Cutinase in aqueous solution at pH 4.5 deactivates following a parallel pathway. At 53degreesC, 88% of the cutinase molecules are in the unfolded conformation, which can aggregate with a reaction order of 3 if the protein concentration is high (greater than or equal to12 muM). The aggregates show a sixfold increase in size as determined by dynamic light scattering. This aggregation process is the first phase observed during a deactivation experiment; however, after significant cutinase depletion and maturation of the aggregates, a first-order step starts to dominate and a second phase independent of the protein concentration is observed. Kinetic partitioning between aggregation and first-order irreversible changes of the unfolded conformation can occur during enzyme deactivation when the equilibrium between the native and the unfolded conformation is shifted and kept toward the unfolded conformation. (C) 2003 Wiley Periodicals, Inc.