Structural Superimposition

Abstact

Many bacterial chlorogenic acid esterases (ChlEs) exhibit atypical temperature behavior, featuring activities that barely change with temperature and activity maxima that fall below the thermal denaturation point. This work focuses on a ChlE fromLactobacillus helveticus (Lh-ChlE), which has a flat temperature dependence. First, it was determined that conformational changes during Lh-ChlE turnover are not rate-limiting and that the overall rate depends on the chemical step at all temperatures. Next, Lh-ChlE's temperature dependence was investigated using a conformational equilibrium model that assumes the existence of a temperature-dependent equilibrium between an active and an inactive conformation and an activation heat capacity model that postulates a difference in heat capacity between the ground and transition states. Although the equilibrium model recapitulates the data well, it yields an unrealistically low inactivation temperature around 280 K. Circular dichroism spectroscopy suggests that Lh-ChlE does not undergo structural changes at that temperature but may undergo small structural transitions at moderately elevated temperature. The activation heat capacity model describes Lh-ChlE behavior well, yielding an activation heat capacity (DeltaCpdouble dagger) of approximately -1 kJ mol-1 K-1. Overall, the results suggest that the atypical temperature behavior of Lh-ChlE likely arises from a negative activation heat capacity. This work illustrates that contrasting thermodynamic models for atypical temperature dependence in enzymes can give rise to similar looking fits, even though they have different underlying physical meaning. Our results furthermore encourage additional analysis of the Lh-ChlE transition state structure to better understand the structural features that cause the enzyme's nonzero activation heat capacity.