On the mechanisms of nitrite utilization by Escherichia coli cells during stationary growth

Periplasmic Nrf nitrite reductase is the main nitrite utilizing enzyme in Escherichia coli cells when concentration of substrate in the medium is low. The model of nitrite utilization by E. coli cells, cultivated during stationary growth in the chemostat, was earlier created. The model took into account the dynamic of the nrf and nir operons, coding enzymes that metabolize and transport nitrite. The model analysis revealed that to describe the kinetic of nitrite accumulation in the chemostat at micromolar nitrite concentrations, additional assumption about higher nitrite utilization rate in the cell then that was estimated from genetic investigations is to be incorporated in the model. In this study it was shown that the discrepancy between genetic and physiological investigation results under low nitrite concentrations may be explained by properties of Nrf enzyme, which catalytic activity and stability depends on its localization in the periplasm, and its secretion to the periplasm is determined on the presence of membrane potential. The model analysis revealed that at low nitrite concentration local change of enzyme concentration during transition from cellular cytoplasm to periplasm under membrane potential influence is sufficient for higher periplasmic Nrf activity achievement, than it can be expected according to nrf operon expression level.