determined for the inactivation reactions throw some light on the relationship between the heat and photochemical processes, and on the mechanisms of the protective action of substrate.