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Science 10 March 1989:
Vol. 243. no. 4896, pp. 1325 - 1330
DOI: 10.1126/science.2646716
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Articles

Science, Vol 243, Issue 4896, 1325-1330
Copyright © 1989 by American Association for the Advancement of Science

articles

Hydrogen tunneling in enzyme reactions

Y Cha, CJ Murray, and JP Klinman

Department of Chemistry, University of California, Berkeley 94720.

Primary and secondary protium-to-tritium (H/T) and deuterium-to-tritium (D/T) kinetic isotope effects for the catalytic oxidation of benzyl alcohol to benzaldehyde by yeast alcohol dehydrogenase (YADH) at 25 degrees Celsius have been determined. Previous studies showed that this reaction is nearly or fully rate limited by the hydrogen-transfer step. Semiclassical mass considerations that do not include tunneling effects would predict that kH/kT = (kD/kT)3.26, where kH, kD, and kT are the rate constants for the reaction of protium, deuterium, and tritium derivatives, respectively. Significant deviations from this relation have now been observed for both primary and especially secondary effects, such that experimental H/T ratios are much greater than those calculated from the above expression. These deviations also hold in the temperature range from 0 to 40 degrees Celsius. Such deviations were previously predicted to result from a reaction coordinate containing a significant contribution from hydrogen tunneling.


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