N-terminal domain unfolds first in the sequential unfolding of papain.

Temperature and Guanidine hydrochloride induced unfolding transitions of papain at pH 2.0 are biphasic implying independent and sequential unfolding of its two domains. To determine the order of unfolding, the active site located in the interface of the domains was labeled with an environment specific fluorescent probe (1,8-IAEDANS). Unfolding of this complex relative to the free protein followed by intrinsic and extrinsic fluorescence measurements suggests that the N domain unfolds initially in the sequential unfolding of domains.