Picosecond primary structural transition of the heme is retarded after nitric oxide binding to heme proteins

We investigated the ultrafast structural transitions of the heme induced by nitric oxide (NO) binding for several heme proteins by subpicosecond time-resolved resonance Raman and femtosecond transient absorption spectroscopy. We probed the heme iron motion by the evolution of the iron-histidine Raman band intensity after NO photolysis. Unexpectedly, we found that the heme response and iron motion do not follow the kinetics of NO rebinding. Whereas NO dissociation induces quasi-instantaneous iron motion and heme doming (<0.6 ps), the reverse process results in a much slower picosecond movement of the iron toward the planar heme configuration after NO binding. The time constant for this primary domed-to-planar heme transition varies among proteins (similar to 30 ps for myoglobin and its H64V mutant, similar to 15 ps for hemoglobin, similar to 7 ps for dehaloperoxidase, and similar to 6 ps for cytochrome c) and depends upon constraints exerted by the protein structure on the heme cofactor. This observed phenomenon constitutes the primary structural transition in heme proteins induced by NO binding.