(113d) Methanol Oxidation Mechanism by Ion-Modified Methanol Dehydrogenase Enzymes for Fuel Cell Applications

Bacterial Methanol Dehydrogenase (MDH) is a water-soluble quinoprotein that oxidizes methanol and other primary alcohols to their corresponding aldehydes. The crystal structure of MDH from Methlylobacterium extorquens [1, 2] and from Methylophilus W3A1 [3-5] has been characterized and it has been determined that the enzyme active center contains a Ca2+ ion, pyrroloquinoline quinone (PQQ) and amino acids. It has been suggested that apart from holding the PQQ molecule in place in the active site, the calcium ion might have an important role in the methanol oxidation mechanism. Some authors [6] have used Ca2+-free MDH enzymes to obtain enzymes containing Sr2+ and Ba2+ in their active sites. Their experimental results have shown that there are no major differences between these enzymes in the interactions between PQQ and the metal ions in the active site. However, even though the Ba2+-modified enzyme has a relative low affinity for methanol, its activation energy for the oxidation reaction is half that of the normal Ca2+-containing MDH enzyme.[6] This result was not expected since the replacement with Ba2+, a weaker Lewis acid, should decrease the activity of the enzyme, and therefore increase its activation energy.[6] This means that Ba2+ has the potential to activate PQQ, but apparently subtle differences in the active site of PQQ-containing enzymes determine the way and the extent to which the activation is expressed. Fuel cells that use bacterial MDH enzyme as anode catalyst are potential attractive power sources, which may produce continuous power output of about 100 mW. Understanding the role of the ion in the active site of MDH, as well as the fuel oxidation mechanism will help to contribute to the design of novel catalysts for fuel cell applications. In this work, the methanol oxidation mechanisms by Ca2+- and Ba2+-MDH enzymes are investigated using Quantum Mechanical and Molecular simulations. Quantum mechanical Density Functional Theory (DFT) simulations are used to obtain geometry, energetic, electronic configurations, and binding energies of small portions of the active site of the enzyme. Equivalent information is obtained on larger systems using Ab Initio Molecular Dynamics. The activation energies for the methanol oxidation reactions are calculated and rates predicted using Transition State Theory. Our findings suggest that the nature of the ion modifies the binding and orientation of methanol with respect to the active site of the enzyme, facilitating the methanol oxidation reaction in the case of having Ba2+ ion in the active site of MDH enzyme.

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