Electron transport reactions of denitrification

Thesis

A study is reported which demonstrates that electron transport to the reductase reactions of denitrification in the bacterium Thiosphaera pantotropha can occur aerobically. Use of dark-type electrodes has demonstrated that the N₂O reductase enzyme of this organism is active under aerobic conditions, and that O₂ and N₂O reduction can occur simultaneously. The reduction of NO₃^- to N₂ gas, even under aerobic conditions, is shown to proceed via NO as an intermediate. It is concluded that the reaction of NO with O₂ must be sufficiently slow that it does not effectively compete with the reduction of NO to N₂O.

The ability of T. pantotropha to catalyse aerobic NO₃^- reduction, the first step of the aerobic denitrification process, is shown to correlate with the expression of a NO₃^- reductase enzyme that is located in the periplasm. This periplasmic enzyme is expressed, and is active, under both aerobic and anaerobic conditions. A membrane bound NO₃^- reductase is also expressed, but only under anaerobic conditions, by this organism. This latter reductase resembles the NO₃^- reductase of Paracoccus denitrificans in respect of both its catalytic properties and the inhibition of activity in intact cells under aerobic conditions. Mutants of T. pantotropha that lack the membrane bound NO₃^- reductase, and not only retain but overproduce the periplasmic enzyme, have been obtained via Tn5 mutagenesis. The periplasmic NO₃^- reductase identified in T. pantotropha bears catalytic and structural similarities to an enzyme previously characterised in some strains of Rhodobacter capsulatus.

The ability of strains of R. capsulatus to reduce NO to N₂O is reported together with evidence that there is a discrete NO reductase in this organism. The electron transport pathway to NO reductase has been elucidated. The first identification of a denitrifying strain of R. capsulatus is reported.

Authors: Bell, L

• Peer review status: Peer reviewed
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford