Homeostatic control of nitric oxide (NO) at nanomolar concentrations in denitrifying bacteria – modelling and experimental determination of NO reductase kinetics in vivo in Paracoccus denitrificans

Homeostatic control of nitric oxide (NO) at nanomolar concentrations appears common among denitrifying bacteria, often ascribed to synchronized expression of nitrite and nitric oxide reductase (Nir and Nor). We questioned whether this is sufficient: using the reported substrate affinities for cytochrome cd₁ nitrite reductase (cNor), our model of batch cultures of Paracoccus denitrificans predicted NO concentrations orders of magnitude higher than measured. We rejected a hypothesis that the homeostatic control is due to a negative feedback by NO on the activity of NirS because the inclusion of such feedback resulted in too slow anaerobic growth and N₂ production. We proceeded by determining the kinetic parameters for cNor in vivo by a carefully designed experiment, allowing the estimation of NO concentration at the cell surface while anoxic cultures depleted low headspace doses of NO. With the new parameters for cNor kinetics in vivo {v = v_max/[1 + K₂/(NO) + K₁ × K₂/(NO)²]; v_max = 3.56 fmol NO cell⁻¹ h⁻¹, K₁ < 1 nM, and K₂ = 34 nM}, the model predicted NO concentrations close to that measured. Thus, enzyme kinetics alone can explain the observed NO homeostasis. Determinations of enzyme kinetic parameters in vivo are not trivial but evidently required to understand and model NO kinetics in denitrifying organisms in soils and aquatic environments.