Dynamics of Quaternary Structure Transitions in R‑State Carbonmonoxyhemoglobin Unveiled in Time-Resolved X‑ray Scattering Patterns Following a Temperature Jump

• Published in: The journal of physical chemistry. B Vol. 122; no. 49; pp. 11488 - 11496
• Main Authors: Cho, Hyun Sun, Schotte, Friedrich, Stadnytskyi, Valentyn, DiChiara, Anthony, Henning, Robert, Anfinrud, Philip
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13-12-2018
• Subjects: Carboxyhemoglobin - chemistry; Erythrocytes - chemistry; Humans; Kinetics; Molecular Dynamics Simulation; Protein Conformation; Scattering, Small Angle; Temperature; Time Factors; X-Ray Diffraction
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.8b07414

It is well-known that tetrameric hemoglobin binds ligands cooperatively by undergoing a ligand-induced T → R quaternary structure transition, a structure–function relationship that has long served as a model system for understanding allostery in proteins. However, kinetic studies of the reverse, R → T quaternary structure transition following photolysis of carbonmonoxyhemoglobin (HbCO) reveal complex behavior that may be better explained by the presence of two different R quaternary structures coexisting in thermal equilibrium. Indeed, we report here time-resolved small- and wide-angle X-ray scattering (SAXS/WAXS) patterns of HbCO following a temperature jump that not only provide unambiguous evidence for more than one R state, but also unveil the time scale for interconversion between them. Since the time scale for the photolysis-induced R → T transition is likely different for different R-states, this structural heterogeneity must be accounted for to properly explain the kinetic heterogeneity observed in time-resolved spectroscopic studies following photolysis of HbCO.