Chimeric beta-EF3-alpha hemoglobin (Psi): energetics of subunit interaction and ligand binding

Coexpression of human alpha- and circularly permuted beta-globins yields a hemoglobin with normal R state but modified T state properties

For the first time, a circularly permuted human beta-globin (cpbeta) has been coexpressed with human alpha-globin in bacterial cells and shown to associate to form alpha-cpbeta hemoglobin in solution. Flash photolysis studies of alpha-cpbeta show markedly biphasic CO and O(2) kinetics with the amplitudes for the fast association phases being dominant due the presence of large amounts of high-affinity liganded hemoglobin dimers. Extensive dimerization of liganded but not deoxygenated alpha-cpbeta was observed by gel chromatography. The rate constants for O(2) and CO binding to the R state forms of alpha-cpbeta are almost identical to those of native HbA (k'(R(CO)) approximately 5.0 microM(-1) s(-1); k'(R(O(2))) approximately 50 microM(-1) s(-1)), and the rate of O(2) dissociation from fully oxygenated alpha-cpbeta is also very similar to that observed for HbA (k(R(O(2))) approximately 21-28 s(-1)). When the equilibrium deoxyHb form of alpha-cpbeta is reacted with CO in rapid mixing experiments, the observed time courses are monophasic and the observed bimolecular association rate constant is approximately 1.0 microM(-1) s(-1), which is intermediate between the R state rate measured in partial photolysis experiments (approximately 5 microM(-1) s(-1)) and that observed for T state deoxyHbA (k'(T(CO)) approximately 0.1 to 0.2 microM(-1) s(-1)). Thus the deoxygenated permutated beta subunits generate an intermediate, higher affinity, deoxyHb quaternary state. This conclusion is supported by equilibrium oxygen binding measurements in which alpha-cpbeta exhibits a P(50) of approximately 1.5 mmHg and a low n-value (approximately 1.3) at pH 7, 20 degrees C, compared to 8.5 mmHg and n approximately 2.8 for native HbA under identical, dilute conditions.

Computationally accessible method for estimating free energy changes resulting from site-specific mutations of biomolecules: systematic model building and structural/hydropathic analysis of deoxy and oxy hemoglobins

A practical computational method for the molecular modeling of free-energy changes associated with protein mutations is reported. The de novo generation, optimization, and thermodynamic analysis of a wide variety of deoxy and oxy hemoglobin mutants are described in detail. Hemoglobin is shown to be an ideal candidate protein for study because both the native deoxy and oxy states have been crystallographically determined, and a large and diverse population of its mutants has been thermodynamically characterized. Noncovalent interactions for all computationally generated hemoglobin mutants are quantitatively examined with the molecular modeling program HINT (Hydropathic INTeractions). HINT scores all biomolecular noncovalent interactions, including hydrogen bonding, acid-base, hydrophobic-hydrophobic, acid-acid, base-base, and hydrophobic-polar, to generate dimer-dimer interface "scores" that are translated into free-energy estimates. Analysis of 23 hemoglobin mutants, in both deoxy and oxy states, indicates that the effects of mutant residues on structurally bound waters (and visa versa) are important for generating accurate free-energy estimates. For several mutants, the addition/elimination of structural waters is key to understanding the thermodynamic consequences of residue mutation. Good agreement is found between calculated and experimental data for deoxy hemoglobin mutants (r = 0.79, slope = 0.78, standard error = 1.4 kcal mol(-1), n = 23). Less accurate estimates were initially obtained for oxy hemoglobin mutants (r = 0.48, slope = 0.47, standard error = 1.4 kcal mol(-1), n = 23). However, the elimination of three outliers from this data set results in a better correlation of r = 0.87 (slope = 0.72, standard error = 0.75, n = 20). These three mutations may significantly perturb the hemoglobin quaternary structure beyond the scope of our structural optimization procedure. The method described is also useful in the examination of residue ionization states in protein structures. Specifically, we find an acidic residue within the native deoxy hemoglobin dimer-dimer interface that may be protonated at physiological pH. The final analysis is a model design of novel hemoglobin mutants that modify cooperative free energy (deltaGc)--the energy barrier between the allosteric transition from deoxy to oxy hemoglobin.