Binding interaction of hypocrellin B to myoglobin: A spectroscopic and computational study

Hypocrellin B (Hyp B), a perylenequinone naturally present in Hypocrella bambusae, is commonly used to treat a variety of diseases. Its versatile role in different biomedical applications necessitates a thorough investigation of its interaction with different biomolecules, particularly enzymes. To address this need, the binding mode of Hyp B to myoglobin (Mb) was studied using UV-visible absorption, emission, and synchronous fluorescence spectroscopies, as well as flexible docking simulations. Analyses of the absorbance and fluorescence data establish that Hyp B quenches tyrosine (Tyr) and tryptophan (Trp) fluorescence via the formation of two unique ground-state complexes on the surface of Mb, with one site being more energetically preferred than the other (the fraction of fluorophores accessible by Hyp B is 0.32). Molecular modeling simulations demonstrate preferential Hyp B binding at the Tyr103 site first, followed by the Trp7 site. In both cases, a ground-state complex is generated through H-bonding interaction between Hyp B and the respective residues, with the Tyr103 complex being more stable than that of the Trp7 complex. Synchronous fluorescence measurements indicate that the microenvironment surrounding Trp7 becomes more hydrophilic upon Hyp B interaction. This is evidenced by a red-shift of the band associated with this residue, while that of Tyr103 remains the same. Electrostatic potential surfaces reveal a more pronounced shift in electron density of Trp7 upon Hyp B binding compared to Tyr103. The binding constant of Hyp B to Mb is 1.21 × 10⁵ M ¹, suggesting a relatively strong interaction between the ligand and enzyme.