NMR paves the way for atomic level descriptions of sparsely populated, transiently formed biomolecular conformers

Abstract

The importance of dynamics to biomolecular function is becoming increasingly clear. A description of the structure-function relationship must, therefore, include the role of motion, requiring a shift in paradigm from focus on a single static 3D picture to one where a given biomolecule is considered in terms of an ensemble of interconverting conformers, each with potentially diverse activities. In this Perspective, we describe how recent developments in solution NMR spectroscopy facilitate atomic resolution studies of sparsely populated, transiently formed biomolecular conformations that exchange with the native state. Examples of how this methodology is applied to protein folding and misfolding, ligand binding, and molecular recognition are provided as a means of illustrating both the power of the new techniques and the significant roles that conformationally excited protein states play in biology.energy landscape | transiently and sparsely populated biomolecular states | invisible states | structure-function paradigmThe field of structural biology emerged from seminal X-ray diffraction studies of biomolecules such as DNA (1), myoglobin (2), hemoglobin (3), and lysozyme (4) that were carried out over 50 years ago. Since these landmark investigations, our knowledge of the relation between structure and function has greatly expanded, driven by significant improvements in both experimental and computational approaches and, of course, by the exponential increase in the number of structures that are now available. Despite tremendous advances, structural biology remains largely focused on studies of the lowest-energy conformational states of biomolecules, and although there are notable exceptions (5), the end game often remains the determination of a single static 3D structure that is then used as a starting point for understanding molecular function. It is becoming increasingly clear, however, that this narrow approach is not sufficient and that a description of molecular structure must take into account conformational fluctuations that can occur over a broad range of both time and length scales.The conformational space that can be explored by a given biomolecule is usually explained by invoking the concept of an energy landscape (6), a multidimensional hypersurface that governs the thermodynamics and kinetics of conformational transitions (7,8). Because biomolecular stability is dictated by the sum of a large number of attractive and repulsive interactions of similar strength, the resultant free-energy landscape is rugged (9), with the global minimum in the surface thought to correspond to the native state of the biomolecule. In addition, there are often local minima that are separated from the global minimum by free-energy barriers that can be overcome by thermal fluctuations. These low-lying conformationally excited states have remained elusive to quantitative investigation because their sparse population and transient nature complicates their study by conventional structural biology techniques that are so powerfu...

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