T state, of hemoglobin

CPR can be used to find continuous paths for complex transitions that might have hundreds of saddle points and need to be described by thousands of path points. Examples of such transitions include the quaternary transition between the R and T states of hemoglobin [57] and the reorganization of the retinoic acid receptor upon substrate entry [58]. Because CPR yields the exact saddle points as part of the path, it can also be used in conjunction with nonnal mode analysis to estimate the vibrational entropy of activation... 

This reaction is driven to the right in tissues by the high COg concentration the equilibrium shifts the other way in the lungs where [COg] is low. Thus, car-bamylation of the N-termini converts them to anionic functions, which then form salt links with the cationic side chains of Arg al41 that stabilize the deoxy or T state of hemoglobin. 

The transition between the T and R states of hemoglobin is also deeply involved in the Bohr effect and cooperativity. Therefore stabilization of either of the two stmctures should diminish these effects, which have important physiologic consequences. The clinical consequences of stabilization are not known. 

For many years hemoglobin was the only allosteric protein whose stereochemical mechanism was understood in detail. However, more recently detailed structural information has been obtained for both the R and the T states of several enzymes as well as one genetic repressor system, the trp-repressor, described in Chapter 8. We will here examine the structural differences between the R and the T states of a key enzyme in the glycolytic pathway, phosphofructokinase. 

Unzai S et al Rate constants for O2 and CO binding to the alpha and beta subunits within the R and T states of human hemoglobin. J Biol Chem 1998 273 23150. 

FIGURE 5-9 Some ion pairs that stabilize the T state of deoxyhemoglobin. (a) A close-up view of a portion of a deoxyhemoglobin molecule in theT state (PDB ID 1HGA). Interactions between the ion pairs His HC3 and Asp FG1 of the 0 subunit (blue) and between Lys C5 of the a subunit (gray) and His HC3 (its cr-carboxyl group) of the )3 subunit are shown with dashed lines. (Recall that HC3 is the carboxyl-terminal residue of the 0 subunit.) (b) The interactions between these ion pairs, and between others not shown in (a), are schematized in this representation of the extended polypeptide chains of hemoglobin. 

Biochemists now know a great deal about the T and R states of hemoglobin, but much remains to be learned about how the T —> R transition occurs. Two models for the cooperative binding of ligands to proteins with multiple binding sites have greatly influenced thinking about this problem. 

Fig. 2. Quaternary structural transition in hemoglobin. (A) Subunit motion of hemoglobin in going from the deoxy (or T conformation) to the oxy (or R conformation) state (B) side views of deoxy and oxy states of hemoglobin. In oxy-Hb, the oiiPi dimer is rotated 15° relative to the a2p2 dimer. [Adapted from Dickerson and Geis (1983) illustration copyright by I. Geis],...

Aime, S., Ascenzi, R, Comoglio, E., et al. (1995) Molecular recognition of R- and T-states of human adult hemoglobin by a paramagnetic Gd(II 1) complex by means of the measurement of solvent water proton relaxation rate. Journal of the American Chemical Society, 117, 9365-9366. 

Mathews. A.J. Olson. J.S. Assignment of rate constants for O2 and CO binding to alpha and beta subunits within R-and T-state human hemoglobin. Methods Enzymol. 1994, 232. 363-386. 

A further check was obtained through the study of the effects of alcohols on the oxygen affinity of hemoglobin at various temperatures. This allowed the determination of non bulk-electrostatic contributions, due to the presence of alcohols, to the standard enthalpy and entropy differences between T and R states of hemoglobin. 

Reactivity. Hemoglobin can exist ia either of two stmctural coaformatioas, corresponding to the oxy (R, relaxed) or deoxy (T, tense) states. The key differences between these two stmctures are that the constrained T state has a much lower oxygen affinity than the R state and the T state has a lower tendency to dissociate into subunits that can be filtered in the kidneys. Therefore, stabilization of the T conformation would be expected to solve both the oxygen affinity and renal excretion problems. 

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