Implication for using heme methyl hyperfine shifts as indicators of heme seating as related to stereoselectivity in the catabolism of heme by heme oxygenase: in-plane heme versus axial his rotation

Distal regulation of heme binding of heme oxygenase-1 mediated by conformational fluctuations

Heme oxygenase-1 (HO-1) is an enzyme that catalyzes the oxidative degradation of heme. Since free heme is toxic to cells, rapid degradation of heme is important for maintaining cellular health. There have been useful mechanistic studies of the HO reaction based on crystal structures; however, how HO-1 recognizes heme is not completely understood because the crystal structure of heme-free rat HO-1 lacks electron densities for A-helix that ligates heme. In this study, we characterized conformational dynamics of HO-1 using NMR to elucidate the mechanism by which HO-1 recognizes heme. NMR relaxation experiments showed that the heme-binding site in heme-free HO-1 fluctuates in concert with a surface-exposed loop and transiently forms a partially unfolded structure. Because the fluctuating loop is located over 17 Å distal from the heme-binding site and its conformation is nearly identical among different crystal structures including catalytic intermediate states, the function of the loop has been unexamined. In the course of elucidating its function, we found interesting mutations in this loop that altered activity but caused little change to the conformation. The Phe79Ala mutation in the loop changed the conformational dynamics of the heme-binding site. Furthermore, the heme binding kinetics of the mutant was slower than that of the wild type. Hence, we concluded that the distal loop is involved in the regulation of the conformational change for heme binding through the conformational fluctuations. Similar to other enzymes, HO-1 effectively promotes its function using the identified distal sites, which might be potential targets for protein engineering.

The orbital ground state of the azide-substrate complex of human heme oxygenase is an indicator of distal H-bonding: implications for the enzyme mechanism

The active site electronic structure of the azide complex of substrate-bound human heme oxygenase 1 (hHO) has been investigated by (1)H NMR spectroscopy to shed light on the orbital/spin ground state as an indicator of the unique distal pocket environment of the enzyme. Two-dimensional (1)H NMR assignments of the substrate and substrate-contact residue signals reveal a pattern of substrate methyl contact shifts that places the lone iron pi-spin in the d(xz) orbital, rather than the d(yz) orbital found in the cyanide complex. Comparison of iron spin relaxivity, magnetic anisotropy, and magnetic susceptibilities argues for a low-spin, (d(xy))(2)(d(yz),d(xz))(3), ground state in both azide and cyanide complexes. The switch from singly occupied d(yz) for the cyanide to d(xz) for the azide complex of hHO is shown to be consistent with the orbital hole determined by the azide pi-plane in the latter complex, which is approximately 90 degrees in-plane rotated from that of the imidazole pi-plane. The induction of the altered orbital ground state in the azide relative to the cyanide hHO complex, as well as the mean low-field bias of methyl hyperfine shifts and their paramagnetic relaxivity relative to those in globins, indicates that azide exerts a stronger ligand field in hHO than in the globins, or that the distal H-bonding to azide is weaker in hHO than in globins. The Asp140 --> Ala hHO mutant that abolishes activity retains the unusual WT azide complex spin/orbital ground state. The relevance of our findings for other HO complexes and the HO mechanism is discussed.

1H NMR study of the effect of variable ligand on heme oxygenase electronic and molecular structure

Heme oxygenase carries out stereospecific catabolism of protohemin to yield iron, CO and biliverdin. Instability of the physiological oxy complex has necessitated the use of model ligands, of which cyanide and azide are amenable to solution NMR characterization. Since cyanide and azide are contrasting models for bound oxygen, it is of interest to characterize differences in their molecular and/or electronic structures. We report on detailed 2D NMR comparison of the azide and cyanide substrate complexes of heme oxygenase from Neisseria meningitidis, which reveals significant and widespread differences in chemical shifts between the two complexes. To differentiate molecular from electronic structural changes between the two complexes, the anisotropy and orientation of the paramagnetic susceptibility tensor were determined for the azide complex for comparison with those for the cyanide complex. Comparison of the predicted and observed dipolar shifts reveals that shift differences are strongly dominated by differences in electronic structure and do not provide any evidence for detectable differences in molecular structure or hydrogen bonding except in the immediate vicinity of the distal ligand. The readily cleaved C-terminus interacts with the active site and saturation-transfer allows difficult heme assignments in the high-spin aquo complex.