X-ray structure determination of a metastable state of carbonmonoxy myoglobin after photodissociation

The x-ray structure of carbon monoxide (CO)-ligated myoglobin illuminated during data collection by a laser diode at the wavelength λ = 690 nm has been determined to a resolution of 1.7 A at T = 36 K. For comparison, we also measured data sets of deoxymyoglobin and CO-ligated myoglobin. In the photon-induced structure the electron density associated with the CO ligand can be described by a tube extending from the iron into the heme pocket over more than 4 A. This density can be interpreted by two discrete positions of the CO molecule. One is close to the heme iron and can be identified to be bound CO. In the second, the CO is dissociated from the heme iron and lies on top of pyrrole ring C. At our experimental conditions the overall structure of myoglobin in the metastable state is close to the structure of a CO-ligated molecule. However, the iron has essentially relaxed into the position of deoxymyoglobin. We compare our results with those of Schlichting et al. [Schlichting, I., Berendzen, J., Phillips, G. N., Jr., and Sweet, R. M. (1994) Nature 317, 808-812], who worked with the myoglobin mutant (D122N) that crystallizes in the space group P6 and Teng et al. [Teng, T. Y., Srajer, V. and Moffat, K. (1994) Nat. Struct. Biol. 1, 701-705], who used native myoglobin crystals of the space group P2₁. Possible reasons for the structural differences are discussed.