UNUSUAL THERMAL-STABILITY OF SOYBEAN PEROXIDASE

Soybean peroxidase (SBP) has an extremely high melting temperature of 90.5 degrees C at pH 8.0 in the presence of 1 mM CaCl2. The enzyme is substantially more thermostable than the peroxidases from horseradish (HRP) and Coprinus cinereus (CIP). SBP denaturation does not precisely fit the two-state denaturation model due to the formation of the apoe nzyme upon initial melting. A pseudo-two-state denaturation can be ass umed, however, and this gives rise to apparent kinetics for irreversib le inactivation. The apparent kinetics indicate that irreversible deac tivation is comprised primarily of enthalpic contributions, with Delta H-deact(double dagger) = 22.4 kcal/mol and T Delta S-deact(double dag ger) = 0.2 kcal/mol at 95 degrees C. Heme transfer studies from the pe roxidases to apomyoglobin indicate that SBP holds onto its heme much m ore tightly than does HRP, and this is consistent with a thermodynamic ally more stable enzyme.