Biomedical subjects

J J Stephanos

Publications and source records attributed to J J Stephanos.

8 recordsLinked to original sources

PubMed1999

Copper(II) Schiff-base complexes and apoglobin stability.

N,N'-Propylene-bis-(N-salicylidene)copper(II) (Cu(Salprn)) explicitly stabilizes apomyoglobin. The optical spectrum of this copper(II) Schiff-base complex of apomyoglobin arises from the electronic excitations of pi *-O-Salprn-->dx2-y2 and N-Salprn-->dx2-y2. Shifts of these transitions with respect to those of the parent complex may be a consequence of hydrophobic solvatochromism or binding of an additional ligand. ESR parameters imply no change in the identity of the first coordination sphere around the copper, while hydrophobic solvatochromism cannot be excluded. Combination of copper(II) Schiff-base complex with apomyoglobin does not inhibit the ability of apomyoglobin to extract hemin from the main component of Glycera dibranchiata hemoglobin. Hemin replaces the copper complex, and the value of the apparent first-order rate constant varies with time. The mechanism involves dissociative and associative interchange pathways. Values of rate constants for transfer of hemin to copper(II) Schiff-base apomyoglobin complex, as well as the change of concentration with time are evaluated.

Animals↗

PubMed1997

Triangular kinetic schemes applied to the stability of a heme-globin complex.

Horse heart apomyoglobin traps the heme released from Aplysia californica myoglobin. The kinetics fit a triangular mechanism for a bipbasic reaction. Laplacian solutions for differential equations appropriate to triangular kinetic schemes involving up to four rate constants are elaborated and confirmed. Two general schemes and two special cases are considered. In the first scheme, a rearrangement of the starting material is concurrent with product formation. In the second scheme, the starting material forms two products in equilibrium at two different rates. A general equation for the absorbance-time curve is derived for these triangular schemes, from which rate constants can be estimated. Changes in instantaneous rate versus time are employed to analyze the absorption versus time plots and the curvature of a first-order rate analysis. Aplysia metmyoglobin equilibrates between slow donor (pentacoordinate, which lacks the axial water molecule) and fast donor (bexacoordinate). No heme release was observed for deoxy, oxy, carbonyl, or azide derivatives of the Aplysia myoglobin, or when the distal HisE7 of the apohemoprotein is replaced by leucine or valine. This suggest a role for hydrophobicity of the active site, and for a trans effect of the axial ligand in determining the stability of the embedded prosthetic heme.

Animals↗

PubMed1996

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.

Animals↗

PubMed1996

Drug-protein interactions: two-site binding of heterocyclic ligands to a monomeric hemoglobin.

The reactivity response of the heme proteins to the heterotropic effectors, purine, caffeine, theophylline, and (C2H5)4N+, have been examined. The heterotropic effectors influence the heme ligation affinities. The heme axial ligation of pyridine and pyrazole have not influenced the hemoglobin's affinity for caffeine and theophylline. The imidazole ligation indicates a mutual interaction between the heme active site and the noncoordinate binding site.

Binding Sites↗

PubMed1990

Kinetic and spectroscopic studies of haemoglobin and myoglobin from Urechis caupo. Distal residue effects.

Seven components of the tetrameric haemoglobin (Hbu) from Urechis caupo were separated by preparative isoelectric focusing and characterized by their absorption spectra and pI values. The helix content and Soret delta epsilon values are reported for several of the components. Temperature-jump O2-binding kinetics of the major components of Hbu show biphasic behaviour, with the majority species having kon = 1.57 x 10(9) mol-1.s-1 and koff = 3.32 x 10(4) s-1. The Fourier-transform i.r. spectrum of pooled Hbu(II)-CO displays a stretching frequency of 1942 cm-1. E.s.r. of Hbu(II)-NO demonstrates evidence of proximal strain similar to that encountered in T-state human haemoglobin. CO-driven reduction of U. caupo methaemoglobin, Hbu(III) and U. caupo metmyoglobin [Mbu(III)] shows much higher rates relative to haemoglobins and myoglobins known to possess a distal histidine residue. Nitrosyl auto-reduction kinetics of Hbu(III)-NO and Mbu(III)-NO are examined. The equilibrium binding constants of several ligands are reported for both Hbu and Mbu, and together with the above kinetic data suggest differences in haem pocket environments between Hbu and Mbu. Reaction of Hbu with 2-chloromercuri-4,6-dinitrophenol demonstrates the presence of one reactive thiol group per globin chain. lambda max. values and the respective molar absorption coefficients for selected ligand-bound states are reported for the major component of Hbu and for Mbu. The majority haem orientation in U. caupo haemoglobin is identical with that of human haemoglobin.

Animals↗

PubMed1990

Spectroscopic and kinetic aspects of Elephas maximus hemoglobin.

In comparison with myoglobin and human and Glycera dibranchiata hemoglobins, the heme distal side amino acid exchanges within the heme environment of elephant tetrameric hemoglobin (Hbe) only slightly affect the electronic and ESR spectra of Hbe(III) and Hbe(II) derivatives, several of which were prepared and characterized by optical and ESR spectroscopy. Addition of 2,3-bisphosphoglycerate [Gri(2,3)P2] or inositol hexakisphosphate to Hbe(II)NO causes tension in the Fe-N(proximal His) bond, although the behaviour differs in detail from that of HbA(II)NO. There are two equilibrium states of Hbe having significantly different kinetics for the Hbe(III)----Hbe(II) reaction of Hbe(III)NO. This autoreduction occurs in the form of two parallel processes, which collapse into one intermediate rate in the presence of Gri(2,3)P2. The temperature dependences of the rates enable deduction of delta H0 and delta S0 for the linked equilibrium, and yield linear Eyring plots for Hbe(III)NO, from which activation parameters were estimated on the basis of a previously described mechanism.

Animals↗

PubMed1990

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

The thermally induced difference spectra of myoglobin (Mb) and Glycera dibranchiata hemoglobin (Hbm) derivatives and of cytochrome-c were recorded between 4 degrees and 30 degrees C in the 390-750 nm range. Thermodynamic parameters were estimated and upper and lower temperature limiting spectra were deduced for the various heme protein derivatives' equilibria. The effective iron d-electron population divides the hemes broadly into two different groups of behavior type. In the first group, Hbm(III)N3, Hbm(III), Mb(III)(H2O), and Cytc(III) show equilibria between two spin states. The weakest coupling between the heme and the globin occurs among the second group, for Hbm(II)CO and Mb(II)CO, which in the higher temperature limit undergoes averaging of the carbonyl tilt, while an axially elongated geometry is probably accessed for Hbm(II)NO and Mb(II)NO. Examples of the less common situation of increased absorption intensity and/or low-spin states at higher temperature were found in both groups. In the case of the methyl thioglycolate low-spin adducts of Hbm(III), an acid/base equilibrium involving thioglycolate deprotonation occurs. Apparent enthalpy-entropy compensation is exhibited by all these heme derivatives, and it is suggested that the delta H degrees and delta S degrees values relate to the intimacy of coupling between the heme structure and the solvent-dependent microconformation of the globin.

Animals↗

PubMed1986

Nitrosyliron(III) hemoglobin: autoreduction and spectroscopy.

Nitrosyl complexes of the iron(III) forms of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm and Hbh), and model iron(II) and iron(III) synthetic porphyrins including octaethylporphyrin (OEP) have been prepared. The iron(III) heme proteins are electron spin (paramagnetic) resonance (ESR) silent, while hexacoordinate solution structures are indicated for [Fe(OEP)(NO)2]ClO4 and for Hbm(II)NO, which has an ESR spectrum similar to that of Mb(II)NO and the hexacoordinate iron(II) model complex Fe(OEP)NO(BzIm). The splitting of the alpha- and beta-bands in the optical spectrum of Mb(III)NO and Hbh(III)NO contrasts markedly with the sharp, single bands observed in that of Hbm-(III)NO. The nondegeneracy of the dxz and dyz orbitals in Mb(III)NO and Hbh(III)NO is attributed to the influence of the distal histidine. Circular dichroism spectra were obtained for Hbm(III)NO, Hbm(II)NO, Hbh(III)NO, Hbh(II)NO, Mb(II)NO, and Mb(III)NO. The vicinal chiral center contribution that governs the heme protein CD leads to low Kuhn anisotropies, which have been used to assign certain electronic transitions. The Hb(III)NO spectrum is not stable but transforms into that of Hb(II)NO. This autoredox process follows kinetics that are first order in FeIIINO. The relative rates of autoreduction (25 degrees C, 1 atm NO) are Mb(III)NO less than Hbm(III)NO less than Hb alpha(III)NO less than HbA(III)NO. At high NO partial pressure or after "recycling" of HbA, the rates of reduction decrease. The first step in the reaction of NO with the ferric heme is the reversible formation of the formally iron(III) adduct. This reacts with another molecule of NO, generating the final heme(II)-NO via nitrosylation of NO itself or of an endogenous nucleophile. Kinetic and spectroscopic evidence shows involvement of trans-heme-(NO)2 in the reaction. The activation parameters delta H and delta S were determined. The overall reaction is photoenhanced.

Animals↗