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Determination of three-dimensional structures of proteins in solution by nuclear magnetic resonance spectroscopy.

Introduction Ever since the early days of biological nuclear magnetic resonance (n.m.r.) spectroscopy, it was appreciated that n.m.r. could in principle be used to determine the three-dimensional structures of proteins in solution at a resolution comparable to that afforded in the crystal state by X-ray diffraction methods. The structural information that can be extracted by n.m.r. arises from two main sources. The first and most important is the interproton nuclear Overhauser effect (NOE) which can be used to demonstrate the proximity of protons in space and to determine their separation (Noggle and Schirmer, 1971). The second is the measurement of three bond coupling constants which can be used to derive approximate torsion angle restraints from Karplus (1963) type relationships. Before such information can be extracted, however, it is essential to assign the 'H-n.m.r. spectrum of the protein. This presents a formidable task and it is only in the past few years with the advent of very high field (500 MHz) n.m.r. spectrometers and new experimental techniques, in particular the whole array of two-dimensional n.m.r. experiments (Ernst et al., 1986), that this has become a feasible proposition. Once a large set of interproton distance and torsion angle restraints has been obtained, it is necessary to use computational techniques to convert this information into cartesian coordinates. This too presents a difficult problem as the n.m.r. data are limited in their number, range {<, 5 A) and accuracy. At the present time, the structures of only a few proteins have been determined by n.m.r. and a summary of these is provided in Table I. In this article, we will present an overview of the various steps involved in solving the three-dimensional structure of a protein in solution by n.m.r. as summarized by the flow chart in Table II.