Studies of invertebrate heme proteins

Binding affinities of monomeric Glycera dibranchiata hemoglobin for some anions and heterocyclic amines have been evaluated and compared with those of sperm whale and horse heart myoglobin. The proteins' affinities for substituted heterocyclic amines are strongly influenced by the steric bulk and flexibility of the aromatic ring. The ligand coordination mode depends on the heme oxidation state, iron(III) amine adducts being more stable than the iron(II) adducts. pH-Dependences of the ESR and optical spectra of the azole adducts reveal iron-linked ionizations and spin-equilibria in the heme pocket. Enthalpy and entropy changes for the binding process were estimated for several ligands, and mutually compensatory behavior is observed globally for [Delta]H [degrees] and [Delta]S [degrees]. Horse heart apomyoglobin traps the heme released from Aplysia californica myoglobin. The kinetics fit a triangular mechanism for a biphasic reaction. Two general schemes and two special cases are considered. Aplysia metmyoglobin equilibrates between slow donor (pentacoordinate, which lacks the axial water molecule), and fast donor (hexacoordinate). No heme release was observed for deoxy, oxy, carbonyl, or azide derivatives of the Aplysia, or when the distal HisE7 of the apohemoprotein is replaced by leucine or valine. The electronic, circular dichroism, and Kuhn anisotropy spectra of the major component Glycera dibranchiata derivatives were studied. Both iron(II) and iron(III) oxidation states are reported. Derivatives include oxy, deoxy, carbonyl, methylthioglycolate, morphilinoethylisocyanide, butyl isocyanide, benzyl isocyanide, aquamet, cyano, azidomet, isocyanate, thiocyanate, selenocyanate, dicyanamide, pyridyl, imidazole and tetrazolate. The effect of the distal histidine, or lack there of, on ligand binding and its associated electronic spectrum are discussed with reference to horse heart myoglobin.