The Structures of Carbonmonoxymyoglobin and its Photoproducts at Cryogenic Temperatures. Tsu-yi Teng, Vukica Srajer and Keith Moffat, Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, IL 60637

X-ray structures of carbonmonoxymyoglobin and its photoproducts produced by laser photolysis have been determined by monochromatic oscillation and Laue diffraction at 20 and 40 K with resolution up to 1.7 Å. The results confirm our earlier brief report¹ that myoglobin and its photoproducts show nearly rigid protein structures at cryogenic temperatures. The drift of photodissociated ligand CO away from its binding site and the partial relaxation of heme and its adjacent residues (heme pocket) can readily be visualized. The drift of CO molecule from its binding site depends on the photolysis/X-ray data collection protocol and varies from 1.0 Å to 2.5 Å. It can be interpreted as unbound ligand moving along the reaction coordinate after photolysis. Our earlier results and those of Schlichting et al.² and Hartmann et al.³ are all located within this range and might represent only a partial picture of a much more complex process of ligand migration following photodissociation. The different photolysis protocols used in these experiments are mainly responsible for the differences among them. It is also important to emphasize careful experimental design (cryogenics, photolysis and X-ray diffraction) in the freeze-trapping experiment.

¹Teng, T.-Y., Srajer, V. & Moffat, K. (1994) Nat. Struct. Biol. 1, 701-705.

²Schlichting, I., Berendzen, J., Phillips, G. N., Jr. & Sweet, R. M. (1994). Nature 371, 808-812.

³Hartmann, H., Zinser, S., Komninos, P., Schneider, R. T., Nienhaus, G. U. & Parak, F. (1996) Proc. Natl. Acad. Sci. USA 93, 7013-7016.