A Specific Set of Heterogeneous Native Interactions Yields Efficient Knotting in Protein Folding

Discrepant Effects of Hydrated and Neat Reline on the Conformational Stability of a Knotted Protein

Although knotted proteins are rare in number, their peculiar topology has long intrigued the scientific community. In this study, we have explored the conformational stability of a trefoil-knotted protein, YbeA, in reline (choline chloride:urea in a 1:2 ratio), a well-characterized deep eutectic solvent, using classical molecular dynamics simulation. Deep eutectic solvents (DESs) are explored as a reliable alternative to conventional solvents, effectively altering a protein's structural stability and activity, either stabilizing its native state or disordering its conformation depending on the relevant interactions involved. Here, using pure and hydrated concentrations of reline, we observe the conflicting effect of the DES on the knotted protein's stability. Our studies at room temperature and elevated temperatures show that in pure reline, the protein is conformationally stable and rigid. In contrast, the protein tends to lose its structural integrity in hydrated reline. The stable knotted topology also gets untied as the protein, solvated in hydrated reline, is exposed to an elevated temperature. Using Minimum Distance Distribution Functions and Kirkwood-Buff Integrals, we analyzed the solvation pattern of the DES constituents around the protein. We expect that this study will lead to more effective strategies in developing tailored solvent systems for comprehending the conformational behavior of knotted proteins.

Effect of a Knot on the Thermal Stability of Protein MJ0366: Insights into Molecular Dynamics and Monte Carlo Simulations

Protein MJ0366 is a hypothetical protein from Methanocaldococcus jannaschii that has a rare and complex knot in its structure. The knot is a right-handed trefoil knot that involves about half of the protein's residues. In this work, we investigate the thermal stability of protein MJ0366 using numerical simulations based on molecular dynamics and Monte Carlo methods. We compare the results with those of a similar unknotted protein and analyze the effects of the knot on the folding and unfolding processes. We show that the knot in protein MJ0366 increases its thermal stability by creating a topological barrier that prevents the protein from unfolding at high temperatures.

Effects of sequence-dependent non-native interactions in equilibrium and kinetic folding properties of knotted proteins

Determining the role of non-native interactions in folding dynamics, kinetics, and mechanisms is a classic problem in protein folding. More recently, this question has witnessed a renewed interest in light of the hypothesis that knotted proteins require the assistance of non-native interactions to fold efficiently. Here, we conduct extensive equilibrium and kinetic Monte Carlo simulations of a simple off-lattice C-alpha model to explore the role of non-native interactions in the thermodynamics and kinetics of three proteins embedding a trefoil knot in their native structure. We find that equilibrium knotted conformations are stabilized by non-native interactions that are non-local, and proximal to native ones, thus enhancing them. Additionally, non-native interactions increase the knotting frequency at high temperatures, and in partially folded conformations below the transition temperatures. Although non-native interactions clearly enhance the efficiency of transition from an unfolded conformation to a partially folded knotted one, they are not required to efficiently fold a knotted protein. Indeed, a native-centric interaction potential drives the most efficient folding transition, provided that the simulation temperature is well below the transition temperature of the considered model system.

Knotting Optimization and Folding Pathways of a Go-Model with a Deep Knot

Formation of protein knots is an intriguing offshoot of the protein folding problem. Since experimental resolution on knot formation is limited, theoretical methods currently provide the most detailed insights into the knotting process. While suitable for shallow knots, molecular dynamics simulations have faced challenges capturing the formation of deep knots in proteins such as the minimally tied trefoil α/β methyltransferase from Thermotoga maritima (MTTTM). To improve the efficiency of MTTTM knotting in Cα Go-model simulations, mutant variants of the MTTTM Go-model were investigated. Through a structure-based analysis of knotted and unknotted states, four residues (K71, R72, E75, V76) were identified to increase the knotting efficiency from 2% to 83% when their contact energies were doubled and dihedral strength around the knot loop increased. The key features of this model are (i) a C-terminal slipknot intermediate that threads the knot in a highly unstructured intermediate, (ii) the inability to knot in native-like intermediate states, and (iii) a minor population in a long-lived trap that cannot knot. Examination of residue 71-76 contacts provides a small set of potential mutants that can directly test the model's validity. In addition, the knotting optimization process developed here has broad applicability in generating knotting-efficient models of other knotted proteins.

A Note on the Effects of Linear Topology Preservation in Monte Carlo Simulations of Knotted Proteins

Monte Carlo simulations are a powerful technique and are widely used in different fields. When applied to complex molecular systems with long chains, such as those in synthetic polymers and proteins, they have the advantage of providing a fast and computationally efficient way to sample equilibrium ensembles and calculate thermodynamic and structural properties under desired conditions. Conformational Monte Carlo techniques employ a move set to perform the transitions in the simulation Markov chain. While accepted conformations must preserve the sequential bonding of the protein chain model and excluded volume among its units, the moves themselves may take the chain across itself. We call this a break in linear topology preservation. In this manuscript, we show, using simple protein models, that there is no difference in equilibrium properties calculated with a move set that preserves linear topology and one that does not. However, for complex structures, such as those of deeply knotted proteins, the preservation of linear topology provides correct equilibrium results but only after long relaxation. In any case, to analyze folding pathways, knotting mechanisms and folding kinetics, the preservation of linear topology may be an unavoidable requirement.

In silico method for identifying the key residues in a knotted protein: with MJ0366 as an example

A simple in silico method for predicting the key residues for knotting and unknotting a knotted protein is put forward, with the residues ranked by the relevance to knotting and unknotting in the annealing molecular dynamics simulations.