Sulfmyoglobin Conformational Change: A Role in the Decrease of Oxy-Myoglobin Functionality

Abstract

This work is focused at understanding the interaction of H₂S with Myoglobin (Mb), in particular the Sulfmyoglobin (SMb) product, whose physiological role is controversial and not well understood. The scattering curves, Guinier, Kratky, Porod and P(r) plots were analyzed for oxy-Mb and oxy-Hemoglobin I (oxyHbI) in the absence and presence of H₂S, using Small and Wide Angle X-ray Scattering (SAXS/WAXS) technique. Three dimensional models were also generated from the SAXS/WAXS data. The results show that SMb formation, produced by oxyMb and H₂S interaction, induces a change in the protein conformation where its envelope has a very small cleft and the protein is more flexible, less rigid and compact. Based on the direct relationship between Mb's structural conformation and its functionality, we suggest that the conformational change observed upon SMb formation plays a contribution to the protein decrease in O₂ affinity and, therefore, on its functionality.

Keywords: SAXS; WAXS; hemoglobin I (HbI); hydrogen sulfide (H2S); myoglobin (Mb); sulfmyoglobin (SMb).

Graphical abstract

Fig. 1

Sulfmyoglobin (SMb) Structure. Myoglobin (A, left) and SMb isomer C (B, right); PDB files 1MBO (Mb) and 1YMC (SMb).

Fig. 2

SAXS and WAXS analysis. Left panel is oxyHbI in the absence (black line) and presence of H₂S (red line). Right panel is oxyMb (black line) and SMb (red line, product of oxyMb and H₂S interaction). A and B; SAXS/WAXS scattering curves (showing q range of 0.19–1.0 Å⁻¹), with their corresponding Guinier Plot (inset; q range =0.030–0.072 Å⁻¹ for HbI and 0.038–0.072 Å⁻¹ for Mb). C and D; P(r) plots normalized to I(0) (q range =0.024–0.442 Å⁻¹ for oxyHbI, 0.039–0.44 Å⁻¹ for oxyHbI in the presence of H₂S, 0.025–0.5 Å⁻¹ for oxyMb and 0.036–0.05 Å⁻¹ for SMb). E and F; Dimensionless Kratky plots (q range =0.02–0.26 Å⁻¹ ). G and H; Porod plots (q range =0.02–0.16 Å⁻¹). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

SAXS/WAXS three dimensional models. A; SAXS/WAXS three-dimensional surfaces of oxyMb (top-left) and SMb (bottom-left). B; Comparison of the experimental SAXS/WAXS data of oxyMb (black-dotted lines) and SMb (red-dotted lines) with the corresponding calculated scattering curves of their theoretical models (solid lines). The q range used for the theoretical models was the same as those used in the P(r) function. The curves were offset for better appraisal (oxyMb has x²=0.8 and R²=0.99984; SMb has x²=2.2 and R²=0.99975). C; Residual plots of the experimental minus the calculated scattering curves of oxyMb (black line) and SMb (red line) theoretical models.

Fig. 4

Comparison of the SAXS/WAXS and crystallographic data. Left panel is oxyMb from the SAXS/WAXS experimental data (red-line) and crystal structure (black-dotted line), PDB file 1MBO. Right panel is SMb from the SAXS/WAXS experimental data (red-line) and crystal structure (black-dotted line), PDB file 1 YMC. A and B; Scattering curves. C and D; The corresponding P(r) plots.