Catalytic monolayer voltammetry and in situ scanning tunneling microscopy of copper nitrite reductase on cysteamine-modified Au(111) electrodes

We have studied the adsorption and electrocatalysis of the redox metalloenzyme blue copper nitrite reductase from Achromobacter xylosoxidans (AxCuNiR) on single-crystal Au(111)-electrode surfaces modified by a self-assembled monolayer of cysteamine. A combination of cyclic voltammetry and in situ electrochemical scanning tunneling microscopy (in situ STM) directly in aqueous acetate buffer, pH 6.0 has been used. High-resolution in situ STM shows that cysteamine packs into ordered domains with strip features of a periodic distance of 11.7 +/- 0.3 Angstrom. No voltammetric signals of the nitrite substrate on this surface could be detected. A strong cathodic catalytic wave appears in the presence of nitrite. The catalytic current follows a Michaelis-Menten pattern with a Michaelis constant of K-m approximate to 44 muM, which is close to the value for AxCuNiR in homogeneous solution. The apparent catalytic rate constant based on a dense monolayer is k(cat) = 6-10 s(-1). This is significantly lower than two reported values of 185 s(-1) and 1400-1900 s(-1) for AxCuNiR in homogeneous solution. In situ STM of adsorbed AxCuNiR on the cysteamine-modified Au(111) surface suggests, however, that the coverage is low and the actual rate constant 120-220 s(-1) is much closer to the values in homogeneous solution. The results show that AxCuNiR can be brought to immobilization in a functional state on suitably modified, well-defined, atomically planar Au(111)-electrode surfaces. This would be important for forthcoming biotechnology at the monolayer and toward the single-molecule level.