Mechanism of substrate inhibition in cytochrome-c dependent NO reductases from denitrifying bacteria (cNORs)

Steady-state kinetics of cytochrome-c dependent denitrifying NO reductases (cNORs) show evidence of substrate inhibition at NO concentrations higher than 10 μM, but the mechanism of inhibition remains unclear. Here, we present low-temperature FTIR photolysis experiments carried out on the NO complex formed by addition of NO to the oxidized cNORs. A differential signal at 1261 cm⁻¹ that downshifts with ¹⁵NO and ¹⁵N¹⁸O is assigned to a ν(NO₂) from a bridging diiron-nitrito complex at the heme-nonheme diron site. Theoretical calculations reproduces observed frequencies and isotope shifts. Our experimental results confirm a prior theoretical study by Blomberg and Siegbahn [Blomberg, M. R., and Siegbahn, P. E. M. Biochemistry 2012, 51, 5173–5186] that proposed substrate inhibition through a radical combination reaction between the diferric μ-oxo group and an NO molecule to form a heme Fe(III)-nitrito-Fe_B(II) inhibitory complex. Stopped-flow experiments suggest that substrate inhibition also occurs after a half-reduction cycle, i.e. when fully-reduced cNOR reduces two NO molecules at the heme-nonheme diferrous active site cluster to produce one N₂O molecule and the diferric cluster. These results support catalytic mechanisms that proceed through isomerization of a diferric-hyponitrite transient complex to produce a bridging diferric μ-oxo group and N₂O without protonation of the putative hyponitrite intermediate.