Quaternary-Linked Changes in Structure and Dynamics That Modulate O2 Migration within Hemoglobin's Gas Diffusion...

Area of Science:

• Biophysics
• Computational Biology
• Protein Dynamics

Background:

• Human hemoglobin (HbA) is crucial for oxygen transport.
• Understanding oxygen diffusion pathways within HbA is key to its function.
• Crystal structures do not fully capture the dynamic nature of gas transport in HbA.

Purpose of the Study:

• To investigate oxygen (O2) diffusion pathways from heme to solvent in human hemoglobin (HbA) using atomistic molecular dynamics simulations.
• To elucidate the role of HbA's quaternary states (T, R, R2) in modulating O2 escape routes.
• To identify the structural and dynamic factors influencing O2 distribution within HbA.

Main Methods:

• Atomistic molecular dynamics simulations of O2 diffusion in HbA tetramers.
• Analysis of transient gas tunnels and Xe binding cavities.
• Examination of subunit dynamics and heme positioning across different quaternary states.
• Mutagenesis simulations (HisE7 to Tryptophan) to study O2 migration adjustments.

Main Results:

• Transient gas tunnels, not observed in crystal structures, facilitate O2 diffusion.
• Tunnel topology is conserved across T, R, and R2 states, but O2 escape portals vary significantly.
• Quaternary structure-dependent changes in subunit dynamics and β-heme position modulate O2 distribution.
• Distal histidine conformation remains closed, yet significant O2 escape occurs via distal portals and interior tunnels.
• Mutations highlight adaptive O2 migration and provide insights into ligand binding kinetics.

Conclusions:

• HbA exhibits significant gas porosity across its T, R, and R2 quaternary states.
• Quaternary state transitions dynamically regulate O2 escape pathways, optimizing oxygen delivery.
• Simulations provide novel mechanistic insights into HbA's allosteric regulation of gas transport.