Solution 1H NMR of the active site of substrate-bound, cyanide-inhibited human heme oxygenase. comparison to the crystal structure of the water-ligated form

The majority of the active site residues of cyanide-inhibited, substrate-bound human heme oxygenase have been assigned on the basis of two-dimensional NMR using the crystal structure of the water-ligated substrate complex as a guide (Schuller, D. J., Wilks, A., Ortiz de Montellano, P. R., and Poulos, T. L. (1999) Nat. Struct. Biol. 6, 860-867). The proximal helix and the N-terminal portion of the distal helix are found to be identical to those in the crystal except that the heme for the major isomer ( approximately 75-80%) in solution is rotated 180 degrees about the alpha-gamma-meso axis relative to the unique orientation in the crystal. The central portion of the distal helix in solution is translated slightly over the heme toward the distal ligand, and a distal four-ring aromatic cluster has moved 1-2 A closer to the heme, which allows for strong hydrogen bonds between the hydroxyls of Tyr-58 and Tyr-137. These latter interactions are proposed to stabilize the closed pocket conducive to the high stereospecificity of the alpha-meso ring opening. The determination of the magnetic axes, for which the major axis is controlled by the Fe-CN orientation, reveals a approximately 20 degrees tilt of the distal ligand from the heme normal in the direction of the alpha-meso bridge, demonstrating that the close placement of the distal helix over the heme exerts control of stereospecificity by both blocking access to the beta, gamma, and delta-meso positions and tilting the axial ligand, a proposed peroxide, toward the alpha-meso position.