Detailed scheme for the photoreaction of wild-type sperm whale MbCO. In particular, the movements of the CO molecule are highlighted. Yellow spheres indicate the locations of the CO for A, B, C, D, and S states. The CO ligated to the heme (A) is dissociated by the laser pulse centered at 532 nm. The photodissociated CO moves from the primary docking site (B) via the Xe4 site (C) to the Xe1 site (D) and finally to the solvent environment (S). The interplay between the distal histidine and the CO generates the conformational substates of A and B. In particular, due to the existence of the two conformational substates of B, the transition from B to C exhibits biphasic kinetics.