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Biomedical subjects

K Moffat

Publications and source records attributed to K Moffat.

At least 73 records · Page 4Linked to original sources
PubMed1979

A structural model for the kinetic behavior of hemoglobin.

The tertiary structures of all liganded hemoglobins in the R state differ in detail. Steric hindrance arising from nonbonded ligand-globin interactions affects the binding of ligands such as CO and cyanide which preferentially form linear axial complexes to heme; these ligands bind in a strained off-axis configuration. Ligands such as O2 and NO, which preferentially form bent complexes, encounter less steric hindrance and can bind in their (preferred) unstrained configuration. Linear complexes distort the ligand pockets in the R state (and by inference, in the T state) more than bent complexes. These structural differences between linear and bent complexes are reflected in the kinetic behavior of hemoglobin. Structural interpretation of this kinetic behavior indicates that the relative contributions of nonbonded ligand-globin interactions and nonbonded heme interactions to transition state free energies differ for linear and bent ligands. The relative contributions of these interactions to the free energy of cooperativity may also differ for linear and bent ligands. Thus the detailed molecular mechanism by which the affinity of heme is regulated differs for different ligands.

Allosteric Regulation↗
PubMed1978

Theoretical considerations of the sensitivity of quantitative subunit hybridization.

The theoretical basis of the quantitative subunit hybridization technique and its ability to measure evolutionary amino acid substitutions is examined. Homospecific:heterospecific enzyme ratios found after subunit reassociation depend upon K1.2, the equilibrium constant for the dissociation of the heterospecific enzyme. It is shown that if this constant is near the geometric mean of the two homospecific enzyme dissociation constants, as it should be in enzymes whose subunits pair isologously, the quantitative subunit hybridization method will not detect most changes in the subunit contact regions of homologous proteins.

Biological Evolution↗
PubMed1976

Ligand binding properties of horse hemoglobins containing deutero- and mesoheme.

The reactions of horse globin reconstituted with proto-, deutero-, and mesoheme have been examined by equilibrium and kinetic methods. In virtually all reactions studied, mesohemoglobin displays the more extreme functional behavior, whereas deuterohemoglobin exhibits behavior which is either very similar to native hemoglobin or intermediate between the two. Our kinetic and equilibrium results indicate that the primary effect of heme modification on the functional properties of hemoglobin is to alter the intrinsic reactivities of the deoxy and liganded conformations. Heme modification does not, however, result in substantial alterations in the conformational equilibrium between the two states. Simple inductive electronic effects of the 2- and 4-substituents of the heme moiety in deutero- and mesohemoglobin are apparently not sufficient to explain the observed equilibrium and kinetic properties completely, which indicates that steric effects of these substituents may also play a role in determining the functional behavior of the hemoglobin molecule.

Animals↗
PubMed1975

Influence of inositol hexaphosphate binding on subunit dissociation in methemoglobin.

The tetramer-dimer equilibria of various forms of methemoglobin have been measured by sedimentation equilibrium to test the hypothesis of Perutz that high spin derivatives can be switched by inositol hexaphosphate (Inos-P6) from the R state to the T state more readily than low spin derivatives. Since transitions from the R state to the T state are accompanied by a decrease in the tetramer-dimer dissociation constant (K4,2), this parameter is a quantitative indicator of the conformational state. Measurements of K4,2 were performed using an analytical ultracentrifuge with absorption optics and a scanner-computer system. Statistical analysis of the sedimentation data indicated that the stoichiometry if Inos-P6 binding is 1 molecule/hemoglobin tetramer and 2 molecules/hemoglobin dimer. The apparent affinity of the dimer sites for Inos-P6 is much lower than the corresponding value for the tetramer site. As a result of the stoichiometries, at low concentrations Inos-P6 shifts the tetramer-dimer equilibrium in favor of the tetramer, but at high concentrations Inos-P6 shifts the equilibrium in favor of the dimer. Te tetramer binding site for Inos-P6 of various liganded forms of hemoglobin appears to be the same as has been established for deoxyhemoglobin, since the effect of Inos-P6 on subunit dissociation is reduced in pyridoxylated derivatives. Values of K4,2 for aquo-, azido- and cyanomethemoglobin in 0.01 M 2,2-bis(hydroxymethyl)-2,2',2''-nitroethanol buffer, pH 6.0/0.1 M NaCl, are all near 2 X 10(-5) M. Upon addition of 50 muM Inos-P6 the values of K4,2 for all three forms are shifted to near 10(-9) M. Since the aquo derivative is high spin, while the azido and cyano derivatives are low spin, the similarity of values for the derivatives in the presence and absence of Inos-P6 indicate that the changes in K4,2 are not spin-spin state dependent. For another high spin derivative, fluoromethemoglobin, such high concentrations of NaF are required that ionic strength effects are encountered. When data at several NaF concentrations are extrapolated to 0.1 M NaF to correct for the ionic strength effects, values of K4,2 of 7 X 10(-6) M and 10(-8) M are obtained for solutions in the absence and in the presence of 50 muM Inos-P6, respectively. Therefore the results with the fluoro derivative, in conjunction with the other forms of methemoglobin, support the view that high spin derivatives do not exhibit a greater response to Inos-P6 than low spin derivatives.

Azides↗