Lamprey hemoglobin - Structural basis of the Bohr effect

Lampreys, among the most primitive living vertebrates, have hemoglobins (Hb s) with self-association and ligand-binding properties very different from those that characterize the alpha(2)beta(2) tetrameric Hbs of higher verteb rates. Monomeric, ligated lamprey Hb self-associates to dimers and tetramer s upon deoxygenation. Dissociation to monomers upon oxygenation accounts fo r the cooperative binding of O-2 and its pH dependence. Honzatko and Hendri ckson (Honzatko, R, B., and Hendrickson, W, A (1986) Proc. Natl. Acad. Sci, U.S.A 83, 8487-8491) proposed that the dimeric interface of the Hb resembl es either the alpha(1)beta(2) interface of mammalian Hbs or the contacts in clam Hb where the E and F helices form the interface. Perutz (Perutz, M. F , (1989) Quart, Rev. Biophys. 2, 139-236) proposed a version of the clam mo del in which the distal histidine swings out of the heme pocket upon deoxyg enation to form a bond with a carboxyl group of a second monomer, The sedim entation behavior and oxygen equilibria of nine mutants of the major Hb com ponent, PMII, from Petromyzon marinus have been measured to test these mode ls. The results strongly support a critical role of the E helix and the AB corner in forming the subunit interface in the dimer and rule out the alpha (1)beta(2) model The pH dependence of both the sedimentation equilibrium an d the oxygen binding of the mutant E75Q indicate that Glu(75) is one of two groups responsible for the Bohr effect. Changing the distal histidine 73 t o glutamine almost completely abolishes the self-association of the deoxy-H b and causes a large increase in O-2 affinity. The recent x-ray crystallogr aphic determination of the structure of deoxy lamprey Hb, reported after th e completion of this work (Heaslet, H.A., and Royer, W, E, (1999) Structure 7, 517-526), shows that the dimer interface does involve the E helix and t he AB corner, supporting the measurements and interpretations reported here .