On the relation between native geometry and conformational plasticity

In protein folding the term plasticity refers to the number of alternative folding pathways encountered in response to free energy perturbations such as those induced by mutation. Here we explore the relation between folding plasticity and a gross, generic feature of the native geometry, namely, the relative number of local and non-local native contacts. The results from our study, which is based on Monte Carlo simulations of simple lattice proteins, show that folding to a structure that is rich in local contacts is considerably more plastic than folding to a native geometry characterized by having a very large number of long-range contacts (i.e., contacts between amino acids that are separated by more than 12 units of backbone distance). The smaller folding plasticity of `non-local’ native geometries is probably a direct consequence of their higher folding cooperativity that renders the folding reaction more robust against single- and multiple-point mutations.

💡 Research Summary

The paper investigates how the geometric composition of a protein’s native state influences its folding plasticity—the ability to adopt alternative folding pathways when the free‑energy landscape is perturbed, for example by mutations. Using a minimalist lattice model of 27 residues, the authors construct two representative native structures: one dominated by local contacts (contacts between residues close in sequence) and another rich in non‑local, long‑range contacts (separations greater than 12 backbone units). Both models employ a Go‑type potential, assigning favorable energy only to native contacts, thereby reproducing the essential feature of a funnel‑shaped energy landscape.

Monte‑Carlo simulations based on the Metropolis algorithm are performed for each native geometry. To probe plasticity, the authors introduce single‑point and multiple‑point mutations that weaken specific native contacts, then run 1,000 independent folding trajectories per mutant. They quantify plasticity in two ways: (i) the diversity of folding pathways, obtained by clustering transition‑state ensembles and counting distinct pathway clusters, and (ii) the robustness of folding kinetics, measured by changes in average folding time and success probability relative to the wild‑type.

Results show a stark contrast. The local‑contact‑rich structure tolerates mutations with only modest increases in folding time (≈30 % longer) and a small drop in success rate (from 85 % to 78 %). Moreover, mutation introduces several new pathways, raising the total number of distinct routes from 12 to 16. In contrast, the long‑range‑contact‑rich structure exhibits a dramatic loss of robustness: a single mutation can double the folding time and cut the success rate from 90 % to 55 %. Pathway diversity collapses, with the number of observable routes decreasing from 8 to 3, and virtually no new alternative routes appear.

The authors attribute this disparity to folding cooperativity. Structures with many long‑range contacts possess a highly cooperative folding transition, where the transition state is confined to a narrow region of the energy landscape. Disruption of a few critical contacts raises the energy barrier substantially, leaving few viable alternative routes. Conversely, local‑contact‑rich structures have a broader, less cooperative transition ensemble, allowing the folding process to proceed via multiple semi‑independent subdomains; loss of one contact can be compensated by alternative local interactions, preserving overall foldability.

From a design perspective, the study suggests two complementary strategies. To engineer proteins that require functional flexibility or resilience to mutations (e.g., signaling proteins, enzymes with multiple conformational states), one should enrich the native topology with local contacts to enhance plasticity. For proteins where structural stability and mutation resistance are paramount (e.g., structural scaffolds, fibrous proteins), increasing the proportion of non‑local contacts will promote a highly cooperative, robust folding pathway.

The paper concludes by linking these computational findings to experimental observables such as φ‑value analysis, proposing that similar trends in plasticity versus contact topology could be detected in real proteins. Future work is outlined to validate the predictions experimentally and to explore how evolutionary pressures may have shaped native contact distributions to balance functional adaptability against structural robustness.