3.2.2. Copper Nitrite Reductase

A common feature of the blue multi-copper oxidases is a blue T<sup>1</sup> center for the electron inlet and a TII or combined TII/TIII catalytic center for the electron "outlet" in the catalytic process (nitrite, or, dioxygen reduction). Such an electrochemical mode of action is only feasible via an efficient (short) intramolecular ET channel. This feature has been mapped in considerable detail for three blue copper enzyme classes, the copper nitrite reductases, bilirubin oxidases, and the laccases. The trimeric blue copper nitrite reductase (CuNiR) is crucial in the global nitrogen cycle, catalyzing the single-electron reduction of nitrite to nitrogen monoxide [140]. CuNiR effects direct bioelectrocatalysis, including an ET relay (Cu<sup>I</sup> ) and a catalytic site (CuII) in each monomer. Ulstrup and associates reported DET-based electrocatalysis of CuNiR from *Achromobacter xylosoxidans* (*Ax*CuNiR) on a cysteamine SAM-modified Au(111) electrode [141]. *In situ* STM displayed single *Ax*CuNiR molecules but, intriguingly, only in the presence of nitrite substrate. Further, the combination of varying alkanethiols with charged, neutral, hydrophilic, and hydrophobic properties showed that mixed hydrophilic/hydrophobic SAMs were the most favorable for facile *Ax*CuNiR electrocatalysis [142]. *In situ* AFM is also reported and disclosed *Ax*CuNiR conformational changes during catalytic reaction [67], with the apparent height of *Ax*CuNiR rising from 4.5 nm in the resting state to 5.5 nm in the nitrite reduction state in the presence of nitrite.
