Induced fit or conformational selection? The case of Na,K-ATPase Pi binding

SANTIAGO E. FARAJ; MÓNICA R. MONTES; ROLANDO C. ROSSI

Buenos Aires

Congreso; 2nd Protein Biophysics at the End of the World; 2017

Ligand binding by macromolecules is crucial for a multitude of physiological processes. In virtually all cases, binding of ligands and conformational changes go hand in hand. The two limiting cases are the induced fit mechanism, in which ligand binding occurs before conformational change, and conformational selection, where the conformational change occurs before ligand binding. It was pointed out that induced fit is the most commonly documented mechanism of recognition, while conformational selection is confined to a handful of cases. This conclusion is based on the fact that, under the rapid equilibrium approximation, the decrease of the rate of approach to equilibrium (kobs) seen at incremental ligand concentration is diagnostic of an induced fit mechanism. In this work we analyze the kinetics of binding of Pi and BeFx (as a Pi-like ligand) to Na,K‑ATPase, and evaluate the possible models to explain the results. We observed that, although kobs decreases with [Pi] and increases with [BeFx], both processes may be explained by a conformational selection mechanism if no rapid equilibrium assumptions are invoked. In this way, we show that the different behavior of kobs is not necessarily the result of a different binding-induced conformational change mechanism, and that it can be readily explained by the much higher affinity of Na,K‑ATPase for BeFx.