Conformational stability and integrity of α-amylase from mung beans: Evidence of kinetic intermediate in GdmCl-induced unfolding

α-Amylase from mung beans (Vigna radiata) being one of the few plant α-amylases purified so far was studied with respect to its conformational stability by CD and fluorescence spectroscopy. The enzyme was shown to bind 3-4 Ca2+ ions, which all are important for its activity. In contrast to other α-amylases no inhibition was observed at high Ca2+ concentrations (100 mM). Depletion of calcium decreased the transition temperature from 87 to 48 °C. Kinetic stopped-flow fluorescence measurements allowed detecting two unfolding phases at > 6 M GdmCl, whereas only one phase was observed at < 5 M GdmCl. These results suggest that the first (reversible) step of unfolding is slower than the second (irreversible) step at low GdmCl concentrations, whereas the rates of these two steps are opposite at high GdmCl concentrations.