Tunneling in enzymes

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11, 125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects and quantum tunneling in enzyme-catalyzed hydrogen transfer. 

Isotope effects and quantum tunneling in enzyme-catalyzed hydrogen transfer. 

Quantum tunneling in enzyme-catalyzed reactions early indications 35 The rule of the geometric mean ( no isotope effects on isotope effects ) The Swain-Schaad relationship 36... 

Secondary isotope effects measure transition-state structure 37 Quantum tunneling in enzyme-catalyzed reactions breakthroughs 42 Experimental phenomenology of quantum tunneling in enzyme-catalyzed... 

Observations that do not definitively indicate tunneling 70 Observations that likely indicate tunneling 71 Models for tunneling in enzyme reactions 72 Bell tunneling 72... 

QUANTUM TUNNELING IN ENZYME-CATALYZED REACTIONS EARLY INDICATIONS... 

Just as in the preceding examples, early indications of tunneling in enzyme-catalyzed reactions depended on the failure of experiments to conform to the traditional expectations for kinetic isotope effects (Chart 3). Table 1 describes experimental determinations of -secondary isotope effects for redox reactions of the cofactors NADH and NAD. The two hydrogenic positions at C4 of NADH are stereochemically distinct and can be labeled individually by synthetic use of enzyme-catalyzed reactions. In reactions where the deuterium label is not transferred (see below), an... 

Table 2 contains, in reverse chronological order, detailed information about the results of 15 experimental studies of tunneling in enzymic reactions, conducted in the last hve years. These examples will be used to explore the range of evidence, reaction types, enzymes, and concepts currently under study. Other progress made during the preceding decade will be referred to in the discussion of these examples and has been treated in a number of reviews. °... 

Table 2 Recent experimental studies of quantum tunneling in enzyme action... 

The detailed description of the models currently being used to describe tunneling in enzyme-catalyzed reactions will be undertaken in a later part of our treatment in which theoretical studies will be reviewed and ideas about the role of protein motions will be a focus. At this point, we will briefly sketch the most commonly used ideas. 

Bahnson, B.J. and Klinman, J.P. (1995). Hydrogen tunneling in enzyme catalysis. Methods Enzymol. 249, 373-397... 

Carpenter BK. In Allemann RK, Scrutton NS, editors. Quantum tunnelling in enzyme catalysed reactions. Cambridge RSC Publishing 2009. Chapter 1. 

Isotope effects and quantum tunneling in enzyme-catalyzed hydrogen transfer Part I. The experimental basis... 

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