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Ordered water molecules as key allosteric mediators in a cooperative dimeric hemoglobin
W E Royer1, A Pardanani, Q H Gibson
1Program in Molecular Medicine, University of Massachusetts Medical Center, Worcester 01605, USA.
Summary
Interfacial water molecules in Scapharca dimeric hemoglobin are crucial for subunit communication. Disrupting these water clusters significantly alters oxygen affinity and cooperativity, revealing their vital role in hemoglobin function.
Area of Science:
- Biochemistry
- Structural Biology
- Protein Dynamics
Background:
- Scapharca dimeric hemoglobin exhibits a unique ordered water cluster at the subunit interface.
- The role of these interfacial water molecules in ligand binding and allosteric communication remains largely unexplored.
Purpose of the Study:
- To investigate the functional significance of interfacial water molecules in Scapharca dimeric hemoglobin.
- To elucidate the contribution of water clusters to oxygen binding affinity and cooperativity.
Main Methods:
- Site-directed mutagenesis of key interface residues (Thr-72 to Val).
- Osmotic stress techniques using various solutes (glycerol, glucose, sucrose).
- Raman spectroscopy to monitor structural changes in deoxy hemoglobin.
Main Results:
- The Thr-72-->Val mutation increased oxygen affinity over 40-fold and enhanced cooperativity by destabilizing two interfacial water molecules.
- Wild-type Scapharca hemoglobin showed increased oxygen affinity under increased osmotic pressure, contrasting with human hemoglobin.
- An estimated six water molecules are lost upon oxygen binding, aligning with crystallographic predictions.
Conclusions:
- Interfacial water molecules play a critical role in mediating communication between subunits in Scapharca dimeric hemoglobin.
- Destabilization of water clusters significantly impacts hemoglobin's allosteric regulation.
- Hemoglobin's response to osmotic pressure is dependent on its specific structural and functional properties.