On the chemistry of pyrrole pigments, LXXXVII: The apomyoglobin heme pocket as a reaction vessel in bile pigment chemistry

The effects of biliverdin on pressure-induced unfolding of apomyoglobin: The specific role of Zn2+ ions

Apomyoglobin (apoMb), a model protein in biochemistry, exhibits a strong propensity to bind various ligands, which makes it a good candidate as a carrier of bioactive hydrophobic drugs. The stability of its hydrophobic pocket determines its potential as a carrier of bioactive compounds. High pressure (HP) is a potent tool for studying protein stability, revealing the specific role of hydrophobic cavities in unfolding. We probed the effects of biliverdin (BV) binding and its complex with Zn²⁺ ions on the structure and HP stability of apoMb. CD spectroscopy and SAXS measurements revealed that BV and BV-Zn²⁺ complexes make the apoMb structure more compact with higher α-helical content. We performed in-situ HP measurements of apoMb intrinsic fluorescence to demonstrate the ability of BV to stabilise apoMb structure at HP conditions. Furthermore, the presence of Zn²⁺ within the apoMb-BV complex significantly enhances the BV stabilisation effect. In-situ visible absorption study of BV chromophore confirmed the ability of Zn²⁺ to increase the stability of apoMb-BV complex under HP: the onset of complex dissociation is shifted by ~100 MPa in the presence of Zn²⁺. By combining HP-fluorescence and HP-visible absorption spectroscopy, our strategy highlights the crucial role of tetrapyrrole-metal complexes in stabilising apoMb hydrophobic pocket.

Myoglobin with modified tetrapyrrole chromophores: binding specificity and photochemistry

Complexes were prepared of horse heart myoglobin with derivatives of (bacterio)chlorophylls and the linear tetrapyrrole, phycocyanobilin. Structural factors important for binding are (i) the presence of a central metal with open ligation site, which even induces binding of phycocyanobilin, and (ii) the absence of the hydrophobic esterifying alcohol, phytol. Binding is further modulated by the stereochemistry at the isocyclic ring. The binding pocket can act as a reaction chamber: with enolizable substrates, apo-myoglobin acts as a 13(2)-epimerase converting, e.g., Zn-pheophorbide a' (13(2)S) to a (13(2)R). Light-induced reduction and oxidation of the bound pigments are accelerated as compared to solution. Some flexibility of the myoglobin is required for these reactions to occur; a nucleophile is required near the chromophores for photoreduction (Krasnovskii reaction), and oxygen for photooxidation. Oxidation of the bacteriochlorin in the complex and in aqueous solution continues in the dark.

Conformation inversion of bilirubin formed by reduction of the biliverdin-human serum albumin complex: evidence from circular dichroism

As shown by circular dichroism spectroscopy, biliverdin preferentially adopts an M-helicity conformation on human serum albumin in aqueous buffer, pH 7.5, whereas biliverdin exhibits only a weak preference for the P-helicity conformation on bovine serum albumin at the same pH. Upon rapid reduction of the complexes with sodium borohydride, P-helicity bilirubin-IX alpha is obtained on the human albumin complex, and M-helicity bilirubin-IX alpha is obtained on the bovine serum albumin complex. Thus, biliverdin in effect undergoes an inversion of chirality upon reduction. Since the reduction did not afford a rubin with the same helicity as that of the verdin, the observations point to a hitherto undetected conformational mobility of albumin-bound bilirubin.