High resolution NMR study of the charge relay system in chymotrypsin

Biomolecular NMR: Past and future

The editors of this special volume suggested this topic, presumably because of the perspective lent by our combined >90-year association with biomolecular NMR. What follows is our personal experience with the evolution of the field, which we hope will illustrate the trajectory of change over the years. As for the future, one can confidently predict that it will involve unexpected advances. Our narrative is colored by our experience in using the NMR Facility for Biomedical Studies at Carnegie-Mellon University (Pittsburgh) and in developing similar facilities at Purdue (1977-1984) and the University of Wisconsin-Madison (1984-). We have enjoyed developing NMR technology and making it available to collaborators and users of these facilities. Our group's association with the Biological Magnetic Resonance data Bank (BMRB) and with the Worldwide Protein Data Bank (wwPDB) has also been rewarding. Of course, many groups contributed to the early growth and development of biomolecular NMR, and our brief personal account certainly omits many important milestones.

Quantum chemical calculations on structural models of the catalytic site of chymotrypsin: comparison of calculated results with experimental data from NMR spectroscopy

Hybrid density functional quantum mechanical calculations were used to study the strength of the hydrogen bond between His(57) N(delta)(1) and Asp(102) O(delta)(1) in chymotrypsin and how it changes along the reaction coordinate. Comparison of experimental shifts with the results of chemical shift calculations on a variety of small molecules, including species containing very strong hydrogen bonds, has validated the overall approach and provided the means for calibrating and correcting the calculated values. Models of the active site of chymotrypsin in its resting state and tetrahedral intermediate state were derived from high-resolution X-ray structures. The distance between His(57) N(delta)(1) and Asp(102) O(delta)(1) in each model was varied between 2.77 A (weak hydrogen bond) and 2.50 A (extremely strong hydrogen bond), and the one-dimensional potential energy surface of the hydrogen-bonded proton (or deuteron/triton) was determined. The zero-point energy, probability distribution, and chemical shift were determined for each distance. Calculated values for NMR chemical shifts, NMR chemical shift differences between (1)H and (3)H, and (2)H/(1)H fractionation factors were compared with published experimental values. Energies provided by the calculations indicated that the hydrogen bond between His(57) N(delta)(1) and Asp(102) O(delta)(1) in the chymotrypsin active site increases in strength by 11 kcal mol(-)(1) in going from the resting state of the enzyme to the tetrahedral intermediate state. This result confirms the hypothesis that the strengthened hydrogen bond plays an important role in lowering the energy of the transition state and, hence, in the catalytic efficiency of the enzyme. Models of the transition state that best fit the experimental data are consistent with a "strong" hydrogen bond between His(57) N(delta)(1) and Asp(102) O(delta)(1) but apparently not a "low-barrier" or "very strong" hydrogen bond.

Evidence for a strong hydrogen bond in the catalytic dyad of transition-state analogue inhibitor complexes of chymotrypsin from proton-triton NMR isotope shifts

We present here the first accurate measurements of 1H (H) versus 3H (T) isotope shift (DeltadeltaT-H = deltaT - deltaH) in a protein. This approach was used to investigate the strength of the hydrogen bond between His57 and Asp102 in the catalytic dyad of chymotrypsin in three transition-state analogue inhibited complexes: N-acetyl-l-phenylalanyl trifluoromethyl ketone (N-AcF-CF3), N-acetyl-l-valyl-l-phenylalanyl trifluoromethyl ketone (N-AcVF-CF3), and N-acetyl-l-leucyl-l--phenylalanyl trifluoromethyl ketone (N-AcLF-CF3). The measured DeltadeltaT-H values for His57 Hdelta1 in these complexes were between -0.63 and -0.68 ppm. These values are consistent with a strong hydrogen bond in each of these complexes, but not with a very strong hydrogen bond, which would be expected to have a DeltadeltaT-H. value near or greater than zero.