Kinetics of CO recombination to the heme in Geobacillus stearothermophilus nitric oxide synthase

We report the kinetics of CO rebinding to the heme in His134Ser, Ile223Val and His134Ser/Ile223Ser mutants of Geobacillus stearothermophilus nitric oxide synthase (gsNOS). The amplitudes of the two observed kinetics phases, which are insensitive to CO concentration, depend on enzyme concentration. We suggest that two forms of gsNOS are in equilibrium under the conditions employed (6.1-27 μM gsNOS with 20 or 100% CO atmosphere). The kinetics of CO rebinding to the heme do not depend on the identity of the NO-gate residues at positions 134 and 223.