Heme oxygenase (HO) catalyzes physiological
heme degradation using O(2) and reducing equivalents to produce
biliverdin,
iron, and CO. Notably, the HO reaction proceeds without product inhibition by CO, which is generated in the
conversion reaction of
alpha-hydroxyheme to
verdoheme, although CO is known to be a potent inhibitor of HO and other
heme proteins. In order to probe how endogenous CO is released from the reaction site, we collected X-ray diffraction data from a crystal of the CO-bound form of the ferrous
heme-HO complex in the dark and under illumination by a red
laser at approximately 35 K. The difference Fourier map indicates that the CO
ligand is partially photodissociated from the
heme and that the photolyzed CO is trapped in a hydrophobic cavity adjacent to the
heme pocket. This hydrophobic cavity was occupied also by
xenon, which is similar to CO in terms of size and properties. Taking account of the affinity of CO for the ferrous
verdoheme-HO complex being much weaker than that for the ferrous
heme complex, the CO derived from
alpha-hydroxyheme would be trapped preferentially in the hydrophobic cavity but not coordinated to the
iron of
verdoheme. This structural device would ensure the smooth progression of the subsequent reaction, from
verdoheme to
biliverdin, which requires O(2) binding to
verdoheme.