The molecular code for hemoglobin allostery revealed by linking the thermodynamics and kinetics of quaternary structural change. 2. Cooperative free energies of (alphaFeCObetaFe)2 and (alphaFebetaFeCO)2 T-state tetramers.
Ligand photodissociation experiments are used to measure the prephotolysis equilibria between doubly liganded R and T quaternary conformers of the symmetric Fe-Co HbCO hybrids, (alpha(FeCO)beta(Co))(2) and (alpha(Co)beta(FeCO))(2). The free energies obtained from these data are used to calculate the cooperative free energies of the (alpha(FeCO)beta(Fe))(2) and (alpha(Fe)beta(FeCO))(2) intermediate CO-ligation states of normal hemoglobin in the T conformation, quantities important to the evaluation of current models of cooperativity. The symmetry rule model, incorporating sequential cooperativity of T-state ligand binding within an alphabeta dimer in addition to the traditional two-state cooperativity of the tetramer, predicts a larger free energy penalty for disturbing both dimers in a doubly liganded T tetramer than would be expected in the two-state model as currently formulated. (Cooperative energy penalties are simply proportional to the number of tetramer-bound ligands in the traditional two-state model.) The value found here for the energies of doubly liganded T microstates in which both dimers are perturbed, 7.9 +/- 0.3 kcal/mol, is consistent with the symmetry rule model but significantly higher than that expected (5-6 kcal/mol) in the two-state model of cooperativity.
Goldbeck
Robert A
RA
Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA. goldbeck@chemistry.ucsc.edu
Esquerra
Raymond M
RM
Kliger
David S
DS
Holt
Jo M
JM
Ackers
Gary K
GK
D000494:Allosteric Regulation; D002248:Carbon Monoxide / Q000378:metabolism*; D003035:Cobalt / Q000378:metabolism; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism*; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D010782:Photolysis; D020836:Protein Structure, Quaternary; D013057:Spectrum Analysis; D013816:Thermodynamics
D006454:Hemoglobins; D008024:Ligands; D003035:Cobalt; D002248:Carbon Monoxide; D007501:Iron