Solid-State 17O NMR as a New Probe to Study Acyl-Enzyme Intermediates
Tang, Aaron W.
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This thesis reports on a combined solid-state 17O NMR and quantum mechanical study of three acyl-enzyme intermediates: p-N,N-dimethylaminobenzoyl-chymotrypsin (DAB-CHT), trans¬¬-¬o-methoxycinnamoyl-chymotrypsin (oMC-CHT), and trans-p-methoxycinnamoyl-chymotrypsin (pMC-CHT). At pH 7.8, the three acyl-enzyme intermediates were found to undergo hydrolysis (deacylation) with the following rate constants: DAB-CHT, 6.4 ± 0.1 × 10-6; oMC-CHT, 2.8 ± 0.1 × 10-4 s-1; pMC-CHT, 8.5 ± 0.3 × 10-3 s-1. Analysis of solid-state 17O NMR spectra obtained under the magic angle spinning (MAS) condition at 21 T for the three acyl-enzyme intermediates yields the following isotropic chemical shift (δiso), quadrupolar coupling constant (CQ), and asymmetry parameter (ηQ): DAB-CHT, δiso = 316 ± 2 ppm, CQ = 10.0 ± 1 MHz, ηQ = 0.6 ± 0.2; oMC-CHT, δiso = 316 ± 2 ppm, CQ = 9.2 ± 1 MHz, ηQ = 0.6 ± 0.2; and pMC-CHT, δiso = 316 ± 2, CQ = 7.0 ± 0.5 MHz, ηQ = 0.8 ± 0.2. With the aid of quantum mechanical calculations, these 17O NMR parameters were interpreted as to reflect the variations in hydrogen bonding interactions between the carbonyl (C=O) functional group of the acyl moiety and the two NH groups from the protein backbone in the so-called oxyanion hole. This work represents the first use of solid-state 17O NMR spectroscopy to study acyl-enzyme intermediates and demonstrates the great potential of this new technique.