Fig. 4: Heme can bind to the SARS-CoV-2 and MHV Spike proteins.

A Homologous proteins and viral receptors between SARS-CoV-2 and MHV-A59. B (i) Visualization of the final MHV S protein-CEACAM1a complex from top view. (ii) Visualization of the final MHV S protein-CEACAM1a complex from side view. (iii) 2D view of the interactions between heme and the NTD binding site on the left, 3D representation of the docked heme in the beta subunit of the MHV sNTD on the right. S protein is shown in gray. CEACAM1a D1 subunit is shown in green. Heme molecules are shown in dark blue spheres. Transparent overlays represent molecular surfaces of MHV S protein and CEACAM1a. C Overlap of heme—MHV S protein complex (in gray, predicted by docking) and experimental biliverdin - SARS-CoV-2 S protein complex (in blue); residues involved in conserved interactions are represented as sticks and labeled. D (i) Visual comparison of the superposed SARS-CoV-2 sNTD (light green) and MHV-sNTD (cyan). (ii) Comparison of the Val188-Asp200 loop in the MHV-sNTD before (dark red) and after (green) manually re-sampling the loop conformation, in the presence of the biliverdin molecule (dark blue) from the SARS-CoV-2 sNTD template (PDB 7BS2). E (i) UV–visible spectra of hemin alone (10 µM, red) or in the presence of increasing amounts of purified SARS-CoV-2 Spike (S1: 1.8 µM in light blue, S2: 3.6 µM in blue, S3: 7.2 µM in dark blue). (ii) Determination of binding affinities (K_d value) between hemin (10 µM) and purified recombinant SARS-CoV-2 Spike protein (S1: 1.8 µM in light blue, S2: 3.6 µM in blue, S3: 7.2 µM in dark blue) by UV–Visible spectrophotometry. Absorbance at 398 nm, which corresponds to Soret band peak, was plotted to determine Kd using GraphPad Software. Results are shown as the mean ± SEM. Data are representative of three independent experiments.