The experimental survey of protein-folding energy landscapes

1. Introduction 22. The macroscopic and microscopic views of protein folding 22.1 The macroscopic view: the experimental folding free-energy profile 22.2 The microscopic view: an underlying energy landscape 33. The micro to macro projection: from an energy landscape to a free-energy profile 64. Global features of the protein folding transition-state ensemble 124.1 Overall transition state location β[Dagger]: a measure of compactness 124.2 What makes folding so robust ? 135. Structural characterization of the transition-state ensemble 165.1 Insights from ϕ-value analysis 166. Deviations from ideality 206.1 β[Dagger] shifts along seemingly robust trajectories 216.2 Anomalous ϕ values, frustration and inhomogeneities 257. Intermediates 288. Detours, traps and frustration 298.1 Premature collapse and non-native trapping 299. Diffusion on the energy landscape and the elementary events of protein folding 3010. Malleability of folding routes: changes of the dominant collective coordinates for folding 3311. The evolution of the shape of the energy landscape 3511.1 Negative design: the hidden dimension of the folding code 3512. Mechanistic multiplicity and evolutionary choice 3613. Acknowledgements 3714. References 38We review what has been learned about the protein-folding problem from experimental kinetic studies. These studies reveal patterns of both great richness and surprising simplicity. The patterns can be interpreted in terms of proteins possessing an energy landscape which is largely, but not completely, funnel-like. Issues such as speed limitations of folding, the robustness of folding, the origin of barriers and cooperativity and the ensemble nature of transition states, intermediate and traps are assessed using the results from several experimental groups highlighting energy-landscape ideas as an interpretive framework.