Soft Interactions, Rather Than Crowder Size, Determine Deviations From Arrhenius-Like Kinetics of Phosphofructokinase-1.

Enzymes need to be robust against a wide range of local perturbations associated with cellular processes such as cell division or external conditions such as temperature. In this study, we investigated human phosphofructokinase-1 platelet isoform (PFKP) in different crowding conditions. Temperature-dependent studies allowed us to i) measure the robustness of enzymatic activity in different environments, ii) decipher the thermodynamic contributions that govern the associated crowding effects. Folding stability and assembly of PFKP changed under crowding as expected by excluded-volume considerations; however, crowder-specific interactions with substrates and the active site region become important when considering catalytic activity. Enzyme kinetics showed Arrhenius-like behavior at low temperatures but strong deviations at elevated temperatures. We found that the temperature robustness in the non-Arrhenius regime is increased by glucose and dextran. Using Macromolecular Rate Theory, we could attribute this effect to the crowders restricting conformational fluctuations leading to the transition state. We found that a delicate balance of enthalpic and entropic crowding effects determines enzymatic activity and temperature robustness in different environments. Deciphering the respective contributions for different enzymes and cell-like conditions will be important to understand the interplay between the local cellular environment and enzyme activity.