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The Physical Basis of Enzymic Catalysis

The hallmarks of catalysis by enzymes are selectivity, specificity, and speed. However, despite their central role, the physical basis of the enormous catalytic power of enzymes is not well understood. Initially, tunnelling was treated through the introduction of a tunnelling correction to transition-state theory.

However, the examination of the temperature dependence of the kinetic isotope effects (KIEs) of several enzymatic hydrogen transfer reactions has led to a collapse of the semi-classical model for hydrogen tunnelling and new models were developed to explain these observations such as environmentally coupled tunnelling in which protein motions are proposed to drive hydrogen tunnelling.

Dihydrofolate reductase and the environmentally coupled tunnelling model

It is central to our understanding of enzyme catalysis to test these models further and contrast them with potential alternatives. This is especially important for the case of temperature dependent KIEs, which are consistent with a model where an active promoting motion leads to a compression of the tunnelling barrier in the reactive state and enhanced tunnelling. Alternative models such as multiple conformational states of the enzymes also must be examined experimentally.

We use the enzyme dihydrofolate reductase (DHFR) as a model system in which to study hydride tunnelling. The dependence on temperature, pH, pressure and solvent of the KIEs from bacterial DHFRs with a range of optimal temperatures are studied. In addition, DHFR structure and dynamics are probed using techniques such as circular dichroism and NMR. Recently we have synthesised a range of fully and partially heavy-isotope labelled proteins to uncover the impact of dynamics on DHFR catalysis.

Selected references:
• J. Javier Ruiz-Pernía, Enas Behiry, Louis Y. P. Luk, E. Joel Loveridge, Iñaki Tuñón,Vicent Moliner and Rudolf K. Allemann, Chemical Science, 7, 3248-3255 (2016). DOI:10.1039/C5SC04209G.
• Louis Y. P. Luk, J. Javier Ruiz-Pernía, Aduragbemi S. Adesina, E. Joel Loveridge, Iñaki Tuñón, Vincent Moliner and Rudolf K. Allemann Angew. Chem. Int. Ed., 54, 9016–9020 (2015). DOI:10.1002/ANIE.201503968.
• Louis Y. P. Luk, J. Javier Ruiz-Pernía, William M. Dawson, E. Joel Loveridge, Iñaki Tuñón, Vicent Moliner and Rudolf K. Allemann J. Am. Chem. Soc., 136, 17317–17323 (2014). DOI:10.1021/ja5102536.