Heat-inactivation studies were carried out on the two primary erythrocyte carbonic anhydrase isoenzymes, CA I and CA II, and the secondary isoenzyme of CA I, CA I (+1). In addition, two genetic variants of human isoenzyme CA I, CA Id Michigan (100 Thr→Lys) and CA If London (102 Glu→Lys), and one variant of isoenzyme CA II, CA IIh (251 Asn→Asp), were similarly analysed. The first-order rate constants and Arrhenius plots for these six enzyme forms showed that (1) isoenzyme CA II is more heat-stable than CA I, (2) isoenzyme CA I (+1) is less heat-stable than CA I, (3) the variants CA IIh and CA If London are less heat-stable than the normal enzymes, and (4) isoenzyme CA Id Michigan is more heat-stable than normal CA I. From the values of the slopes of the Arrhenius plots, the energy of activation (Ea) for each isoenzyme and isoenzyme variant was determined, and the following thermodynamic activation parameters were calculated at 55°C: the free energy of activation (ΔG‡), the activation enthalpy (ΔH‡) and the activation entropy (ΔS‡). The ΔG‡ for the enzymes shows a relative constancy with compensating variation in ΔH‡ and ΔS‡. When the values for ΔH‡ are plotted against ΔS‡, an increase in ΔH‡ involves a concomitant increase in ΔS‡.
