Temperature-induced denaturation and renaturation of triosephosphate isomerase from Saccharomyces cerevisiae: evidence of dimerization coupled to refolding of the thermally unfolded protein.

The thermal denaturation of the dimeric enzyme triosephosphate isomerase (TIM) from Saccharomyces cerevisiae was studied by spectroscopic and calorimetric methods. At low protein concentration the structural transition proved to be reversible in thermal scannings conducted at a rate greater than 1.0 degrees C min(-1). Under these conditions, however, the denaturation-renaturation cycle exhibited marked hysteresis. The use of lower scanning rates lead to pronounced irreversibility. Kinetic studies indicated that denaturation of the enzyme likely consists of an initial first-order reaction that forms thermally unfolded (U) TIM, followed by irreversibility-inducing reactions which are probably linked to aggregation of the unfolded protein. As judged from CD measurements, U possesses residual secondary structure but lacks most of the tertiary interactions present in native TIM. Furthermore, the large increment in heat capacity upon denaturation suggests that extensive exposure of surface area occurs when U is formed. Above 63 degrees C, reactions leading to irreversibility were much slower than the unfolding process; as a result, U was sufficiently long-lived as to allow an investigation of its refolding kinetics. We found that U transforms into nativelike TIM through a second-order reaction in which association is coupled to the regain of secondary structure. The rate constants for unfolding and refolding of TIM displayed temperature dependences resembling those reported for monomeric proteins but with considerably larger activation enthalpies. Such large temperature dependences seem to be determinant for the occurrence of kinetically controlled transitions and thus constitute a simple explanation for the hysteresis observed in thermal scannings.