Kinetic studies of unfolding process of aspergillopepsin II by pH-jump methods

pH-induced unfolding process of the acid proteinase aspergillopepsin II was studied kinetically by stopped-flow methods using circular dichroism and small angle X-ray scattering. Native aspergillopepsin II consists of two polypeptide chains, a light chain and a heavy chain, which are bound noncovalently to each other at acidic pH. Above neutral pH the two chains are known to dissociate and swell due to electrostatic repulsion inside the molecule. The present studies suggested that the polypeptide chains dissociate first and that the dissociated heavy chain expands subsequently. This two-phased pathway is consistent with the accumulation of an intermediate species in the pH-induced unfolding of the enzyme.