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

Role of propionates in substrate binding to heme oxygenase from Neisseria meningitidis: a nuclear magnetic resonance study

Heme oxygenase (HO) cleaves hemin into biliverdin, iron, and CO. For mammalian HOs, both native hemin propionates are required for substrate binding and activity. The HO from the pathogenic bacterium Neisseria meningitidis (NmHO) possesses a crystallographically undetected C-terminal fragment that by solution (1)H nuclear magnetic resonance (NMR) is found to fold and interact with the active site. One of the substrate propionates has been proposed to form a salt bridge to the C-terminus rather than to the conventional buried cationic side chain in other HOs. Moreover, the C-terminal dipeptide Arg208His209 cleaves spontaneously over ~24 h at a rate dependent on substituent size. Two-dimensional (1)H NMR of NmHO azide complexes with hemins with selectively deleted or rearranged propionates shows that all bind to NmHO with a structurally conserved active site as reflected in optical spectra and NMR nuclear Overhauser effect spectroscopy cross-peak and hyperfine shift patterns. In contrast to mammalian HOs, NmHO requires only a single propionate interacting with the buried terminus of Lys16 to exhibit full activity and tolerates the existence of a propionate at the exposed 8-position. The structure of the C-terminus is qualitatively retained upon deletion of the 7-propionate, but a dramatic change in the 7-propionate carboxylate (13)C chemical shift upon C-terminal cleavage confirms its role in the interaction with the C-terminus. The stronger hydrophobic contacts between pyrroles A and B with NmHO contribute more substantially to the substrate binding free energy than in mammalian HOs, "liberating" one propionate to stabilize the C-terminus. The functional implications of the C-terminus in product release are discussed.

Influence of substrate modification and C-terminal truncation on the active site structure of substrate-bound heme oxygenase from Neisseriae meningitidis. A 1H NMR study

Heme oxygenase (HO), from the pathogenic bacterium N. meningitidis(NmHO), which secures host iron, shares many properties with mammalian HOs but also exhibits some key differences. The crystal structure appears more compact, and the crystal-undetected C-terminus interacts with substrate in solution. The unique nature of substrate-protein, specifically pyrrole-I/II-helix-2, peripheral interactions in NmHO are probed by 2D (1)H NMR to reveal unique structural features controlling substrate orientation. The thermodynamics of substrate orientational isomerism are mapped for substrates with individual vinyl → methyl → hydrogen substitutions and with enzyme C-terminal deletions. NmHO exhibits significantly stronger orientational preference, reflecting much stronger and selective pyrrole-I/II interactions with the protein matrix, than in mammalian HOs. Thus, replacing bulky vinyls with hydrogens results in a 180° rotation of substrate about the α,γ-meso axis in the active site. A "collapse" of the substrate pocket as substrate size decreases is reflected in movement of helix-2 toward the substrate as indicated by significant and selective increased NOESY cross-peak intensity, increase in steric Fe-CN tilt reflected in the orientation of the major magnetic axis, and decrease in steric constraints controlling the rate of aromatic ring reorientation. The active site of NmHO appears "stressed" for native protohemin, and its "collapse" upon replacing vinyls by hydrogen leads to a factor ~10(2) increase in substrate affinity. Interaction of the C-terminus with the active site destabilizes the crystallographic protohemin orientation by ~0.7 kcal/mol, which is consistent with optimizing the His207-Asp27 H-bond. Implications of the active site "stress" for product release are discussed.

1H NMR study of the influence of mutation on the interaction of the C-terminus with the active site in heme oxygenase from Neisseria meningitidis: implications for product release

The HO from the pathogenic bacterium Neisseria meningitidis, NmHO, possesses C-terminal His207, Arg208, and His209 residues that are undetected in crystal structures. NMR found the C-terminus ordered and interacting with the active site and shown to undergo a spontaneous cleavage of the C-terminal Arg208-His209 bond that affects the product off rate. A preliminary model for the interaction based on the wild-type (WT) NmHO complexes has been presented [Liu, Y., Ma, L.-H., Satterlee, J. D., Zhang, X., Yoshida, T., and La Mar, G. N. (2006) Biochemistry 45, 3875-3886]. Two-dimensional (1)H NMR data of resting-state, azide-inhibited substrate complexes of the three C-terminal truncation mutants (Des-His209-, Des-Arg208His209-, and Des-His207Arg208His209-NmHO) confirm the previous proposed roles for His207 and Arg208 and reveal important additional salt bridges involving the His209 carboxylate and the side chains of both Lys126 and Arg208. Deletion of His209 leads to a qualitatively retained C-terminal geometry, but with increased separation between the C-terminus and active site. Moreover, replacing vinyls with methyls on the substrate leads to a decrease in the separation between the C-terminus and the active site. The expanded model for the C-terminus reveals a less stable His207-Arg208 cis peptide bond, providing a rationalization for its spontaneous cleavage. The rate of this spontaneous cleavage is shown to correlate with the proximity of the C-terminus to the active site, suggesting that the closer interaction leads to increased strain on the already weak His207-Arg208 peptide bond. The relevance of the C-terminus structure for in vitro studies, and the physiological function of product release, is discussed.

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.