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Transition-State Vibrational Analysis and Isotope Effects for COMT-Catalyzed Methyl Transfer.

Maite Roca1, Ian H Williams2

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Journal of the American Chemical Society
|August 20, 2020
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Summary
This summary is machine-generated.

This study analyzes isotopic partition-function ratios (IPFRs) for methyl-transfer reactions, finding smaller computational models accurately predict hydrogen IPFRs but not carbon-14 IPFRs. Ponderal effects influence IPFRs, with protein contributions to the transition vector being minimal.

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Area of Science:

  • Computational Chemistry
  • Enzyme Catalysis
  • Biophysical Chemistry

Background:

  • Catechol O-methyltransferase (COMT) catalyzes methyl transfer reactions.
  • Isotopic partition-function ratios (IPFRs) are crucial for understanding reaction mechanisms and isotopic effects.
  • Hybrid quantum mechanics/molecular mechanics (QM/MM) methods are employed to model complex enzymatic systems.

Purpose of the Study:

  • To analyze IPFRs for transition structures (TSs) in COMT-catalyzed methyl transfer.
  • To critically assess the ability of smaller computational models (Hessians) to reproduce IPFR trends.
  • To investigate the influence of ponderal effects and protein residues on IPFRs.

Main Methods:

  • Computation of IPFRs for TSs using hybrid QM/MM methods.
  • Analysis of smaller extracted Hessians against larger QM/MM Hessians for IPFR accuracy.
  • Calculation of harmonic frequencies, mean-square amplitudes, and potential energy scans to assess anharmonicity.
  • Investigation of ponderal effects by varying system mass.

Main Results:

  • A 6-atom Hessian accurately reproduced alpha-tritium (α-T3) IPFRs but not alpha-carbon-14 (α-14C) IPFRs.
  • Ponderal effects show α-14C IPFRs tend toward a minimum and α-T3 IPFRs toward a maximum with increasing system mass.
  • The transition vector is primarily composed of motions within the methyl donor and acceptor, with minimal contribution from protein residues.
  • Valence force constants differ significantly from previous estimates, challenging the compression hypothesis.

Conclusions:

  • Smaller computational models are insufficient for accurately predicting all isotopic effects in COMT catalysis.
  • Ponderal effects play a significant role in determining IPFRs, particularly for heavier isotopes.
  • The catalytic mechanism does not appear to be strongly influenced by protein residue interactions or compression effects.