Hemoglobin Site-mutants Reveal Dynamical Role of Interhelical H-bonds in the Allosteric Pathway: Time-resolved UV Resonance Raman Evidence for Intra-dimer Coupling

The dynamical effect of eliminating specific tertiary H-bonds in the hemoglobin (Hb) tetramer has been investigated by site-directed mutagenesis and time-resolved absorption and ultraviolet resonance Raman (UVRR) spectroscopy. The Trpα14⋯Thrα67 and Trpβ15⋯Serβ72 H-bonds connect the A and E helices in the α and β chains, and are proposed to break in the earliest protein intermediate (Rdeoxy) following photo-deligation of HbCO, along with a second pair of H-bonds involving tyrosine residues. Mutation of the acceptor residues Thrα67 and Serβ72 to Val and Ala eliminates the A–E H-bonds, but has been shown to have no significant effect on ligand-binding affinity or cooperativity, or on spectroscopic markers of the T-state quaternary interactions. However, the mutations have profound and unexpected effects on the character of the Rdeoxy intermediate, and on the dynamics of the subsequent steps leading to the T state. Formation of the initial quaternary contact (RT intermediate) is accelerated, by an order of magnitude, but the locking-in of the T state is delayed by a factor of 2. These rate effects are essentially the same for either mutation, or for the double mutation, suggesting that the αβ dimer behaves as a mechanically coupled dynamical unit. Further evidence for intra-dimer coupling is provided by the Rdeoxy UVRR spectrum, in which either or both mutations eliminate the tyrosine difference intensity, although only tryptophan H-bonds are directly affected. A possible mechanism for mechanical coupling is outlined, involving transmission of forces through the α1β1 (and α2β2) interface. The present observations establish that quaternary motions can occur on the ∼100 ns time-scale. They show also that a full complement of interhelical H-bonds actually slows the initial quaternary motion in Hb, but accelerates the locking in of the T-contacts.