Orientation of carbon monoxide and structure-function relationship in carbonmonoxymyoglobin

Fourier transform infrared spectroscopy of the CO stretch bands in carbonmonoxymyoglobin (MbCO) reveals three major bands implying that MbCO exists in three major substates, A0, A1, and A3. After photolysis at low temperatures the CO is in the heme pocket, and the resulting CO stretch bands represent the B substates. Photoselection experiments determine the orientation of CO in the A (bound) and B (photolyzed) substates: Small fractions of MbCO are photolyzed at 10 K with linearly polarized light at 540 nm. The resulting linear dichroism in the A and B IR bands yields the tilt angle between the heme normal and CO. The average angles are as follows: alpha (A0) = 15 degrees +/- 3 degrees; alpha (A1) = 28 degrees +/- 2 degrees, and alpha (A3) = 33 degrees +/- 4 degrees. The A bands are inhomogeneously broadened; the angle alpha shows a wavenumber dependence within the A bands. The wavenumber dependence is interpreted as a distribution of the tilt angle within the individually inhomogeneous A substates, thus providing a structural parameter to characterize the distribution of the conformational substates. The B substates exhibit no induced linear dichroism; in the photolyzed substates the ligand is randomly oriented with respect to the heme plane. The present results together with earlier data on static and kinetic properties of CO binding to Mb establish relations among spectroscopic, structural, energetic, and functional parameters.