Hydrogen-bonding in enzyme catalysis. Fourier-transform infrared detection of ground-state electronic strain in acyl-chymotrypsins and analysis of the kinetic consequences
- 15 September 1990
- journal article
- Published by Portland Press Ltd. in Biochemical Journal
- Vol. 270 (3) , 627-637
- https://doi.org/10.1042/bj2700627
Abstract
I.r. difference spectra are presented for 3-(indol-3-yl)acryloyl-, cinnamoyl-, 3-(5-methylthien-2-yl)acryloyl-, dehydrocinnamoyl- and dihydrocinnamoyl-chymotrypsins at low pH, where the acyl-enzymes are catalytically inactive. At least two absorption bands are seen in each case in the ester carbonyl stretching region of the spectrum. Cinnamoyl-chymotrypsin substituted at the carbonyl carbon atom with 13C was prepared. A difference spectrum in which 13C-substituted acyl-enzyme was subtracted from [12C]acyl-enzyme shows two bands in the ester carbonyl region and thus confirms the assignment of the features to the single ester carbonyl group. The frequencies of the ester carbonyl bands are interpreted in terms of differential hydrogen-bonding. In each case a lower-frequency relatively narrow band is assigned to a productive potentially reactive binding mode in which the carbonyl oxygen atom is inserted in the oxyanion hole of the enzyme active centre. The higher-frequency band, which is broader, is assigned to a non-productive binding mode in each case, where a water molecule bridges from the carbonyl oxygen atom to His-57; this mode is equivalent to the crystallographically determined structure of 3-(indol-3-yl)acryloyl-chymotrypsin, i.e. the Henderson structure. A difference spectrum of dihydrocinnamoyl-chymotrypsin taken at higher pH shows resolution of a feature centred upon 1731 cm-1, which is assigned to a non-bonded conformer in which the carbonyl oxygen atom is not hydrogen-bonded. Perturbation of the protein spectrum in the presence of acyl groups is interpreted in terms of enhanced structural rigidity. It is reported that the ester carbonyl region of the difference spectrum of cinnamoyl-subtilisin is complicated by overlap of features that arise from protein perturbation. Measurements of carbonyl absorption frequencies in a number of solvents of the methyl esters of the acyl groups used to make acyl-enzymes have permitted determination of the apparent dielectric constants experienced by carbonyl groups in the enzyme active centre as well as a discussion of the effects of polarity. The ester carbonyl bond strengths of the various conformations were estimated by using simple harmonic oscillator theory and an empirical relation between the force constants and bond strengths. The fractional bond breaking induced by hydrogen-bonding was used to calculate rate enhancement factors by using absolute reaction rate theory.(ABSTRACT TRUNCATED AT 400 WORDS)Keywords
This publication has 22 references indexed in Scilit:
- Direct observation of the titration of substrate carbonyl groups in the active site of .alpha.-chymotrypsin by resonance Raman spectroscopyBiochemistry, 1989
- Prediction of electrostatic effects of engineering of protein chargesNature, 1987
- Direct observation of substrate distortion by triosephosphate isomerase using Fourier transform infrared spectroscopyBiochemistry, 1980
- Crystal structure of elastase–substrate complex at −55 °CNature, 1976
- Binding Energy, Specificity, and Enzymic Catalysis: The Circe EffectPublished by Wiley ,1975
- Dependence of the kinetic parameters for elastase-catalyzed amide hydrolysis on the length of peptide substratesBiochemistry, 1973
- Isolation and characterization of carboxypeptidases A and B from activated pancreatic juiceBiochemistry, 1971
- Structure of crystalline α-chymotrypsinJournal of Molecular Biology, 1970
- Are the structure and function of an enzyme the same in aqueous solution and in the wet crystal?Journal of Molecular Biology, 1970
- The Determination of the Concentration of Hydrolytic Enzyme Solutions: α-Chymotrypsin, Trypsin, Papain, Elastase, Subtilisin, and Acetylcholinesterase1Journal of the American Chemical Society, 1966