A Model for the Unusual Kinetics of Thermal Denaturation of Rubredoxin

Abstract

The thermal denaturation of the simple, redox-active iron protein rubredoxin is characterized by a slow, irreversible decay of the characteristic red color of the iron center at elevated temperatures in the presence of oxygen at pH 7.8. The denaturation rate is essentially constant and the time period for complete bleaching is nearly independent of protein concentration. These two characteristics of the kinetics can be fit by a simple self-catalyzed kinetics model consisting of the combination of a first-order decay and catalysis by some product of that decay, i.e., dP/dt=k ₁[A]+(k ₂[P][A])/(K _m+[A]), where A is native rubredoxin, P, is unspecified product, k ₁ is a first-order rate constant, and k ₂ and K _m are the catalytic constants. In order for the second term to be of this simple form over the full course of a decay, the model must include the condition that the reaction is effectively irreversible. This model has properties which suggest other biological roles in regulation (changes in k ₁ or k ₂ can dramatically modulate the kinetics), in timing (titer-independent fixed reaction time), and in self-activation reactions. At one extreme (k₁ ≫ k₂) the kinetics becomes exponential, but at the other extreme (k₂ ≫ k₁) they show a dramatic and rapid terminal increase after a lag period. Some obvious possible roles in the kinetics of programmed cell death, prion disease, and protease autoactivation are discussed.