Time-resolved Absorption and UV Resonance Raman Spectra Reveal Stepwise Formation of T Quaternary Contacts in the Allosteric Pathway of Hemoglobin

Raman spectroscopy of protein pharmaceuticals

Recent advances in optical and spectroscopic technologies have enabled a plethora of Raman spectrometers that are suitable for studies of protein pharmaceuticals. Highly sensitive Raman spectrometers have overcome the handicap of the fundamentally weak Raman effect that hampered their applications to protein pharmaceuticals in the past. These Raman spectrometers can now routinely measure protein therapeutics at the low concentration of 1 mg/mL, which is on par with other spectroscopic methods such as CD, fluorescence and FTIR spectroscopies. In this article, various Raman techniques that can be used for protein pharmaceutical studies are reviewed. Novel Raman marker of proteins discovered from fundamental studies of protein complexes are examined along with established Raman spectra and structure correlations. Examples of Raman spectroscopic studies of protein pharmaceuticals are demonstrated. Future applications of Raman spectroscopy to protein pharmaceuticals are discussed.

Modulation of reactivity and conformation within the T-quaternary state of human hemoglobin: the combined use of mutagenesis and sol-gel encapsulation

A range of conformationally distinct functional states within the T quaternary state of hemoglobin are accessed and probed using a combination of mutagenesis and sol-gel encapsulation that greatly slow or eliminate the T --> R transition. Visible and UV resonance Raman spectroscopy are used to probe the proximal strain at the heme and the status of the alpha(1)beta(2) interface, respectively, whereas CO geminate and bimolecular recombination traces in conjunction with MEM (maximum entropy method) analysis of kinetic populations are used to identify functionally distinct T-state populations. The mutants used in this study are Hb(Nbeta102A) and the alpha99-alpha99 cross-linked derivative of Hb(Wbeta37E). The former mutant, which binds oxygen noncooperatively with very low affinity, is used to access low-affinity ligated T-state conformations, whereas the latter mutant is used to access the high-affinity end of the distribution of T-state conformations. A pattern emerges within the T state in which ligand reactivity increases as both the proximal strain and the alpha(1)beta(2) interface interactions are progressively lessened after ligand binding to the deoxy T-state species. The ligation and effector-dependent interplay between the heme environment and the stability of the Trp beta37 cluster in the hinge region of the alpha(1)beta(2) interface appears to determine the distribution of the ligated T-state species generated upon ligand binding. A qualitative model is presented, suggesting that different T quaternary structures modulate the stability of different alphabeta dimer conformations within the tetramer.

Dynamics of Allostery in Hemoglobin: Roles of the Penultimate Tyrosine H bonds

The tyrosine residues adjacent to the C termini of the hemoglobin (Hb) subunits, alphaY140 and betaY145, are expected to play important structural roles, because the C termini are the loci of T-state quaternary salt-bridges, and because the tyrosine side-chains bridge the H and F helices via H bonds to the alphaV93 and betaV98 carbonyl groups. These roles have been investigated via measurements of oxygen binding, (1)H NMR spectra, resonance Raman (RR) spectra, and time-resolved resonance Raman (TR(3)) spectra on site mutants in which the Hcdots, three dots, centeredF H bonds are eliminated by replacing the tyrosine residues with phenylalanine. The TR(3) spectra confirm the hypothesis, based on TR(3) studies of wild-type Hb, that the Hcdots, three dots, centeredF H bonds break and then re-form during the sub-microsecond phase of the R-T quaternary transition. The TR(3) spectra support the inference from other mutational studies that the alphabeta dimers act as single dynamic units in this early phase, motions of the E and F helices being coupled tightly across the dimer interface. Formation of T quaternary contacts occurs at about the same rate in the mutants as in HbA. However, these contacts are weakened substantially by the Y/F substitutions. Equilibrium perturbations are apparent also, especially for the alpha-subunits, in which relaxation of the Fe-His bond, strengthening of the Acdots, three dots, centeredE interhelical H bond, and weakening of the "switch" quaternary contact in deoxyHb are all apparent. Structural effects are less marked for the beta-chain Y/F replacement, but the Bohr effect is reduced by 25%, indicating that the salt-bridge and H bond interactions of the adjacent C terminus are loosened. The alpha-chain replacement reduces the Bohr effect much more, consistent with the global perturbations detected by the structure probes.

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 Trp alpha 14...Thr alpha 67 and Trp beta 15...Ser beta 72 H-bonds connect the A and E helices in the alpha and beta 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 Thr alpha 67 and Ser beta 72 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 alpha beta 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 alpha(1)beta(1) (and alpha(2)beta(2)) interface. The present observations establish that quaternary motions can occur on the approximately 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.