Metals content of Glossoscolex paulistus extracellular hemoglobin: Its peroxidase activity and the importance of these ions in the protein stability

Biorecognition of hydrogen peroxide using a novel electrochemical platform designed with Glossoscolex paulistus giant hemoglobin

The giant extracellular hemoglobin of the annelid Glossoscolex paulistus (HbGp; 3.6 MDa) is a valuable and underexplored supramolecular hemoprotein system for the biorecognition of reactive oxygen species. In this work, an efficient and simple electrochemical platform was designed for analyzing H₂O₂, using HbGp covalently immobilized on Nafion®-modified glassy carbon electrode, named as HbGp/Nafion/GCE. Voltammetric and spectroscopic studies revealed the importance of prior modification of the electrodic support with the conducting polymer to obtain satisfactory hemoglobin electroactivity, as well as a biocompatible microenvironment for its immobilization. In terms of biological activity, it was observed a greater reactivity of the biomolecule in acidic medium, enabling the detection of the analyte by a quasi-reversible mechanism, whose kinetics was limited by analyte diffusion. In the presence of H₂O₂, the native structure of hemoglobin (oxy-HbGp (Fe²⁺)) oxidizes to ferryl-HbGp (Fe⁴⁺) and this redox reaction can be monitored on HbGp/Nafion/GCE with a detection limit of 8.5 × 10^‒7  mol L^-1. In addition to high sensitivity, the electrochemical biosensor also provided reproducible, consistent, and accurate measurements. The electroanalytical method showed an appropriate performance to quantify different levels of H₂O₂ in milk samples, proving the potential of HbGp/Nafion/GCE for this purpose.

Oligomeric stability of Glossoscolex paulistus hemoglobin as a function of the storage time

Glossoscolex paulistus hemoglobin structure is composed of 144 globin chains and 36 polypeptide chains lacking the heme group, with a total molecular mass of 3600 kDa. The current study focuses on the oxy-HbGp oligomeric stability, as a function of the storage time, at pH 7.0, using dynamic light scattering, analytical ultracentrifugation (AUC), optical absorption and size exclusion chromatography (SEC). HbGp stored in Tris-HCl buffer, pH 7.0, at 4 °C, for two years remains in the native form, while 4-6 years HbGp stocks present typical hemichrome species absorption spectra. AUC and SEC analyses show that the contribution of HbGp-subunits, such as, dodecamer (abcd)₃, tetramer abcd, trimer abc and monomer d, increases with the protein aging due to the lower stability of the HbGp with the time. The dissociation and the oxidation of the iron noted for the older protein solutions indicate that HbGp storage for periods of time longer than two years changes its ability to carry oxygen. Despite the reduction of HbGp stability and oxygen carrying capacity with aging, the protein stability is still larger as compared to mammalian hemoglobins. Thus, the extracellular hemoglobins are quite stable and resistant to the auto-oxidation process, making them of interest for biotechnological applications.

Specific heme binding to heme regulatory motifs in iron regulatory proteins and its functional significance

Iron regulatory proteins (IRPs) control iron metabolism in mammalian cells by binding to the iron-responsive element (IRE) in the target mRNA. Heme regulatory motifs (HRMs) are conserved in the two IRP homologues IRP1 and IRP2 that specifically bind to two and three heme equivalents, respectively; however, only the heme binding to the iron-dependent degradation (IDD) domain of IRP2 causes heme-mediated oxidation, which does not occur in IRP1. Therefore, the functional significance of conserved HRMs outside the IDD domain is yet unclear. In this study, spectroscopic heme titration with IRP mutants confirmed heme binding to each HRM in IRPs, and the effect of heme binding to HRMs on IRE binding was examined. Native polyacrylamide gel electrophoresis analysis revealed that heme binding to HRMs near the IRE binding site inhibits complex formation between IRPs and IRE without oxidative modification, indicating that the function of HRMs varies outside and within the IDD domain. However, the formation of a typical reactive oxygen species (ROS), hydrogen peroxide, was spectroscopically detected in both heme-bound IRPs. Comparing the heme environmental structures surrounding HRMs, the flexible conformation and many amino acid residues sensitive to ROS of the IDD domain were suggested to promote specific oxidation by the generated hydrogen peroxide. Thus, heme binding to HRM near the IRE binding site sterically interferes with IRE binding, while HRM in the IDD domain facilitates specific heme-mediated oxidation of the protein moiety and the protein degradation via the ubiquitin-proteasome system, resulting in the inhibition of IRE binding.

Interaction of Glossoscolex paulistus extracellular hemoglobin with hydrogen peroxide: Formation and decay of ferryl-HbGp

The giant extracellular hemoglobin from earthworm Glossoscolex paulistus (HbGp) reacts with hydrogen peroxide, displaying peroxidase activity in the presence of guaiacol. The formation of ferryl-HbGp (compound II) from the peroxidase cycle was studied in the present work. The hypervalent ferryl-HbGp species was formed directly by the reaction of oxy-HbGp and hydrogen peroxide. The oxy-HbGp heme groups (144) under different excess of H₂O₂, relative to heme, showed an influence in the total amount of ferryl-HbGp at the end of the reaction. The ferryl-HbGp was formed with second order rate constant of 27.1±0.5M^-1s^-1, at pH7.0 and 25°C. The increase of the pH value to 8.0 induces both faster formation and decay of ferryl-HbGp, together with oligomeric dissociation induced by the presence of H₂O₂, as observed by DLS. This effect of dissociation increases the heme exposure and decreases the ferryl-HbGp stability, affecting the rate constant as a parallel reaction. At pH7.0, high excess of H₂O₂, above 1:5 oxy-HbGp heme: H₂O₂, produces the aggregation of the protein. Our results show for the first time, for an extracellular giant hemoglobin, the possible effects of oxidative stress induced by hydrogen peroxide.