Ultrafast heme-ligand recombination in truncated hemoglobin HbO from Mycobacterium tuberculosis: A ligand cage

► We studied ultrafast ligand dynamics in truncated hemoglobin HbO from M. tuberculosis. ► CO, NO, and O2 all have a very low probability to escape from the heme pocket. ► Fast CO geminate rebinding is robust against changes in close aromatic residues. ► Molecular dynamics simulations suggest origins of caging.