Sensing Nitrite through a Pseudoazurin-Nitrite Reductase Electron Transfer Relay

Nitrite is converted to nitric oxide by haem or copper‐containing enzymes in denitrifying bacteria during the process of denitrification. In designing an efficient biosensor, this enzymic turnover must be quantitatively assessed. The enzyme nitrite reductase from Alcaligenes faecalis contains a redox‐active blue copper centre and a nonblue enzyme‐active copper centre. It can be covalently tethered to modified gold‐electrode surfaces in configurations in which direct electron transfer is possible. A surface cysteine mutant of the enzyme can be similarly immobilised on bare electroactive gold substrates. Under such circumstances, however, electron transfer cannot be effectively coupled with substrate catalytic turnover. In using either the natural redox partner, pseudoazurin, or ruthenium hexammine as an “electron‐shuttle” or “conduit” between enzyme and a peptide‐modified electrode surface, the coupling of electron transfer to catalysis can be utilised in the development of an amperometric nitrite sensor. Design of an efficient biosensor: Natural pseudoazurin and ruthenium hexammine are used as electron relay mediators in the generation of an amperometric nitrite sensing system. The pseudoazurin redox centre is able to undergo facile electron transfer at a thiolated hexapeptide‐modified gold electrode and exchanges electrons through association with a copper‐containing nitrite reductase (CuNiR; the picture shows the hexapeptide–pseudoazurin–CuNiR system).