Thermochromism of heme adducts of Glycera hemoglobin and some other monomeric heme proteins

Iron ligand recognition by monomeric hemoglobins

Binding affinities of monomeric Glycera dibranchiata hemoglobin for some anions and heterocyclic amines, including imidazoles, pyrazole, triazole and tetrazole 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, the higher affinities of stronger Brønsted-Lowry bases reflecting their essentially sigma-donor character. The bifunctional molecule morpholinoethylisocyanide acts as a redox-state-dependent ambidentate ligand, binding as an N-donor to iron(III), but as a C-donor to iron(II). pH-Dependences of the ESR and optical spectra of the azole adducts reveal iron-linked ionisations and spin-equilibria in the heme pocket. Enthalpy and entropy changes for the binding process were estimated for several ligands, and mutually compensatory behaviour is observed globally for delta H degree and delta S degree. At the compensation temperature theta, the binding affinities of monomeric Glycera dibranchiata hemoglobin and sperm whale myoglobin are similar and associated with free energy changes delta G degree (theta) approximately -9 +/- 1 kJ mol-1 for the heterocyclic and anionic ligands.

Low-temperature thermochromism marks a change in coordination for the metal ion in manganese superoxide dismutase

We have observed thermochromism (temperature-dependent absorption) for anion complexes of manganese superoxide dismutase indicating a change in coordination number for the metal complex at low temperatures. The ligand field spectra for the Mn(III) ion, characteristic of five-coordination for the azide complex at 295 K, cleanly convert to spectra reflecting six-coordination at low temperature, with a midpoint for the transition near 200 K. The active site structure is temperature-dependent, a relatively rigid, distorted octahedral low-temperature Mn complex melting with dehydration (or displacement of one of the protein ligands) to form a five-coordinated complex under physiological conditions. Thermodynamic parameters for the transition estimated from van't Hoff analysis (delta HvH = 5 kcal/mol; delta SvH = 22 cal/mol K) are consistent with reduced chemical binding and increased fluxionality at room temperature. This thermochromism of MnSD demonstrates the existence of distinct isomeric forms of the active site metal complex, whose relative stability depends on the degree of vibrational excitation. The marginal destabilization of the six-coordinate anion complex under physiological conditions suggests that the enzyme may thermally control the stability of intermediates in a dissociative displacement mechanism for substrate binding and redox.