Comparative study of three lactate oxidases from Aerococcus viridans for biosensing applications

A comparison between engineered and commercially available l-lactate oxidases from Aerococcus viridans was conducted for biosensing applications. Enzymes were adsorbed onto the surfaces of graphite electrodes modified with multi-walled carbon nanotubes. Thermostable l-lactate oxidases were cloned with a (i) N-, (ii) a C-terminal His-tag and (iii) a wild-type enzyme. Subsequently to the heterologous expression in Escherichia coli and purification, we determined the kinetic parameters of these enzymes in solution. The kinetics of the wild-type, of the N-terminally His-tagged enzyme and of the commercial l-lactate oxidase from A. viridans were studied with a classical Michaelis-Menten as well as with a substrate inhibition model, while the enzyme carrying a C-terminal His-tag showed no activity. The active enzymes were used to fabricate and comparatively investigate multi-walled carbon nanotubes-based biosensors. The enzyme kinetic results were compared with electrochemical studies. By using both spectrophotometric and amperometric techniques, the inhibition phenomenon fits better to the data especially those data related with Lox-His-N. The electrochemical data of the fabricated enzymatic biosensors showed that the N-terminally His-tagged l-lactate oxidase performed best on carboxyl-modified carbon nanotubes. The sensor based on this engineered enzyme showed the highest sensitivity and lowest detection limit in the range of l-lactate concentration 0-1 mM as well as long term stability over one month.