Temperature effects on the allosteric transition of ATP sulfurylase from Penicillium chrysogenum

The effects of temperature on the initial velocity kinetics of allosteric ATP sulfurylase from Penicillium chrysogenum were measured. The experiments were prompted by the structural similarity between the C-terminal regulatory domain of fungal ATP sulfurylase and fungal APS kinase, a homodimer that undergoes a temperature-dependent, reversible dissociation of subunits over a narrow temperature range. Wild-type ATP sulfurylase yielded hyperbolic velocity curves between 18 and 30°C. Increasing the assay temperature above 30°C at a constant pH of 8.0 increased the cooperativity of the velocity curves. Hill coefficients (n_H) up to 1.8 were observed at 42°C. The bireactant kinetics at 42°C were the same as those observed at 30°C in the presence of PAPS, the allosteric inhibitor. In contrast, yeast ATP sulfurylase yielded hyperbolic plots at 42°C. The P. chrysogenum mutant enzyme, C509S, which is intrinsically cooperative (n_H = 1.8) at 30°C, became more cooperative as the temperature was increased yielding n_H values up to 2.9 at 42°C. As the temperature was decreased, the cooperativity of C509S decreased; n_H was 1.0 at 18°C. The cumulative results indicate that increasing the temperature increases the allosteric constant, L, i.e., promotes a shift in the base-level distribution of enzyme molecules from the high MgATP affinity R state toward the low MgATP affinity T state. As a result, the enzyme displays a true "temperature optimum" at subsaturating MgATP. The reversible temperature-dependent transitions of fungal ATP sulfurylase and APS kinase may play a role in energy conservation at high temperatures where the organism can survive but not grow optimally.