D-Amino acid oxidase bio-functionalized platforms: Toward an enhanced enzymatic bio-activity

The purpose of this work is to study the adsorption process and surface bio-activity of His-tagged D-amino acid oxidase (DAAO) from Rhodotorula gracilis (His(6)-RgDAAO) as the first step for the development of an electrochemical bio-functionalized platform. With such a purpose this work comprises: (a) the His(6)-RgDAAO bio-activity in solution determined by amperometry, (b) the adsorption mechanism of His(6)-RgDAAO on bare gold and carboxylated modified substrates in the absence (substrate/COO) and presence of Ni(II) (substrate/COO- + Ni(II)) determined by reflectometry, and (c) the bio-activity of the His(6)-RgDAAO bio-functionalized platforms determined by amperometry. Comparing the adsorption behavior and bio-activity of His(6)-RgDAAO on these different solid substrates allows understanding the contribution of the diverse interactions responsible for the platform performance. His(6)-RgDAAO enzymatic performance in solution is highly improved when compared to the previously used pig kidney (pk) DAAO. His(6)-RgDAAO exhibits an amperometrically detectable bio-activity at concentrations as low as those expected on a bio-functional platform; hence, it is a viable bio-recognition element of D-amino acids to be coupled to electrochemical platforms. Moreover, His(6)-RgDAAO bio-functionalized platforms exhibit a higher surface activity than pkDAAO physically adsorbed on gold. The platform built on Ni(II) modified substrates present enhanced bio-activity because the surface complexes histidine-Ni(II) provide with site-oriented, native-like enzymes. The adsorption mechanism responsible of the excellent performance of the bio-functionalized platform takes place in two steps involving electrostatic and bio-affinity interactions whose prevalence depends on the degree of surface coverage. (C) 2015 Elsevier B.V. All rights reserved.