New enzymes for the hybrid enzymatic and organic electrocatalytic cascade for the complete oxidation of glycerol
Sofiene Abdellaouia, David P. Hickeya, Matthew S. Sigmana and Shelley D. Minteera
aDepartment of Chemistry, University of Utah, 315 S 1400 E Rm 2020, Salt Lake City, Utah, 84112, USA.
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 acids2,3. 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.