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Wiley InterScience | |||||||
 European Journal of BiochemistryVolume 269 Issue 13, Pages 3096 - 3102 Published Online: 4 Aug 2009 FEBS, 2004
Abstract | References | Full Text: HTML, PDF (Size: 253K) | Related Articles | Citation Tracking  MINIREVIEW A new conceptual framework for enzyme catalysis Hydrogen tunneling coupled to enzyme dynamics in flavoprotein and quinoprotein enzymes Definitions: Strictly, the term 'semiclassical'[1] rather than 'classical' is used to indicate the difference in zero point vibrational energies of C–H and C–D bonds in studies using the kinetic isotope effect as a probe of quantum tunneling. In this review, we have used the term classical to indicate over-the-barrier transfer to avoid confusion on the part of a reader less familiar with the concepts of quantised vibrational energy states. Quantum tunneling allows the hydrogen to travel through the barrier. This is made possible by wave–particle duality. A particle cannot pass through – it must pass over-the-barrier. However, wave–particle duality also gives the hydrogen wave-like properties, and this allows it to pass through a region (i.e. the barrier) from which a particle would be excluded. See reference [2] for a more detailed description of quantum tunneling. Copyright FEBS, 2002 KEYWORDS H-tunneling • transition state theory • protein dynamics • flavoprotein • quinoprotein • kinetic isotope effect • computational simulation • quantum mechanics • stopped-flow kinetics • molecular mechanics ABSTRACTRecent years have witnessed high levels of activity in identifying enzyme systems that catalyse H-transfer by quantum tunneling. Rather than being restricted to a small number of specific enzymes as perceived initially, it has now become an accepted mechanism for H-transfer in a growing number of enzymes. Furthermore, H-tunneling is driven by the thermally induced dynamics of the enzyme. In some of those enzymes that break stable C–H bonds the reaction proceeds purely by quantum tunneling, without the need to partially ascend the barrier. Enzymes studied that fall into this category include the flavoprotein and quinoprotein amine dehydrogenases, which have proved to be excellent model systems. These enzymes have enabled us to study the relationship between barrier shape and reaction kinetics. This has involved studies with 'slow' and 'fast' substrates and enzymes impaired by mutagenesis. A number of key questions now remain, including the nature of the coupling between protein dynamics and quantum tunneling. The wide-ranging implications of quantum tunneling introduce a paradigm shift in the conceptual framework for enzyme catalysis, inhibition and design. (Received 7 March 2002, revised 21 May 2002, accepted 6 June 2002) | 
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