X-RAY STRUCTURE DETERMINATION OF A METASTABLE STATE OF CARBONMONOXY MYOGLOBIN AFTER PHOTODISSOCIATION

The x-ray structure of carbon monoxide (CO)ligated myoglobin illuminat ed during data collection by a laser diode at the wavelength lambda = 690 nm has been determined to a resolution of 1.7 A at T = 36 K. For c omparison, we also measured data sets of deoxymyoglobin and CO-ligated myoglobin. In the photon-induced structure the electron density assoc iated 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 int erpreted 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, th e 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 mole cule. However, the iron has essentially relaxed into the position of d eoxymyoglobin. We compare our results with those of Schlichting et al. [Schlichting, I., Berendzen, J., Phillips, G, N., Jr., & Sweet, R. M. (1994) Nature 317, 808-812], who worked with the myoglobin mutant (D1 22N) that crystallizes in the space group P6 and Teng et al., [Teng, T . Y., Srajer, V. & Moffat, K. (1994) Nat. Struct. Biol. 1, 701-705], w ho used native myoglobin crystals of the space group P2(1). Possible r easons for the structural differences are discussed.