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The kinetics of thermal unfolding of apo- and holo-Chromobacterium violaceum phenylalanine hydroxylase (cPAH) was investigated using circular dichroism (CD) over the temperature range 44-76 degrees C. In addition to the native cofactor (FeII), the unfolding kinetics of holo-cPAH was characterized using ZnII and CoII as cofactors. Kinetic profiles for apo- and holo-cPAH showed a single-phase exponential rise in the CD signal at lambda=222 nm and a first-order dependence on protein concentration. The extrapolated unfolding rate constants (ku) at ambient temperature followed the order apo>Fe>Zn>>Co. Transition-state analysis of the activation parameters revealed an isokinetic correlation, which suggests a common mechanism for the enzyme variants. The values of the entropy of activation (DeltaS++) for apo- and Fe-cPAH were negative but small: -34+/-24 and -32+/-18 J mol(-1) K(-1), respectively. On the other hand, DeltaS++ values for Zn- and Co-cPAH were large and positive: 54+/-9 and 175+/-27 J mol(-1) K(-1), respectively. Therefore, at higher temperatures the unfolding rates of Zn- and Co-cPAH are affected significantly by entropy, while the unfolding rates of apo- and Fe-cPAH are dominated by enthalpy even at higher temperatures. The rate of unfolding of holo-cPAH did not depend on excess metal concentrations and maintained single-phase kinetic profiles, refuting the occurrence of adventitious metal binding and the notion that unfolding occurs via apo-cPAH exclusively. Isothermal titration calorimetry (ITC) was employed to measure cPAH binding affinities for Fe, Zn, and Co as well as the enthalpy of metal coordination. Dissociation constants (Kd) decreased in the order Fe>Zn>Co. The non-native metals, Zn and Co, were bound more tightly than Fe. The activation enthalpy for unfolding (DeltaH++) displayed a linear correlation with the enthalpy of metal binding obtained from ITC measurements (DeltaHITC). On this basis, a common mechanism (transition state) is suggested for this family of metal cofactors, and the varying enthalpy of activation arises from the differing stabilities of enzyme variants having different metal cofactors.

Phenylalanine hydroxylase (PAH), a non-heme iron enzyme, is responsible for the phenylalanine conversion to tyrosine. Its malfunction causes phenylketonuria (PKU). To better understand how protein structure and folding profiles are affected by the metal cofactor, we investigated the chemical (un)folding of apo- and holo-PAH from Chromobacterium violaceum (cPAH) using circular dichroism (CD) and analytical ultracentrifugation (AUC). Holo-cPAH shows a two-state unfolding transition. In contrast, the unfolding profile for apo-cPAH reveals a three-state (un)folding pathway and accumulation of an intermediate (apo-cPAH(I)). This intermediate is also observed in refolding experiments. Fluorescence studies are consistent with the CD findings. The intermediate apo-cPAH(I) and unfolded state(s) of apo- and holo-cPAH(U) have been characterized by analytical ultracentrifugation (AUC). At 2.4 and 2.8 M GuHCl, 90% of the signal for apo-cPAH has a weight average sedimentation coefficient in water at 20°C (s20,w) of about 48 S, representing multiple aggregate species made of multiple monomers of cPAH. Aggregate formation for apo-cPAH is also confirmed by dynamic light scattering and electron microscopy giving a hydrodynamic radius (R(H)) of 41 nm for apo-cPAH(I) versus 3.5 nm for the native protein.