Ensemble-based characterization of unbound and bound states on protein energy landscape

TL;DR

This study uses ensemble-based analysis to compare bound and unbound protein states, revealing insights into their energy landscapes, conformational selection mechanisms, and the role of entropy and vibrations in protein binding.

Contribution

It provides a detailed characterization of protein energy landscapes and conformational ensembles, highlighting the overlap between bound and unbound states and supporting conformational selection as a binding mechanism.

Findings

Bound and unbound spectra often significantly overlap.

Conformational selection is the likely binding mechanism for most proteins studied.

Unbound states generally have higher energy and entropy than bound states.

Abstract

Characterization of protein energy landscape and conformational ensembles is important for understanding mechanisms of protein folding and function. We studied ensembles of bound and unbound conformations of six proteins to explore their binding mechanisms and characterize the energy landscapes in implicit solvent. First, results show that bound and unbound spectra often significantly overlap. Moreover, the larger the overlap the smaller the RMSD between bound and unbound conformational ensembles. Second, the analysis of the unbound-to-bound changes points to conformational selection as the binding mechanism for four of the proteins. Third, the center of the unbound spectrum has a higher energy than the center of the corresponding bound spectrum of the dimeric and multimeric states for most of the proteins. This suggests that the unbound states often have larger entropy than the bound states considered outside of the complex. Fourth, the exhaustively long minimization, making small intra-rotamer adjustments, dramatically reduces the distance between the centers of the bound and unbound spectra as well as the spectra extent. It condenses unbound and bound energy levels into a thin layer at the bottom of the energy landscape with the energy spacing that varies between 0.8-4.6 and 3.5-10.5 kcal/mol for the unbound and bound states correspondingly. At the same time, the energy gap between the two lowest states in the full ensemble varies between 0.9 and 12.1 kcal/mol. Finally, the analysis of protein energy fluctuations showed that protein vibrations itself can excite the inter-state transitions, thus supporting the conformational selection theory.