New Enzymes for the Hybrid Enzymatic and Organic Electrocatalytic Cascade for the Complete Oxidation of Glycerol

New enzymes for the hybrid enzymatic and organic electrocatalytic cascade for the complete oxidation of glycerol

Sofiene Abdellaoui^a, David P. Hickey^a, Matthew S. Sigman^a and Shelley D. Minteer^a

^aDepartment of Chemistry, University of Utah, 315 S 1400 E Rm 2020, Salt Lake City, Utah, 84112, USA.

minteer@chem.utah.edu

The complete electrochemical oxidation of the biofuel, glycerol, to CO₂ using a hybrid enzymatic and organic catalytic system has been demonstrated¹. This system combines an organic catalyst, 4-amino-(2,2,6,6-Tetramethylpiperidin-1-yl)oxy (TEMPO-NH₂) with oxalate oxidase (OxOx), resulting in the complete electrochemical oxidation of glycerol at a carbon electrode (Figure 1). This hybrid approach consists of five initial oxidative steps (by TEMPO), resulting in the oxidation of glycerol to mesoxalic acid (1 → 6, Figure 1). A combination of OxOx and TEMPO then facilitates the oxidation of mesoxalic (6) acid to glyoxalic acid (7), oxalic acid (8), and finally, CO₂.

Nevertheless, this system is partly limited, by the weak overlap in pH profiles of OxOx and TEMPO, the first working at acidic pH (pH 4.0) and the latter preferring alkaline pH. Moreover, the electrons generated in the cascade reaction by enzymatic oxidative steps are not mediated to the electrode (4 / 16 electrons) and the OxOx substrate range is limited.

In order to improve this system, we are investigating new enzymes to replace OxOx. Like this latter, Oxalate decarboxylase (Oxdc) belongs to the cupin superfamily of proteins and incorporates mononuclear manganese ions coordinated by four amino acids^2,3. This enzyme is able to transform oxalate to formate and CO₂, with O₂ acting as a unique cofactor which can be used to catalyze a side oxidation reaction with o-phenylenediamine⁴ or 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS). This study also aims to investigate other enzymes working at higher pH and the ability to work with redox mediators as dehydrogenases.

Figure 1: Electrocatalytic Oxidation Cascade of Glycerol by TEMPO and Oxalate Oxidase¹

(1)           Hickey, D. P.; McCammant, M. S.; Giroud, F.; Sigman, M. S.; Minteer, S. D. Journal of the American Chemical Society 2014, 136, 15917.

(2)           Moomaw, E. W.; Angerhofer, A.; Moussatche, P.; Ozarowski, A.; Garcia-Rubio, I.; Richards, N. G. J. Biochemistry 2009, 48, 6116.

(3)           Tanner, A.; Bowater, L.; Fairhurst, S. A.; Bornemann, S. Journal of Biological Chemistry 2001, 276, 43627.

(4)           Emiliani, E.; Bekes, P. Archives of biochemistry and biophysics 1964, 105, 488.