Investigating the Methanol Oxidation Mechanism by Mg+2 and Sr+2-Containing Methanol Dehydrogenase Enzyme

   Global revolution in technologies demands for power sources with high energy density, low cost and portability. One of the promising alternatives to current power sources being studied these days are the Proton Exchange Membrane (PEM) fuel cells [1]. They use expensive anode catalysts like platinum which easily get poisoned by carbon monoxide, a byproduct of oxidation reactions that take place in fuel cells [1]. A solution to these problems is using biofuel cells which use enzymatic catalysts in place of expensive catalysts and are more eco friendly in nature [2]. Methanol dehydrogenase (MDH) is one such enzyme used as an anodic catalyst for a methanol-fed biofuel cell producing enough power for small electronic device applications [3]. Understanding the oxidation mechanism at the active site is necessary to study the power output limitations associated with this MDH fuel cell. The active site of MDH contains a Ca⁺² ion which acts as a Lewis acid during the oxidation mechanism [4].

Previously, it was observed that the energy barrier for the oxidation of methanol was influenced by the atomic size of the metal ion in the active site of MDH [5]. Thus we expect that there might be a change in the barrier if Mg⁺² and Sr⁺² are placed instead of Ca⁺² as they are the same periodic group elements. Quantum mechanical (Density Functional Theory) simulations are performed to obtain geometry, energy and electronic configurations of small portions of the active site and the activation energies for the various methanol oxidation steps have been calculated. The geometries and free energy barriers obtained from the rate-limiting steps with these cations are compared with the previous results [5]. This comparison could help us in determining whether these ion-modified MDH would be more/less active for the oxidation of methanol.

References:

1. Cleghorn S. J. C., Ren X., Springer T. E., Wilson M. S., Zawodzinski C., Zawodzinski T. A., and Gottesfeld S., "Pem Fuel Cells for Transportation and Stationary Power Generation Applications". Int. J. Hydrogen Energy, 22, 1997.

2. Palmore G. T. R. and Whitesides G. M., "Microbial and Enzymatic Biofuel Cells", Enzymatic   Conversion of Biomass for Fuels Production, J.O.B. M.E. Himmel, and R.P. Overend, Washington, DC., 1994.

3. Zhang X. C., Ranta A., and Halme A, "Effect of Different Catalytic Oxidants on the Performance of a Biocatalytic Methanol Fuel Cell", in Proceedings 204th Meeting of The Electrochemical Society, 2003.

4.  C. Anthony, “Methanol Dehydrogenase, a PQQ-Containing Quinoprotein Dehydrogenase”, Kluwer Academic/ Plenum Publishers, New York, 2000.

5. Idupulapati, N.B., Mainardi, D.S., “Coordination and binding of ions in Ca2+- and Ba2+-containing methanol dehydrogenase and interactions with methanol”, Journal of Molecular Structure: THEOCHEM, 901 (1-3), 2009.

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