Achieving Stable Partial Denitrification by Selective Inhibition of Nitrite Reductase with the Biosafe Aprotic Solvent DMSO

The recently proposed partial denitrification (PD), terminating nitrate reduction to nitrite, has been regarded as a promising alternative to nitrite supplying for anammox bacteria. The most important aspect of the PD process for engineering application is the stable and continuous supply of nitrite. However, the activity of nitrate reductase is often higher than that of nitrite reductase (NIR), making it difficult to accumulate nitrite during the denitrification process. Herein, a strategy for achieving efficient and stable partial denitrification using the biosafe additive dimethyl sulfoxide (DMSO) was constructed, and the mechanism of DMSO inhibiting NIR was analyzed. DMSO addition reduced the expression of <i>NIR</i> gene, and 1% DMSO addition can significantly inhibit NIR enzyme activity to achieve a stable PD process. When the DMSO concentration increased to 3.5%, the NIR enzyme activity was almost inhibited with the enzyme activity of only 0.95 mg nitrite/min. However, the addition of DMSO has almost no inhibitory effect on the nitrate reductase (NAR) enzyme. The affinity constant of DMSO with the NAR enzyme is -2.4 kcal/mol, while the affinity constant of DMSO with the NIR enzyme is as high as -3.1 kcal/mol. DMSO shows a higher affinity for NIR. Moreover, DMSO and nitrite occupy the same catalytic cavity in the NIR enzyme, which is the fundamental reason why DMSO selectively inhibits the NIR enzyme. This study provides a new idea for realizing efficient and stable partial denitrification function.