NEGATIVE ACTIVATION ENTHALPIES IN THE KINETICS OF PROTEIN-FOLDING

Although the rates of chemical reactions become faster with increasing temperature, the converse may be observed with protein-folding reacti ons, The rate constant for folding initially increases with temperatur e, goes through a maximum, and then decreases, The activation enthalpy is thus highly temperature dependent because of a large change in spe cific heat (Delta C-p). Such a Delta C-p term is usually presumed to b e a consequence of a large decrease in exposure of hydrophobic surface s to water as the reaction proceeds from the denatured state to the tr ansition state for folding: the hydrophobic side chains are surrounded by ''icebergs'' of water that melt with increasing temperature, thus making a large contribution to the C-p of the denatured state and a sm aller one to the more compact transition state. The rate could also be affected by temperature-induced changes in the conformational populat ion of the ground state: the heat required for the progressive melting of residual structure in the denatured state will contribute to Delta C-p. By examining two proteins with different refolding mechanisms, w e are able to find both of these two processes; barley chymotrypsin in hibitor 2, which refolds from a highly unfolded state, fits well to a hydrophobic interaction model with a constant Delta C-p of activation, whereas barnase, which refolds from a more structured denatured state , deviates from this ideal behavior.