Combining experimental and theoretical methods to learn about the reactivity of gas-processing metalloenzymes

After enzymes were first discovered in the late XIX century, and for the first seventy years of enzymology, kinetic experiments were the only source of information about enzyme mechanisms. Over the following fifty years, these studies were taken over by approaches that give information at the molecular level, such as crystallography, spectroscopy and theoretical chemistry (as emphasized by the Nobel Prize in Chemistry awarded last year to M. Karplus, M. Levitt and A. Warshel). In this review, we thoroughly discuss the interplay between the information obtained from theoretical and experimental methods, by focussing on enzymes that process small molecules such as H2 or CO2 (hydrogenases, CO-dehydrogenase and carbonic anhydrase), and that are therefore relevant in the context of energy and environment. We argue that combining theoretical chemistry (DFT, MD, QM/MM) and detailed investigations that make use of modern kinetic methods, such as protein film voltammetry, is an innovative way of learning about individual steps and/or complex reactions that are part of the catalytic cycles. We illustrate this with recent results from our labs and others, including studies of gas transport along substrate channels, long range proton transfer, and mechanisms of catalysis, inhibition or inactivation.