Hydroxyl-ion-induced subunit dissociation of east cytoplasmic pyruvate decarboxylase. A circular dichroism study.

Cytoplasmic pyruvate decarboxylase (EC 4.1.1.1, from Saccharomyces carlsbergensis) exhibits in its circular dichroic spectrum in the 250--320-nm range a multiple two-signal band. This couplet disappears on increasing the pH up to pH 8.5. Two classes of two protons each can be quantified by these spectral changes. The first class dissociates rapidly and the apparent pK is 7.84. The thermodynamic data are delta G = 87.7 kJ mol-1, delta H = + 56.0 kJ mol-1, delta S = - 108 J mol-1 K-1, very characteristic for the deprotonation of an amino-acid side chain. The second class of the protons has the following thermodynamic data: delta G = 88.3 kJ mol-1, delta H = - 64.3 kJ mol-1, delta S = - 520 J mol-1 K-1 which, in conjunction with kinetic reasoning and in view of enzyme stoichiometry and symmetry, suggests a conformational equilibrium exposing the second two protons. Th enzyme dissociates into two dimeric subunits. This dissociation step is considered to be rate-determining for the overall process. The data are kp = 1.4 . 10(-3), delta H not equal to = + 128.3 kJ mol-1, delta S not equal = + 136 J mol-1 K-1. If there is a conformational equilibrium, the rate constant of product formation kp will be modified by a factor beta = kc/(1 + Kc) (0 < beta less than or equal to 1) where Kc is the conformational equilibrium constant. The subunit dissociation appears to be controlled by the enthalpy of activation indicating that a number of interactions, i.e. ionic, hydrogen and hydrophobic bridges, are to be broken. Optimal conditions for the preparation of the apo-enzyme are derived from the data.