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BIOLOGICAL SCIENCES / BIOPHYSICS
Allosteric action in real time: Time-resolved crystallographic studies of a cooperative dimeric hemoglobin

James E. Knapp, Reinhard Pahl, Vukica rajer^,,¶, and William E. Royer, Jr.^,¶

Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01655; and Consortium for Advanced Radiation Sources and Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, IL 60637

Edited by Robin M. Hochstrasser, University of Pennsylvania, Philadelphia, PA, and approved April 3, 2006 (received for review October 28, 2005)

Protein allostery provides mechanisms for regulation of biological function at the molecular level. We present here an investigation of global, ligand-induced allosteric transition in a protein by time-resolved x-ray diffraction. The study provides a view of structural changes in single crystals of Scapharca dimeric hemoglobin as they proceed in real time, from 5 ns to 80 µs after ligand photodissociation. A tertiary intermediate structure forms rapidly (<5 ns) as the protein responds to the presence of an unliganded heme within each R-state protein subunit, with key structural changes observed in the heme groups, neighboring residues, and interface water molecules. This intermediate lays a foundation for the concerted tertiary and quaternary structural changes that occur on a microsecond time scale and are associated with the transition to a low-affinity T-state structure. Reversal of these changes shows a considerable lag as a T-like structure persists well after ligand rebinding, suggesting a slow T-to-R transition.

allosteric protein transitions | intersubunit communication | kinetics

Conflict of interest statement: No conflicts declared.

This paper was submitted directly (Track II) to the PNAS office.

Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, www.pdb.org (PDB ID codes 2GRF for M37V, 2GRH for M37V-CO, and 2GRZ for 5-ns photoproduct).

^¶To whom correspondence may be addressed. E-mail: william.royer{at}umassmed.edu or v-srajer{at}uchicago.edu

© 2006 by The National Academy of Sciences of the USA

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This article has been cited by other articles in HighWire Press-hosted journals:

				R. Elber A Milestoning Study of the Kinetics of an Allosteric Transition: Atomically Detailed Simulations of Deoxy Scapharca Hemoglobin Biophys. J., May 1, 2007; 92(9): L85 - L87. [Abstract] [Full Text] [PDF]

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