title: Role of Heme Pocket Water in Allosteric Regulation of Ligand Reactivity in Human Hemoglobin.
abstract: Water molecules can enter the heme pockets of unliganded myoglobins and hemoglobins, hydrogen bond with the distal histidine, and introduce steric barriers to ligand binding. The spectrokinetics of photodissociated CO complexes of human hemoglobin and its isolated α and β chains were analyzed for the effect of heme hydration on ligand rebinding. A strong coupling was observed between heme hydration and quaternary state. This coupling may contribute significantly to the 20-60-fold difference between the R- and T-state bimolecular CO binding rate constants and thus to the modulation of ligand reactivity that is the hallmark of hemoglobin allostery. Heme hydration proceeded over the course of several kinetic phases in the tetramer, including the R to T quaternary transition. An initial 150 ns hydration phase increased the R-state distal pocket water occupancy, nw(R), to a level similar to that of the isolated α (∼60%) and β (∼10%) chains, resulting in a modest barrier to ligand binding. A subsequent phase, concurrent with the first step of the R → T transition, further increased the level of heme hydration, increasing the barrier. The final phase, concurrent with the final step of the allosteric transition, brought the water occupancy of the T-state tetramer, nw(T), even higher and close to full occupancy in both the α and β subunits (∼90%). This hydration level could present an even larger barrier to ligand binding and contribute significantly to the lower iron reactivity of the T state toward CO.
authors.affiliation: Department of Chemistry and Biochemistry, San Francisco State University , San Francisco, California 94132, United States.
authors.forename: Raymond M
authors.initials: RM
authors.lastname: Esquerra
authors.affiliation: Department of Chemistry and Biochemistry, San Francisco State University , San Francisco, California 94132, United States.
authors.forename: Bushra M
authors.initials: BM
authors.lastname: Bibi
authors.affiliation: Department of Chemistry and Biochemistry, San Francisco State University , San Francisco, California 94132, United States.
authors.forename: Pooncharas
authors.initials: P
authors.lastname: Tipgunlakant
authors.affiliation: Department of Biochemistry and Cell Biology and W. M. Keck Center for Computational Biology, Rice University , Houston, Texas 77005, United States.
authors.forename: Ivan
authors.initials: I
authors.lastname: Birukou
authors.affiliation: Department of Biochemistry and Cell Biology and W. M. Keck Center for Computational Biology, Rice University , Houston, Texas 77005, United States.
authors.forename: Jayashree
authors.initials: J
authors.lastname: Soman
authors.affiliation: Department of Biochemistry and Cell Biology and W. M. Keck Center for Computational Biology, Rice University , Houston, Texas 77005, United States.
authors.forename: John S
authors.initials: JS
authors.lastname: Olson
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Santa Cruz, California 95064, United States.
authors.forename: David S
authors.initials: DS
authors.lastname: Kliger
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Santa Cruz, California 95064, United States.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Goldbeck
mesh_terms: D000494:Allosteric Regulation; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000737:chemistry*; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D010782:Photolysis; D020836:Protein Structure, Quaternary; D014867:Water / Q000737:chemistry; D055542:alpha-Globins / Q000737:chemistry; D055544:beta-Globins / Q000737:chemistry
chemical_list: D006454:Hemoglobins; D008024:Ligands; D055542:alpha-Globins; D055544:beta-Globins; D014867:Water; D006418:Heme