T-state hemoglobin with four ligands bound.
Flash photolysis kinetics have been measured for ligand recombination to hemoglobin (Hb) in the presence of two effectors: bezafibrate (Bzf) and inositol hexakisphosphate (IHP). The combined influence of the two independent effectors leads to predominantly T-state behavior. Samples equilibrated with 0.1 atm of CO are fully saturated, yet after photodissociation they show only T-state bimolecular recombination rates at all photolysis levels; this indicates that the allosteric transition from R to T occurs before CO rebinding and that the allosteric equilibrium favors the T-state tetramer with up to three ligands bound. Since all four ligands bind at the rate characteristic for the T-state, the return transition from T to R must occur after the fourth ligand was bound. At 1 atm of CO, rebinding to the initial R state competes with the allosteric transition resulting in a certain fraction of CO bound at the rate characteristic for the R state; this fraction is greater the smaller the percentage dissociation. Under 1 atm of oxygen, samples are not more than 93% saturated and show mainly T-state kinetics. The results show that all four hemes can bind oxygen or CO ligands in the T structure. The fraction of the kinetics occurring as geminate is less for partially liganded (T-state) samples than for fully liganded (R-state) Hb.
Marden
M C
MC
INSERM U299, Hôpital de Bicêtre, France.
Kister
J
J
Bohn
B
B
Poyart
C
C
D000495:Allosteric Site; D001629:Bezafibrate / Q000494:pharmacology*; D002263:Carboxyhemoglobin / Q000378:metabolism; D006441:Hemoglobin A / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008024:Ligands; D010108:Oxyhemoglobins / Q000378:metabolism; D010782:Photolysis; D010833:Phytic Acid / Q000494:pharmacology*; D011485:Protein Binding
D008024:Ligands; D010108:Oxyhemoglobins; D010833:Phytic Acid; D006441:Hemoglobin A; D002263:Carboxyhemoglobin; C029774:oxyhemoglobin A; D001629:Bezafibrate