The ability to use conformational flexibility is a hallmark of enzyme function. Here we show that protein motions and catalytic activity in a RNase are coupled and display identical solvent isotope effects. Solution NMR relaxation experiments identify a cluster of residues, some distant from the active site, that are integral to this motion. These studies implicate a single residue, histidine-48, as the key modulator in coupling protein motion with enzyme function. Mutation of H48 to alanine results in loss of protein motion in the isotope-sensitive region of the enzyme. In addition, kcat decreases for this mutant and the kinetic solvent isotope effect on kcat, which was 2.0 in WT, is near unity in H48A. Despite being located 18 Å from the enzyme active site, H48 is essential in coordinating the motions involved in the rate-limiting enzymatic step. These studies have identified, of Ϸ160 potential exchangeable protons, a single site that is integral in the rate-limiting step in RNase A enzyme function.Carr-Purcell-Meiboom-Gill dispersion ͉ enzyme dynamics ͉ NMR ͉ protein motions ͉ RNase A C onformational motions in enzymes play an essential role in their function and are often the rate-limiting step to overall catalytic throughput (1-5). Many enzymes have sufficiently evolved such that the bond-making and -breaking steps are fast relative to the ability of the enzyme to undergo a conformational change, and, thus, steps other than the chemical transformation of substrate are rate-limiting (ref. 6 and references therein). In systems such as these, understanding enzyme function requires characterization of the relevant time-dependent protein fluctuations from the timeaveraged three-dimensional structure. The ability of solution NMR spectroscopy to detect, with atomic resolution, motions over a wide timescale (picoseconds to seconds) makes it an ideal experimental technique to characterize conformational motions in proteins that can ultimately impact drug design, de novo enzyme construction, and enzyme engineering. In particular, relaxation-compensated Carr-Purcell-Meiboom-Gill (rcCPMG) dispersion experiments (7) are capable of informing on the kinetics, thermodynamics, and structural changes of protein motions in the microsecond-tomillisecond timescale.RNase A is an enzyme example in which a conformational change is the bottleneck to overall conversion of substrate to product (see ref. 3 for a review). RNase A catalyzes the cleavage of single-stranded RNA and does not require metal ions or cofactors. This enzyme has been studied in great detail as a model for protein folding, structure, and stability (8, 9). In addition, homologs of RNase A have important cytotoxic and antitumor properties (10). The rate-limiting step for the RNase A reaction is a protein conformational change that is coupled to the product release step (1, 11). This conformational change involves multiple amino acid residues throughout the protein, including those distant from the active site (12-14). These mobile regions include two loops: loop 1,...

We have recently discovered an allosteric switch in Ras, bringing an additional level of complexity to this GTPase whose mutants are involved in nearly 30% of cancers. Upon activation of the allosteric switch, there is a shift in helix 3/loop 7 associated with a disorder to order transition in the active site. Here, we use a combination of multiple solvent crystal structures and computational solvent mapping (FTMap) to determine binding site hot spots in the “off” and “on” allosteric states of the GTP-bound form of H-Ras. Thirteen sites are revealed, expanding possible target sites for ligand binding well beyond the active site. Comparison of FTMaps for the H and K isoforms reveals essentially identical hot spots. Furthermore, using NMR measurements of spin relaxation, we determined that K-Ras exhibits global conformational dynamics very similar to those we previously reported for H-Ras. We thus hypothesize that the global conformational rearrangement serves as a mechanism for allosteric coupling between the effector interface and remote hot spots in all Ras isoforms. At least with respect to the binding sites involving the G domain, H-Ras is an excellent model for K-Ras and probably N-Ras as well. Ras has so far been elusive as a target for drug design. The present work identifies various unexplored hot spots throughout the entire surface of Ras, extending the focus from the disordered active site to well-ordered locations that should be easier to target.

Conformational flexibility of the enzyme architecture is essential for biological function. These structural transitions often encompass significant portions of the enzyme molecule. Here, we present a detailed study of functionally relevant RNase A dynamics in the wild type and a D121A mutant form by NMR spin-relaxation techniques. In the wild-type enzyme, the dynamic properties are largely conserved in the apo, enzyme-substrate, and enzyme-product complexes. In comparison, mutation of aspartic acid 121 to alanine disrupts the timing of active-site dynamics, the product-release step, and global conformational changes, indicating that D121 plays a significant role in coordinating the dynamic events in RNase A. In addition, this mutation results in 90% loss of catalytic activity despite the absence of direct participation of D121 in the chemical reaction or in interactions with the substrate. These data suggest that one role of this conserved residue is to facilitate important millisecond protein dynamics.

Chaperones TAPBPR and tapasin associate with class I major histocompatibility complexes (MHC-I) to promote optimization (editing) of peptide cargo. Here, we use solution NMR to investigate the mechanism of peptide exchange. We identify TAPBPR-induced conformational changes on conserved MHC-I molecular surfaces, consistent with our independently determined X-ray structure of the complex. Dynamics present in the empty MHC-I are stabilized by TAPBPR and become progressively dampened with increasing peptide occupancy. Incoming peptides are recognized according to the global stability of the final pMHC-I product and anneal in a native-like conformation to be edited by TAPBPR. Our results demonstrate an inverse relationship between MHC-I peptide occupancy and TAPBPR binding affinity, wherein the lifetime and structural features of transiently bound peptides control the regulation of a conformational switch located near the TAPBPR binding site, which triggers TAPBPR release. These results suggest a similar mechanism for the function of tapasin in the peptide-loading complex.

Biological function of proteins relies on conformational transitions and binding of specific ligands. Protein-ligand interactions are thermodynamically and kinetically coupled to conformational changes in protein structures as conceptualized by the models of pre-existing equilibria and induced fit. NMR spectroscopy is particularly sensitive to complex ligandbinding modes-NMR line-shape analysis can provide for thermodynamic and kinetic constants of ligand-binding equilibria with the site-specific resolution. However, broad use of line shape analysis is hampered by complexity of NMR line shapes in multi-state systems. To facilitate interpretation of such spectral patterns, I computationally explored systems where isomerization or dimerization of a protein (receptor) molecule is coupled to binding of a ligand. Through an extensive analysis of multiple exchange regimes for a family of three-state models, I identified signature features to guide an NMR experimentalist in recognizing specific interaction mechanisms. Results also show that distinct multistate models may produce very similar spectral patterns. I also discussed aggregation of a receptor as a possible source of spurious three-state line shapes and provided specific suggestions for complementary experiments that can ensure reliable mechanistic insight.

The phosphorylation-specific peptidyl-prolyl isomerase Pin1 catalyzes the isomerization of the peptide bond preceding a proline residue between cis and trans isomers. To best understand the mechanisms of Pin1 regulation, rigorous enzymatic assays of isomerization are required. However, most measures of isomerase activity require significant constraints on substrate sequence and only yield rate constants for the cis isomer, kcatcis and apparent Michaelis constants, KMApp. By contrast, NMR lineshape analysis is a powerful tool for determining microscopic rates and populations of each state in a complex binding scheme. The isolated catalytic domain of Pin1 was employed as a first step towards elucidating the reaction scheme of the full-length enzyme. A 24-residue phosphopeptide derived from the amyloid precurser protein intracellular domain (AICD) phosphorylated at Thr668 served as a biologically-relevant Pin1 substrate. Specific 13C labeling at the Pin1-targeted proline residue provided multiple reporters sensitive to individual isomer binding and on-enzyme catalysis. We have performed titration experiments and employed lineshape analysis of phosphopeptide 13C-1H constant time HSQC spectra to determine Kcatcis, Kcattrans, KDcis, and KDtrans for the catalytic domain of Pin1 acting on this AICD substrate. The on-enzyme equilibrium value of [E·trans]/[E·cis] = 3.9 suggests that the catalytic domain of Pin1 is optimized to operate on this substrate near equilibrium in the cellular context. This highlights the power of lineshape analysis for determining the microscopic parameters of enzyme catalysis, and demonstrates the feasibility of future studies of Pin1-PPIase mutants to gain insights on the catalytic mechanism of this important enzyme.

Ras and its homologues are central to regulation of a multitude of cellular processes. Ras in complex with GTP binds and activates its downstream signaling partners. (31)P NMR studies indicated that the Ras-GTP conformation is heterogeneous on a millisecond time scale, but details of its conformational dynamics remain unknown. Here we present evidence that the conformational exchange process in human H-Ras complexed with GTP mimic GppNHp is global, encompassing most of the GTPase catalytic domain. The correlated character of conformational dynamics in Ras opens opportunities for understanding allosteric effects in Ras function.

This work examines the robustness of fitting of parameters describing conformational exchange (k(ex), p(a/b), and Deltaomega) processes from CPMG relaxation dispersion data. We have analyzed the equations describing conformational exchange processes for the intrinsic inter-dependence of their parameters that leads to the existence of multiple equivalent solutions, which equally satisfy the experimental data. We have used Monte-Carlo simulations and fitting to the synthetic data sets as well as the direct 3-D mapping of the parameter space of k(ex), p(a/b), and Deltaomega to quantitatively assess the degree of the parameter inter-dependence. The demonstrated high correlation between parameters can preclude accurate dynamics parameter estimation from NMR spin-relaxation data obtained at a single static magnetic field. The strong parameter inter-dependence can readily be overcome through acquisition of spin-relaxation data at more than one static magnetic field thereby allowing accurate assessment of conformational exchange properties.