Showing papers on "Cooperativity published in 1971"
TL;DR: 2,3-Diphosphoglycerate markedly reduces the affinity of Hb A to the 1st, 2nd, and 3rd molecules of oxygen without affecting the affinity to the 4th molecule, which is similar to the oxygen affinity of the isolated β chains.
106 citations
TL;DR: In this paper, the tetramer-dimer dissociation constant of both HbCO and deoxy-Hb increases dramatically above pH 10.0 and the value of K4,2 for Hb CO is significantly greater than the value for deoxyHb in the pH range of 7 to 11.0.
88 citations
TL;DR: It is shown that cholesterol inhibits the chain motion of lecithin above its gel-liquid crystal transition temperature (T,), completely removing the transition when present in equimolar quantities.
71 citations
TL;DR: A comparison of the obtained results for PGA and PL with the molecular theories of the helix‐coil transitions shows that the role of dipole‐dipole interactions of nonneighboring peptide groups is greatly overestimated in these theories, leading to a considerable enthalpy contribution to the free energy of initiating helical regions which is not observed in the experiment.
Abstract: The helix–coil transitions for poly-L-lysine (PL) were investigated by the methods of spectropolarimetry, viscometry and potentiometric titration in 02M NaCl at different temperatures as well as in 02MNaBr, 1MKCl, and in mixtures of 02MNaCl or NaBr with methanol at room temperature The enthalpy and entropy differences between the helical and coillike states of uncharged PL molecules in 02M NaCl were determined from the potentiometric titration curves The cooperativity parameters σ for PL in different solvents were determined by two methods (from the sharpness of the transition and from the dependence of the intrinsic viscosity on the helical content in the transition region) In 02MNaCl σ has a value of (23 ± 05) × 10−4 and does not depend on temperature, ie, the cooperativity of the helix-coil transition, as for PGA, is mainly of an entropy origin (the initiating of the helical region is accompanied by the entropy decrease ΔSi = −12 eu/mole of helical regions) A comparison of the obtained results for PGA and PL with the molecular theories of the helix-coil transitions shows that the role of dipole-dipole interactions of nonneighboring peptide groups is greatly overestimated in these theories, leading to a considerable enthalpy contribution to the free energy of initiating helical regions which is not observed in the experiment
57 citations
TL;DR: The helix‐coil transitions for poly(L‐glutamic acid) (PGA) in 0.2M NaCl and in its mixture with dioxane were studied by the methods of spectropolarimetry, viscometry, and potentiometric titration at different temperatures from 8 to 50°C.
Abstract: The helix-coil transitions for poly(L-glutamic acid) (PGA) in 0.2M NaCl and in its mixture with dioxane were studied by the methods of spectropolarimetry, viscometry, and potentiometric titration at different temperatures from 8 to 50°C. The enthalpy and entropy differences between the helical and coillike states of uncharged PGA molecules were determined from the curves of potentiometric titration. The temperature dependence of the cooperativity parameter σ was determined by two methods: from the sharpness of transition and from the dependence of the intrinsic viscosity on the helical content in the transition region. In 0.2MNaCl, σ= (2.5 ± 0.5) × 10−3 and practically does not depend on temperature, i.e., the cooperativity of the helix-coil transition is connected mainly with the entropy decrease in initiating helical regions (ΔSi ≈ −12 is mole of helical regions). On the contrary, initiation of a helical region in the water-organic solvent mixture is accompanied by a considerable enthalpy increase.
56 citations
TL;DR: The model accurately predicts the equilibrium behavior of this protein without considering aggregation above the level of the dimer, and Physiologically, higher aggregates may not have functional importance.
53 citations
TL;DR: Equilibrium dialysis measurements and optical melting curve data have been used to study the formation and stability of a number of complexes between polynucleotides and complementary monomers, and it appears that the ideal composition is a 2:1 hydrogen‐bonded complex, but the actual stoichiometry is such that each base on the random polyn DNA binds less than one‐half of a molecule of its complementary monomer.
Abstract: Equilibrium dialysis measurements and optical melting curve data have been used to study the formation and stability of a number of complexes between polynucleotides and complementary monomers. The cooperativity parameter, (dθ/d ln c)θ = 0.5, where θ is the fraction of U or C residues complexed, and c is the concentration of free monomer has been measured as 1.4 for the 2:1 poly U:d-adenosine-complex, and 2.05 for the 2:1 poly C:d-guanosiue complex at pH 7. The variation of Tm with c for several complexes has been used to calculate their partial molar enthalpies of formation at the midpoint of the transition: in 1.0 MNa + at pH 7, for the 2:1 complex of poly-U with 2-amino-adenine, this is − 18.7 kcal/mole of 2-amino-adenine, for poly-U with adenosine it is − 18.7 kcal/ mole; for poly-C with dG, it is − 16.8 kcal/mole. These results do not agree very well with calorimetric integral heats of reaction reported in the literature.33 Complexes with random copolymers were also studied. The random copolymer, poly-UC, can form a mixed complex with dG and either dA or 2-amino-adenosine; the binding of dG is enhanced by an adenine derivative and vice versa.Similarly, poly AC can form a mixed complex with dG and 3-methyl-xanthine. In each case, it appears that the ideal composition is a 2:1 hydrogen-bonded complex, but the actual stoichiometry is such that each base on the random polynucleotide binds less than one-half of a molecule of its complementary monomer. Poly UG can bind dG and dA, but in a less cooperative and specific way.
47 citations
TL;DR: There is no indication here that cooperativity improves the Hodgkin-Huxley assumption of independent "subunits" and evidence is found against the suggestion that there is cooperativity between K(+) channels arranged in patches of a two-dimensional lattice.
Abstract: We use a tetrahedral model of four interacting protein subunits to represent the K+ channel or gate in the squid nerve membrane. The kinetic predictions, with varying degrees of cooperativity, are compared with experimental observations, especially those of Hodgkin and Huxley (J. Physiol. 117, 500, 1952) and of Cole and Moore (Biophys. J. 1, 1, 1960). The tentative conclusion reached is that if there is any cooperativity present it must be rather weak. There is no indication here that cooperativity improves the Hodgkin-Huxley assumption of independent “subunits”. Other related models will be discussed in Part III. We also find evidence against the suggestion that there is cooperativity between K+ channels arranged in patches of a two-dimensional lattice.
40 citations
TL;DR: The peptides were found to have an equal intrinsic helix‐forming tendency which, with only two exceptions, was not raised by the formation of tertiary structure, and these results agree with the demonstrated absence of stable intermediates in protein unfolding equilibria.
Abstract: We have determined the ultraviolet optical rotatory properties of the cyanogen bromide peptides of myoglobin and reduced, S-carboxymethylated α-lactalbumin in both aqueous and aqueous alcohol solutions. Similar measurements were also made on the tryptic digests of apomyoglobin. In aqueous solutions the α-helicity of the various peptides was between 5 and 15%, while in concentrated ethanol solutions the helicity could be increased significantly, but never to more than about 55%. The maximum helicity attained by the various peptides at high ethanol concentrations, as well as the cooperativity of the coil-to-helix transition (reflected in the slope at the steep portion of the helicity-alcohol concentration curves), does not depend on the number of residues in the peptide in the manner expected. We have used a model which treats proline residues as absolute helix breakers, thus introducing the concept of effective chain length. By applying available theories of helix–coil transitions of short-chain polypeptides to this model, one can satisfactorily describe most of the data on the myoglobin peptides. Significantly, it was possible to predict the helicity of acid-denatured apomyoglobin from the behavior of the shorter fragments. By using the model, the peptides were found to have an equal intrinsic helix-forming tendency which, with only two exceptions, was not raised by the formation of tertiary structure. The exceptions were apomyoglobin and peptide 56–131, which show, respectively, a considerable and a very small helicity attributable to tertiary structure formation in water at neutral pH. These results agree with the demonstrated absence of stable intermediates in protein unfolding equilibria. The results offer a further correlation between helical structure in the native molecule and the tendency to helix formation in isolated peptides. The results do not support the hypothesis that small folded regions are responsible for initiating the folding of the molecule, and an alternate description is proposed which envisages approximately half-folded structures at the rate limiting step in the folding reaction. Helix formation in the 33-residue C-terminal peptide of α-lactalbumin was found to be as easy as in the myoglobin peptides. If the proposed structural analogy between lysozyme and α-lactalbumin is correct, then this is a case where helix formation occurs in a peptide which is not helical in the native protein. On the other hand, an α-lactalbumin peptide corresponding to a region which has β-structure in lysozyme did not lend to form α-helices.
39 citations
TL;DR: Inositol hexaphosphate reduces the affinity of human adult hemoglobin to the 4th oxygen molecule as well as the affinity to the 1st, 2nd, and 3rd molecules, which seems to be quite different from that of DPG or ATP.
38 citations
TL;DR: The effects of concentration of substrates on the activity of pea-seed phosphofructokinase were examined by determining substrates saturation curves and replotting these according to the Hill equation and the relevance of the results to the possible regulatory role of the enzyme in plant carbohydrate metabolism is discussed.
TL;DR: The cooperativity in ligand binding which is exhibited by lamprey hemoglobin can be explained by this linkage between ligation and subunit association-dissociation reactions, which are used to formulate a model which accounts for the efficient transport of oxygen in vivo by these animals.
TL;DR: It had to be determined whether the free dimer may undergo an equivalent conformational transition; that is, whether the dimer possesses the functional interactions responsible for cooperativity in the tetramer11–13.
Abstract: HAEMOGLOBIN is a tetramer (two α and two β subunits) in natural conditions in erythrocytes. I have studied the relationship between quaternary structure and haemoglobin function to determine the sequence of molecular events within the tetramer that bring about the conformational change (from constrained to unconstrained structures1,2) responsible for cooperative ligand binding3. In most technically accessible conditions, however, the tetramer exists in a dissociation-association equilibrium with dimer4–10. To elucidate the tetramer mechanism, therefore, it had to be determined whether the free dimer may undergo an equivalent conformational transition; that is, whether the dimer possesses the functional interactions responsible for cooperativity in the tetramer11–13.
TL;DR: The thermodynamic equilibrium of the association of macromolecular subunits to larger particles is described by the concept of successive binding to existing aggregates and two theoretical models are discussed in detail.
TL;DR: The observed change in β-β interactions even in the absence of DPG suggests that contacts between the two β chains contribute to cooperativity as well as a bridge formed by the DPG molecule.
TL;DR: Calculations are presented for the general case which restricts the (p‐s) association to occur only with ( p‐p) associated nucleotide units and the nature of the binding curve in relation to partitioning the total interaction energy (F + P + S + W) among the parameters is discussed.
Abstract: The matrix method is used for calculating the grand partition function for the reaction: 2 polymer + solute = complex. The homogeneous polymers are assumed to have two types of sites within each nucleotide unit: sites for the polymer–polymer association, i.e., (p-p) sites; sites for polymer–solute association i.e., (p-s) sites. The respective binding parameters, P and F, and nearest-neighbor interaction parameters, W and S, are assumed independent. Complications due to ring entropy are avoided by rest riding the model to one-solute-stack systems, which are physically realizable when the reciprocal of the solute cooperativity parameter is much larger than the number of nucleotides in the polymer. The 4 × 4 generating matrix is shown to be a tensor product of two 2 × 2 matrices, each the generating matrix of a particular type of site. The scalar product of the 4 × 4 matrix is shown to be equivalent to the scalar product of a 2 × 2 matrix in the weak interaction limit, W ≈ 0. Calculations are presented for the general case which restricts the (p-s) association to occur only with (p-p) associated nucleotide units. The nature of the binding curve in relation to partitioning the total interaction energy (F + P + S + W) among the parameters is discussed. Also presented is a criterion for neglecting possible states in the calculation of the grand partition function.