title: Cooperative mechanism in the homodimeric myoglobin from Nassa mutabilis.
abstract: Oxygen binding and spectroscopic properties of the homodimeric myoglobin (Mb) from the prosobranchia sea snail Nassa mutabilis have been investigated. Oxygen equilibrium curves are pH-independent and cooperative with P50 = 5 +/- 1 mmHg and n approximately 1.5. Circular dichroism spectra of the oxygenated and deoxygenated form of N. mutabilis Mb are superimposable between 190 and 250 nm, suggesting a mechanism for cooperative ligand binding that does not involve changes in the alpha-helical content of the whole protein. The oxygen dissociation process is biphasic and pH-dependent, with different pKa values (=6.7 +/- 0.2 and 8.5 +/- 0.3) for the two phases. Moreover, the activation energy is essentially the same for both oxygen dissociation processes (Ea = 56.4 +/- 2.1 kJ/mol for the fast phase, and Ea = 53.8 +/- 1.9 kJ/mol for the slow phase), indicating that the rate difference for O2 dissociation between the diliganded and the monoliganded species is mostly dependent on a variation of the activation entropy. Ferrous nitrosylated N. mutabilis Mb shows, at alkaline and neutral pH, axial and rhombic X-band EPR signals, respectively, which display below pH 6 a three-hyperfine pattern typical of five-coordination. The results presented here suggest that in N.mutabilis Mb the kinetic control of cooperativity operates through a mechanism never observed before in other hemoproteins, which requires a ligand-linked large enhancement for the value of the oxygen association process in a molecule not undergoing changes in quaternary structure.
authors.affiliation: Department of Experimental Medicine and Biochemical Sciences, University of Roma Tor Vergata, Via di Tor Vergata 135, 00133 Roma, Italy. coletta@seneca.ccd.utovrm.it
authors.forename: M
authors.initials: M
authors.lastname: Coletta
authors.forename: P
authors.initials: P
authors.lastname: Ascenzi
authors.forename: F
authors.initials: F
authors.lastname: Polizio
authors.forename: G
authors.initials: G
authors.lastname: Smulevich
authors.forename: R
authors.initials: R
authors.lastname: del Gaudio
authors.forename: M
authors.initials: M
authors.lastname: Piscopo
authors.forename: G
authors.initials: G
authors.lastname: Geraci
mesh_terms: D000818:Animals; D049872:Bivalvia; D002942:Circular Dichroism; D006454:Hemoglobins / Q000737:chemistry; D008024:Ligands; D009211:Myoglobin / Q000737:chemistry* / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D012908:Snails
chemical_list: D006454:Hemoglobins; D008024:Ligands; D009211:Myoglobin; C083082:dimeric hemoglobin, Scapharca inaequivalvis; D010100:Oxygen