Dissecting Ubiquitin Folding Using the Self-Organized Polymer Model

TL;DR

This study uses a coarse-grained simulation model to analyze ubiquitin folding mechanisms at different pH levels and temperatures, revealing multiple pathways, metastable states, and pH-dependent folding behaviors consistent with experimental data.

Contribution

It provides detailed insights into ubiquitin folding pathways, metastable states, and the influence of pH and temperature using the Self-Organized-Polymer model, advancing understanding beyond experimental limitations.

Findings

Ubiquitin folds via multiple pathways involving nucleation-collapse and diffusion-collision mechanisms.

Folding pathways and intermediates are pH-dependent and involve metastable states.

Transition state structures are homogeneous, globular, and native-like.

Abstract

Folding of Ubiquitin (Ub) is investigated at low and neutral pH at different temperatures using simulations of the coarse-grained Self-Organized-Polymer model with side chains. The calculated radius of gyration, showing dramatic variations with pH, is in excellent agreement with scattering experiments. At $T_m$ Ub folds in a two-state manner at low and neutral pH. Clustering analysis of the conformations sampled in equilibrium folding trajectories at $T_m$, with multiple transitions between the folded and unfolded states, show a network of metastable states connecting the native and unfolded states. At low and neutral pH, Ub folds with high probability through a preferred set of conformations resulting in a pH-dependent dominant folding pathway. Folding kinetics reveal that Ub assembly at low pH occurs by multiple pathways involving a combination of nucleation-collapse and diffusion collision mechanism. The mechanism by which Ub folds is dictated by the stability of the key secondary structural elements responsible for establishing long range contacts and collapse of Ub. Nucleation collapse mechanism holds if the stability of these elements are marginal, as would be the case at elevated temperatures. If the lifetimes associated with these structured microdomains are on the order of hundreds of $\mu sec$ then Ub folding follows the diffusion-collision mechanism with intermediates many of which coincide with those found in equilibrium. Folding at neutral pH is a sequential process with a populated intermediate resembling that sampled at equilibrium. The transition state structures, obtained using a $P_{fold}$ analysis, are homogeneous and globular with most of the secondary and tertiary structures being native-like. Many of our findings are not only in agreement with experiments but also provide missing details not resolvable in standard experiments.