Human heme oxygenase-1 (hHO-1) is a critical enzyme responsible for the catabolism of free heme, a process essential to cellular homeostasis and protection against oxidative stress. The enzyme catalyzes the oxygen-dependent degradation of heme into biliverdin, ferrous iron, and carbon monoxide, with high regioselectivity directed toward the -meso-carbon of the porphyrin ring. This specificity is governed by a complex hydrogen bond network in the distal heme pocket, involving water molecules and the conserved aspartic acid residue D140. To investigate the structural and dynamic features of this active site, resonance Raman (rR) spectroscopy was employed using cyanide ligands as probes. Cyanide is an excellent probe due to its stability compared to physiologically relevant dioxygen adducts and peroxo/hydroperoxo intermediates. Its anionic nature allows strong hydrogen-bonding interactions, offering insight into off-axis distortions induced by the protein environment.
The Fe-C-N fragment in cyanide adducts of heme proteins typically adopts a linear geometry; however, steric, electrostatic, and hydrogen-bonding interactions within the distal pocket can induce tilting or bending. In this study, wild-type hHO-1 and its D140A, D140N, and D140E mutants were analyzed using natural abundance CN⁻ and isotopically labeled analogues (¹³CN⁻, ¹⁵N⁻, and ¹³C¹⁵N⁻). Deconvolution of rR spectra revealed two key vibrational modes: the (Fe-C) stretching mode at 454 cm⁻¹ and the (Fe-C-N) bending mode at 376 cm⁻¹.Epithienamycin A Biological Activity The spectral patterns, particularly the monotonic downshift of the (Fe-C) mode with increasing isotope mass and the higher frequency of (Fe-C) relative to (Fe-C-N), confirmed a tilted conformation of the Fe-C-N linkage.FTO Antibody Epigenetic Reader Domain Notably, this tilt is more pronounced than in other histidine-ligated heme proteins, indicating a unique orientation that positions the exogenous ligand toward the -meso-carbon—a feature crucial for the enzyme’s regioselective oxidation mechanism.PMID:35185550
Further analysis of the D140 mutants showed that while all retained the tilted geometry, they exhibited increased bending contributions, suggesting perturbations in the hydrogen bond network caused by the mutations. These findings imply that D140 plays a central role in maintaining the precise geometry required for catalytic activity. The persistence of the tilted configuration across pH conditions (pH 7.4 and 10) indicates the robustness of the distal site scaffold, which remains intact even when the distal water ligand is replaced by hydroxide. This stability supports the role of the hydrogen bond network not only in substrate positioning but also in proton transfer during catalysis.
These results are consistent with previous NMR studies showing a tilted conformer in hHO-1, yet contrast with crystallographic reports of both bent and tilted states in rat heme oxygenase. The discrepancy may arise from differences in experimental time scales—NMR captures dynamic equilibria, while X-ray structures may reflect metastable states. The current rR data, sensitive to multiple conformers, detected only one dominant tilted state, suggesting that the bent form observed in crystals might represent a transient pre-catalytic intermediate rather than the biologically relevant structure. Overall, this work underscores the importance of the distal site architecture in directing ligand orientation and highlights cyanide as a powerful tool for probing the functional dynamics of heme enzymes.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
