Accessing Structure, Dynamics and Function of Biological Macromolecules by NMR Through Advances in Isotope Labeling

NMR spectroscopy has become an indispensable tool for high-resolution structure determination of biomolecules at physiological conditions both in solutions and solids. Currently, NMR is routinely used to study the structure and dynamics of high molecular weight biomolecules in sizes ranging up to ~ 50–100 kDa and to evaluate complexes as large as 500–1 MDa. The latest advances in spectrometer technology, methodologies and advents in newer and highly innovative NMR active isotope-labeling strategies now enable us to overcome an earlier speculated size barrier of ~ 20 kDa for de novo structure determination. Of these, developments in NMR active isotope-labeling strategies are of great significance as they allow reduction in spectral crowding and yield selective spin correlations. Moreover, NMR isotope enrichment schemes permit exploitation of heteronuclear magnetization transfer pathways for enhanced sensitivity and selectivity. Functionally relevant sites or domains in very large complexes can also be selectively evaluated by specific labeling strategies in which other regions are masked. Further, labeling schemes can be effectively used to favourably overcome deleterious relaxation effects. Recently evolved labeling strategies include uniform labeling, perdeuteration, specific labeling of an amino acid or a side chain, selective deuteration or protonation, segmental labeling and biosynthesis of biomolecules in various organisms, cell lines and cell-free systems. The present review is aimed at introducing various NMR isotope labeling strategies and discusses their impact in widening the scope of biomolecular NMR spectroscopy driven structural biology.