Incidental conformational switching in an allosteric enzyme

This study challenges the classical MWC/KNF two-state paradigm of allostery, which links enzyme structure and function through transitions between low-affinity (T) and high-affinity (R) states. Using yeast chorismate mutase (CM) as a model, the authors demonstrate that this enzyme exhibits dynamic sampling of the R-state in the presence of the activator tryptophan, consistent with a conformational selection (CS) mechanism. However, they identify multiple exceptions where conformational status and catalytic activity are decoupled: some CM variants that remain nearly exclusively in the T conformation show maximal activity, while others predominantly in the R conformation are weakly active.

Quantitative comparisons between experimental data and a parameterized CS model reveal deviations of up to two orders of magnitude, ruling out the simplest two-state model for substrate affinity modulation. The authors propose that T-to-R switching in CM is 'incidental'—a byproduct of an evolved energy landscape that permits access to the substrate-bound pose but does not mechanistically determine affinity. Instead, allosteric regulation in CM may be driven by local features of the ground-state ensemble that operate independently of global T/R status.

This work adds to a growing list of exceptions to the traditional allosteric paradigm and emphasizes that observing a pre-sampled active conformation does not prove a two-state mechanism. The findings underscore the need for deeper ensemble-based perspectives in protein engineering and allostery research.