TCR binding to peptide-MHC stabilizes a flexible recognition interface.
The binding of TCRs to their peptide-MHC ligands is characterized by a low affinity, slow kinetics, and a high degree of cross-reactivity. Here, we report the results of a kinetic and thermodynamic analysis of two TCRs binding to their peptide-MHC ligands, which reveal two striking features. First, significant activation energy barriers must be overcome during both association and dissociation, suggesting that conformational adjustments are required. Second, the low affinity of binding is a consequence of highly unfavorable entropic effects, indicative of a substantial reduction in disorder upon binding. This is evidence that the TCR and/or peptide-MHC have flexible binding surfaces that are stabilized upon binding. Such conformational flexibility, which may also be a feature of primary antibodies, is likely to contribute to cross-reactivity in antigen recognition. (+info)
Comparison of the entropy technique with two other techniques for detecting disease clustering using data from children with high blood lead levels.
The entropy technique was compared with two other case-control techniques for detecting disease clustering using data on blood lead levels of children who were patients at the King/Drew Medical Center in South-Central Los Angeles in 1991 to 1994. The other two methods are the nearest neighbor technique (NNT) and Moran's IPOP technique, a variation of Moran's I test, in which rates are adjusted for population size. Four different blood lead levels (15 microg/dl, 20 microg/dl, 30 microg/dl, 35 microg/dl) were used as cutoff levels to designate cases. Persons with blood lead levels greater than or equal to the cutoff level were designated as cases. The authors found significant clustering for all four cutoff levels using the entropy method, and for the first three cutoff levels using the NNT. They found significant clustering with Moran's IPOP for some scales for two of the cutoff levels. While performance of the entropy technique and the NNT were independent of scale, that of Moran's IPOP was highly scale-dependent. (+info)
Electrostatic-undulatory theory of plectonemically supercoiled DNA.
We present an analytical calculation of the electrostatic interaction in a plectonemic supercoil within the Poisson-Boltzmann approximation. Undulations of the supercoil strands arising from thermal motion couple nonlinearly with the electrostatic interaction, giving rise to a strong enhancement of the bare interaction. In the limit of fairly tight winding, the free energy of a plectonemic supercoil may be split into an elastic contribution containing the bending and torsional energies and an electrostatic-undulatory free energy. The total free energy of the supercoil is minimized according to an iterative scheme, which utilizes the special symmetry inherent in the usual elastic free energy of the plectoneme. The superhelical radius, opening angle, and undulation amplitudes in the radius and pitch are obtained as a function of the specific linking difference and the concentration of monovalent salt. Our results compare favorably with the experimental values for these parameters of Boles et al. (1990. J. Mol. Biol. 213:931-951). In particular, we confirm the experimental observation that the writhe is a virtually constant fraction of the excess linking number over a wide range of superhelical densities. Another important prediction is the ionic strength dependence of the plectonemic parameters, which is in reasonable agreement with the results from computer simulations. (+info)
Structural bases of lectin-carbohydrate affinities: comparison with protein-folding energetics.
We have made a comparative structure based analysis of the thermodynamics of lectin-carbohydrate (L-C) binding and protein folding. Examination of the total change in accessible surface area in those processes revealed a much larger decrease in free energy per unit of area buried in the case of L-C associations. According to our analysis, this larger stabilization of L-C interactions arises from a more favorable enthalpy of burying a unit of polar surface area, and from higher proportions of polar areas. Hydrogen bonds present at 14 L-C interfaces were identified, and their overall characteristics were compared to those reported before for hydrogen bonds in protein structures. Three major factors might explain why polar-polar interactions are stronger in L-C binding than in protein folding: (1) higher surface density of hydrogen bonds; (2) better hydrogen-bonding geometry; (3) larger proportion of hydrogen bonds involving charged groups. Theoretically, the binding entropy can be partitioned into three main contributions: entropy changes due to surface desolvation, entropy losses arising from freezing rotatable bonds, and entropic effects that result from restricting translation and overall rotation motions. These contributions were estimated from structural information and added up to give calculated binding entropies. Good correlation between experimental and calculated values was observed when solvation effects were treated according to a parametrization developed by other authors from protein folding studies. Finally, our structural parametrization gave calculated free energies that deviate from experimental values by 1.1 kcal/mol on the average; this amounts to an uncertainty of one order of magnitude in the binding constant. (+info)
Simulation of electron-proton coupling with a Monte Carlo method: application to cytochrome c3 using continuum electrostatics.
A new method is presented for simulating the simultaneous binding equilibrium of electrons and protons on protein molecules, which makes it possible to study the full equilibrium thermodynamics of redox and protonation processes, including electron-proton coupling. The simulations using this method reflect directly the pH and electrostatic potential of the environment, thus providing a much closer and realistic connection with experimental parameters than do usual methods. By ignoring the full binding equilibrium, calculations usually overlook the twofold effect that binding fluctuations have on the behavior of redox proteins: first, they affect the energy of the system by creating partially occupied sites; second, they affect its entropy by introducing an additional empty/occupied site disorder (here named occupational entropy). The proposed method is applied to cytochrome c3 of Desulfovibrio vulgaris Hildenborough to study its redox properties and electron-proton coupling (redox-Bohr effect), using a continuum electrostatic method based on the linear Poisson-Boltzmann equation. Unlike previous studies using other methods, the full reduction order of the four hemes at physiological pH is successfully predicted. The sites more strongly involved in the redox-Bohr effect are identified by analysis of their titration curves/surfaces and the shifts of their midpoint redox potentials and pKa values. Site-site couplings are analyzed using statistical correlations, a method much more realistic than the usual analysis based on direct interactions. The site found to be more strongly involved in the redox-Bohr effect is propionate D of heme I, in agreement with previous studies; other likely candidates are His67, the N-terminus, and propionate D of heme IV. Even though the present study is limited to equilibrium conditions, the possible role of binding fluctuations in the concerted transfer of protons and electrons under nonequilibrium conditions is also discussed. The occupational entropy contributions to midpoint redox potentials and pKa values are computed and shown to be significant. (+info)
Polymer-in-a-box mechanism for the thermal stabilization of collagen molecules in fibers.
Collagen molecules in solution unfold close to the maximum body temperature of the species of animal from which the molecules are extracted. It is therefore vital that collagen is stabilized during fiber formation. In this paper, our concept that the collagen molecule is thermally stabilized by loss of configurational entropy of the molecule in the fiber lattice, is refined by examining the process theoretically. Combining an equation for the entropy of a polymer-in-a-box with our previously published rate theory analysis of collagen denaturation, we have derived a hyperbolic relationship between the denaturation temperature, Tm, and the volume fraction, epsilon, of water in the fiber. DSC data were consistent with the model for water volume fractions greater than 0.2. At a water volume fraction of about 0.2, there was an abrupt change in the slope of the linear relationship between 1/Tm and epsilon. This may have been caused by a collapse of the gap-overlap fiber structure at low hydrations. At more than 6 moles water per tripeptide, the enthalpy of denaturation on a dry tendon basis was independent of hydration at 58.55 +/- 0.59 J g-1. Between about 6 and 1 moles water per tripeptide, dehydration caused a substantial loss of enthalpy of denaturation, caused by a loss of water bridges from the hydration network surrounding the triple helix. At very low hydrations (less than 1 mole of water per tripeptide), where there was not enough water to form bridges and only sufficient to hydrogen bond to primary binding sites on the peptide chains, the enthalpy was approximately constant at 11.6 +/- 0.69 J g-1. This was assigned mainly to the breaking of the direct hydrogen bonds between the alpha chains. (+info)
Origins of the temperature dependence of hammerhead ribozyme catalysis.
The difficulties in interpreting the temperature dependence of protein enzyme reactions are well recognized. Here, the hammerhead ribozyme cleavage was investigated under single-turnover conditions between 0 and 60 degrees C as a model for RNA-catalyzed reactions. Under the adopted conditions, the chemical step appears to be rate-limiting. However, the observed rate of cleavage is affected by pre-catalytic equilibria involving deprotonation of an essential group and binding of at least one low-affinity Mg2+ion. Thus, the apparent entropy and enthalpy of activation include contributions from the temperature dependence of these equilibria, precluding a simple physical interpretation of the observed activation parameters. Similar pre-catalytic equilibria likely contribute to the observed activation parameters for ribozyme reactions in general. The Arrhenius plot for the hammerhead reaction is substantially curved over the temperature range considered, which suggests the occurrence of a conformational change of the ribozyme ground state around physiological temperatures. (+info)
Thermodynamic analysis of chain-melting transition temperatures for monounsaturated phospholipid membranes: dependence on cis-monoenoic double bond position.
Unsaturated phospholipid is the membrane component that is essential to the dynamic environment needed for biomembrane function. The dependence of the chain-melting transition temperature, T(t), of phospholipid bilayer membranes on the position, n(u), of the cis double bond in the glycerophospholipid sn-2 chain can be described by an expression of the form T(t) = T(t)(c)(1 + h'(c)|n(u) - n(c)|)/(1 + s'(c)|n(u) - n(c)|), where n(c) is the chain position of the double bond corresponding to the minimum transition temperature, T(t)(c), for constant diacyl lipid chain lengths. This implies that the incremental transition enthalpy (and entropy) contributed by the sn-2 chain is greater for whichever of the chain segments, above or below the double-bond position, is the longer. The critical position, n(c), of the double bond is offset from the center of the sn-2 chain by an approximately constant amount, deltan(c) approximately 1. 5 C-atom units. The dependence of the parameters T(t)(c), h'(c), and s'(c) on sn-1 and sn-2 chain lengths can be interpreted consistently when allowance is made for the chain packing mismatch between the sn-1 and sn-2 chains. The length of the sn-2 chain is reduced by approximately 0.8 C-atom units by the cis double bond, in addition to a shortening by approximately 1.3 C-atom units by the bent configuration at the C-2 position. Based on this analysis, a general thermodynamic expression is proposed for the dependence of the chain-melting transition temperature on the position of the cis double bond and on the sn-1 and sn-2 chain lengths. The above treatment is restricted mostly to double-bond positions close to the center of the sn-2 chain. For double bonds positioned closer to the carboxyl or terminal methyl ends of the sn-2 chain, the effects on transition enthalpy can be considerably larger. They may be interpreted by the same formalism, but with different characteristic parameters, h'(c) and s'(c), such that the shorter of the chain segments makes a considerably smaller contribution to the calorimetric properties of the chain-melting transition. (+info)