ABSTRACT
I. Enzymic H-Tunneling and Kinetic Isotope Effects ........................................................ 671
A. Stopped-Flow Methods to Access the Half-Reactions
of Flavoenzymes and Quinoproteins ....................................................................... 672
II. Interpreting Temperature Dependence of Isotope Effects in Terms
of H-Tunneling................................................................................................................. 673
III. H-Tunneling in Flavoenzymes PETN Reductase and MR ............................................. 675
IV. H-Tunneling in TTQ-Dependent MADH and AADH .................................................... 678
V. Computational Studies of Substrate Oxidation in TTQ-Dependent
Amine Dehydrogenases ................................................................................................... 679
VI. H-Tunneling in Flavoprotein Amine Dehydrogenases: TSOX and
Engineering Gated Motion in TMADH .......................................................................... 682
VII. Concluding Remarks........................................................................................................ 685
Acknowledgments ........................................................................................................................ 685
References..................................................................................................................................... 685
Kinetic isotope effects (KIEs) are powerful probes of H-transfer reactions and have provided
evidence for nonclassical transfer of the H nucleus in enzymes (see Chapter 28 by Kohen in this
volume for a detailed discussion of the use of KIEs to identify tunneling regimes). Early
studies of H-transfer by quantum tunneling focused on deviations from values predicted by
semi-classical models (in which zero point energies, but not tunneling, have been taken into
account): KIEs, Swain-Schaad relationships
[exp
. 3:26; where k
, k
, and k
are the
rates of transfer for protium, deuterium and tritium, respectively] or Arrhenius prefactor ratios
(q1 for a reaction proceeding purely by tunneling, ,1 for moderate tunneling). For a more
detailed discussion see, for example, Chapter 28 by Kohen in this volume. Early examples in
which H-tunneling was inferred from measurements of KIEs include yeast alcohol
dehydrogenase,
bovine serum amine oxidase,
horse liver alcohol dehydrogenase,
and
monoamine oxidase.