Biophysical and kinetic characterization of HemAT, an aerotaxis receptor from Bacillus subtilis

HemAT from Bacillus subtilis is a new type of heme protein responsible for sensing oxygen. The structural and functional properties of the full-length HemAT protein, the sensor domain (1-178), and Tyr-70 mutants have been characterized. Kinetic and equilibrium measurements reveal that both full-length HemAT and the sensor domain show two distinct O(2) binding components. The high-affinity component has a K(dissociation) approximately 1-2 microM and a normal O(2) dissociation rate constant, k(O2) = 50-80 s(-1). The low-affinity component has a K(dissociation) approximately 50-100 microM and a large O(2) dissociation rate constant equal to approximately 2000 s(-1). The low n-value and biphasic character of the equilibrium curve indicate that O(2) binding to HemAT involves either independent binding to high- and low-affinity subunits in the dimer or negative cooperativity. Replacement of Tyr-70(B10) with Phe, Leu, or Trp in the sensor domain causes dramatic increases in k(O2) for both the high- and low-affinity components. In contrast, the rates and affinity for CO binding are little affected by loss of the Tyr-70 hydroxyl group. These results suggest highly dynamic behavior for the Tyr-70 side chain and the fraction of the "up" versus "down" conformation is strongly influenced by the nature of the iron-ligand complex. As a result of having both high- and low-affinity components, HemAT can respond to oxygen concentration gradients under both hypoxic (0-10 microM) and aerobic (50-250 microM) conditions, a property which could, in principle, be important for a robust sensing system. The unusual ligand-binding properties of HemAT suggest that asymmetry and apparent negative cooperativity play an important role in the signal transduction pathway.     

Kinetics of [3H]muscimol binding to the GABAA receptor in bovine brain membranes

The binding of the GABA receptor agonist [3H]muscimol to membrane preparations from bovine cerebral cortex has been investigated in equilibrium and kinetic experiments. Equilibrium binding curves are biphasic and suggest that [3H]muscimol binds to both high-affinity (Kd approximately 10 nM) and low-affinity (Kd approximately 0.5 microM) sites. Binding to each class of sites is inhibited by GABA and by the specific GABAA receptor antagonist bicuculline. The kinetics of [3H]muscimol binding have been measured by using both manual filtration assays and an automated rapid filtration technique which permits the measurement of ligand dissociation on subsecond time scales. Association and dissociation curves are biphasic at all concentrations of [3H]muscimol studied, even under conditions of low receptor saturation when no significant occupancy of the low-affinity sites would be expected. These results cannot be simply explained by the presence of two populations of binding sites in the membrane preparations but suggest the existence of two forms of the monoliganded receptor. Dissociation constants for these two forms have been estimated to be 16 and 82 nM at 23 degrees C. At higher ligand concentrations, kinetic measurements have suggested that the binding of [3H]muscimol to low-affinity sites is accompanied by a slow conformational change of the receptor-ligand complex.     

Variations in binding affinities of the peripheral nicotinic acetylcholine receptor can be attributed to endogenous acetylcholine

Equilibrium binding studies performed with fresh membrane fragments from Torpedo marmorata reveal a low affinity for [3H]acetylcholine with an equilibrium dissociation constant in the micromolar range and no indication of cooperative interactions. The low binding affinity is an artifact caused by the presence of endogenous acetylcholine and is not related to the active conformation of the receptor. Endogenous acetylcholine is identified by its interaction with acetylcholine esterase and choline kinase. It is present in presynaptic vesicles as shown in electron micrographs. Leakage of these synaptosomes is of the order of 300 pmol acetylcholine per g tissue as determined by means of binding studies performed with [3H]acetylcholine. In the absence of endogenous acetylcholine, equilibrium binding studies show a high affinity for [3H]acetylcholine and a slight cooperativity of sites (K1D = 30nM; K2D = 10nM). The addition of detergents, local anesthetics or alcohols to a further increase in affinity and to a decrease in cooperativity (K1D = 11nM; K2D = 5nM). No low-affinity binding can be detected in the micromolar range.     

Formyl peptide chemotaxis receptors on the rat neutrophil: experimental evidence for negative cooperativity

To examine the existence of negative cooperativity among formyl peptide chemotaxis receptors, steady-state binding of f Met-Leu-[3H]Phe to viable rat neutrophils and their purified plasma membranes was measured and the data were subjected to statistical analysis and to computer curve fitting using the NONLIN computer program. Curvilinear, concave upward Scatchard plots were obtained. NONLIN and statistical analysis of the binding data indicated that a two-saturable-sites model was preferable to a one-saturable-site model and statistically valid by the F-test (P less than .010). In addition, Hill coefficients of 0.80 +/- 0.02 were obtained. Kinetic dissociation experiments using purified plasma membranes showed evidence of site-site interactions of the destabilizing type (negative cooperativity). Thus, unlabeled f Met-Leu-Phe accelerated the dissociation of f Met-Leu-[3H]Phe under conditions where no rebinding of radioligand occurred. The rate of dissociation of f Met-Leu-[3H]Phe from the plasma membranes was dependent on the fold excess of unlabeled f Met-Leu-Phe used in the dilution medium; at the highest concentration tested (10,000-fold excess), the dissociation rate was more than double the dissociation rate seen with dilution alone. In addition, occupancy-dependent affinity was ascertained directly by studying the effect of increasing fractional receptor saturation with labeled ligand on the dissociation rate of the receptor-bound labeled ligand. These data showed that the f Met-Leu-[3H]Phe dissociation rate was dependent on the degree of binding site occupancy over the entire biologically relevant range of formyl peptide concentrations. Furthermore, monitoring of the time course of dissociation of the receptor/f Met-Leu-[3H]Phe receptor/f Met-Leu-[3H]Phe complex as a function of receptor occupancy revealed that receptor affinity for f Met-Leu-Phe remained occupancy-dependent during the entire time of dissociation examined (up to 10 min). Finally, the average affinity profile of the equilibrium binding data demonstrated a 60% decrease in receptor affinity in changing from the high affinity to the low affinity conformation.     

The hormonal regulation and interaction of concanavalin A and insulin binding in R3230AC mammary adenocarcinoma cells

To investigate the effects of concanavalin A on insulin binding to R323AC mammary carcinomas, initial experiments were performed to characterize binding of concanavalin A. Concanavalin A binding was found to be specific and saturable. Equilibrium binding experiments demonstrated that addition of low concentration of concanavalin A enhanced the binding of [³H]concanavalin A, suggestive of positively cooperative interactions. Binding of concanavalin A was responsive to hormonal alterations; tumor cells from diabetic rats showed enhanced binding of concanavalin A and insulin compared to cells from intact rats and administration of insulin to diabetic rats returned concanavalin A and insulin binding to levels seen in controls. Incubation of tumor cells with concanavalin A prior to addition of ¹²⁵I-labelled insulin resulted in a reduction of insulin-binding capacity; succinyl-concanavalin A did not affect binding of insulin. The percent inhibition of insulin binding by concanavalin A was highest at the lower insulin concentrations, providing a linearized Scatchard plot that yielded a calculated K_d value comparable to the low-affinity portion of the curvilinear Scatchard plot for insulin binding. The dissociation rate of bound insulin depended on receptor occupancy. Addition of concanavalin A after insulin binding reached equilibrium resulted in increased insulin binding hormone concentrations, decreased rates of dissociation of insulin and a loss of the correlation between receptor occupancy and dissociation rates. Concanavalin A alone demonstrated an insulin-like effect on glucose transport, which in these tumor cells represents a decrease in transport of 3-O-methylglucose. These suggest that binding of both concanavalin A and insulin to cells from this hormonally responsive neoplasm is under insulin regulation and demonstrates similar characteristics to those reported for a variety of normal cells. Furthermore, the interaction between concanavalin A and the cell membranes affects the affinity of the insulin receptor for insulin and appears to decrease the observed negative cooperativity.     

Positive cooperativity of ryanodine binding to the calcium release channel of sarcoplasmic reticulum from heart and skeletal muscle

Ryanodine is a specific ligand for the calcium release channel which mediates calcium release in excitation-contraction coupling in muscle. In this study, ryanodine binding in sarcoplasmic reticulum from heart muscle and skeletal muscle is further compared and correlated with function. The new findings include the following: (1) Two types of binding, high affinity (KD1 approximately 5-10 nM) and low affinity (KD2 approximately 3 microM), can now be discerned for the skeletal muscle receptor. KD1 is approximately the same as and KD2 of similar magnitude to that previously reported for heart. (2) The dissociation rates for the high-affinity binding have been directly measured for both heart and skeletal muscle (t1/2 approximately 30-40 min). These rates are more rapid than previously reported (t1/2 approximately 14 h). (3) KD1's obtained from the ratio of the dissociation and association rate constants agree with the dissociation constant measured by equilibrium binding Scatchard analysis. (4) Ryanodine binding to the low-affinity site can be correlated with a decrease in the dissociation rate constant (k-1) of the high-affinity site, and thereby in the apparent dissociation constant (KD1). The inhibition constant (KI) for inhibiting the high-affinity off rate obtained from a double-reciprocal plot of the change in off rate vs [ryanodine] is practically the same in heart (0.66 microM) and skeletal muscle (0.64 microM) and in the range of the KD2. The binding of cold ryanodine to the low-affinity site appears to lock the bound [3H]ryanodine onto the high-affinity site rather than to exchange with it. Thus, in this sense, the ryanodine receptor exhibits "positive cooperativity".(ABSTRACT TRUNCATED AT 250 WORDS)     

Negatively cooperative binding of high-density lipoprotein to the HDL receptor SR-BI

Scavenger receptor class B, type I (SR-BI), is a high-density lipoprotein (HDL) receptor, which also binds low-density lipoprotein (LDL), and mediates the cellular selective uptake of cholesteryl esters from lipoproteins. SR-BI also is a coreceptor for hepatitis C virus and a signaling receptor that regulates cell metabolism. Many investigators have reported that lipoproteins bind to SR-BI via a single class of independent (not interacting), high-affinity binding sites (one site model). We have reinvestigated the ligand concentration dependence of (125)I-HDL binding to SR-BI and SR-BI-mediated specific uptake of [(3)H]CE from [(3)H]CE-HDL using an expanded range of ligand concentrations (<1 μg of protein/mL, lower than previously reported). Scatchard and nonlinear least-squares model fitting analyses of the binding and uptake data were both inconsistent with a single class of independent binding sites binding univalent lipoprotein ligands. The data are best fit by models in which SR-BI has either two independent classes of binding sites or one class of sites exhibiting negative cooperativity due to either classic allostery or ensemble effects ("lattice model"). Similar results were observed for LDL. Application of the "infinite dilution" dissociation rate method established that the binding of (125)I-HDL to SR-BI at 4 °C exhibits negative cooperativity. The unexpected complexity of the interactions of lipoproteins with SR-BI should be taken into account when interpreting the results of experiments that explore the mechanism(s) by which SR-BI mediates ligand binding, lipid transport, and cell signaling.     

Characteristics of Partially Purified Nerve Growth Factor Receptor

Receptors for the nerve growth factor protein (NGF) have been isolated from three cell types [embryonic chicken sensory neurons (dorsal root sensory ganglia; DRG), rat pheochromocytoma (PC12) and human neuroblastoma (LAN-1) cells] and have been shown to be similar with respect to equilibrium dissociation constants. The present results demonstrate that there are multiple molecular weight species for NGF receptors from DRG neurons and PC12 cells. NGF receptors can be isolated from DRG as four different molecular species of 228, 187, 125, and 112 kilodaltons, and PC12 cells as three molecular species of 203, 118, and 107 kilodaltons. The NGF receptors isolated from DRG show different pH-binding profiles for high- and low-affinity binding. High-affinity binding displays a bell-shaped pH profile with maximum binding between pH 7.0 and 7.9, whereas low-affinity binding is constant between pH 5.0 and 9.1, with a twofold greater binding at pH 3.6. At 22 degrees C, the association rate constant was found to be 9.5 +/- 1.0 X 10(6) M-1 s-1. Two dissociation rate constants were observed. The fast dissociating receptor has a dissociation rate constant of 3.0 +/- 1.5 X 10(-2) s-1, whereas the slow dissociating receptor constant was 2.4 +/- 1.0 X 10(-4) s-1. The equilibrium dissociation constants calculated from the ratio of dissociation to association rate constants are 2.5 X 109-11) M for the high-affinity receptor (type I) and 3.2 X 10(-9) M for the low-affinity receptor (type II). These values are the same as those determined by equilibrium experiments on the isolated receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Leukotriene B4 binding sites in guinea-pig alveolar macrophages

Leukotriene B4 binding sites were investigated in alveolar macrophages obtained from guinea-pigs by brochoalveolar lavage. Analysis of the binding data was compatible with a two-receptors model. Best-fit computer-assisted evaluation of the results yielded a KD = 0.33 +/- 0.18 nM with 618 +/- 138 binding sites/cell for the high-affinity receptor, and KD = 52.9 +/- 12.3 nM with 95,400 +/- 37,900 sites/cell for the low-affinity binding site. Study of the dissociation rate of labelled ligand induced by dilution only and by dilution plus excess unlabelled ligand showed no differences in the two situations. These data suggest that the finding of two receptors is not due to negative cooperativity. Since most studies failed to demonstrate two distinct LTB4-binding proteins, the present results reinforces the hypothesis of LTB4 receptors in guinea-pig alveolar macrophages being a single protein with interchangeable affinity states.     

Effect of phospholipases and proteases on the [3H]N6-(R)-phenylisopropyladenosine ([3H]R-PIA) binding to A1 adenosine receptors from pig cerebral cortex.

The effect of phospholipases and proteases on the membrane-bound and solubilized A1 adenosine receptor has been studied. Phospholipids modulate the [3H]N6-(R)-phenylisopropyladenosine binding to A1 adenosine receptors in crude membranes and in soluble preparations, because changes in the phospholipid environment decrease both the binding capacity and the affinity for the ligand. It has become clear that 1) there is co-solubilization of receptor and phospholipids; 2) the phospholipid requirements are different for the coupled and the uncoupled receptor; 3) a net charge in the polar head produced by phospholipase D prevents the agonist binding to the receptor-G protein complex; alternatively, when the whole polar head is removed by phospholipase C the uncoupled receptor is altered; and 4) the protease action upon the receptor suggests that receptor coupled to G protein is more protected by the membrane than the uncoupled receptor. In kinetic experiments performed on membranes it was demonstrated that phospholipase C and trypsin increased the Kd value of the high-affinity state by modifying both k1 and k-1. In contrast they only modified the dissociation constant of the low-affinity state. In conclusion it should be noted that phospholipids play a key role for the binding of R-PIA to A1 adenosine receptor. Also, a different disposition within the membrane of the coupled and uncoupled receptor is encountered.     

A1 Adenosine Receptor-G Protein Coupling in Bovine Brain Membranes: Effects of Guanine Nucleotides, Salt, and Solubilization

The effects of guanine nucleotides, NaCl, and solubilization on the interaction of antagonists and agonists with the A1 adenosine receptor of bovine brain membranes were studied using the high-affinity antagonist radioligand [3H]xanthine amine congener ([3H]XAC). In membranes, guanine nucleotides and NaCl had no effect on [3H]XAC saturation curves. Using agonist (R)-phenylisopropyladenosine (R-PIA) competition curves versus [3H]XAC, it was demonstrated that agonists could differentiate two affinity states having high and low affinity for agonist and that guanine nucleotides shifted the equilibrium to an all-low-affinity state that was indistinguishable from the low-affinity state in the absence of guanine nucleotides. In contrast, NaCl decreased agonist affinity by a distinctly different mechanism characterized by a parallel rightward shifted agonist curve such that R-PIA still recognized two affinity states albeit of lower affinity than in the absence of salt. R-PIA competition curves in the presence of both guanine nucleotides and salt were still shallow but were shifted far to the right, and two very low affinity states were discerned. On solubilization, guanine nucleotides in a reversible, concentration-dependent manner increased antagonist ([3H]XAC) but not agonist (R-N6-[3H]phenylisopropyladenosine) binding. This was consequent to a change in maximal binding capacity. R-PIA competition curves (versus [3H]XAC) in solubilized preparations demonstrated that agonist could still differentiate two agonist specific affinity states which were modulated by guanine nucleotides. In the presence of guanine nucleotides all the receptors were shifted to a uniform low-affinity state. In contrast, NaCl had no effect on agonist affinity as determined by agonist competition curves in a solubilized receptor preparation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyrotropin receptors in normal human thyroid. Nonclassical binding kinetics not explained by the negative cooperativity model

Saturation analysis of equilibrium binding of iodinated thyrotropin (125I-TSH) to normal human thyroid preparations yielded linear Scatchard plots under non-physiological conditions of pH 6.0 or 20 mM Tris/acetate buffer, pH 7.4. The apparent equilibrium dissociation constant of this binding was approximately 10(-8) M. By contrast, nonlinear plots were obtained under standard conditions of pH 7.4 and 40 mM Tris/acetate buffer. Resolution of the components of these curves by computer analysis revealed the presence of at least two classes of binding sites, one of which is of a low capacity and high affinity (approximately 10(-10) M) consistent with receptor binding. The other component is of a high capacity and lower affinity. Binding to non-target tissues of muscle, parathyroid, mammary carcinoma, and placenta was only demonstrable at pH 6.0 or in 20 mM Tris/acetate buffer, pH 7.4, yielding linear Scatchard plots with similar binding affinity (approximately 10(-8)M) to normal thyroid but much reduced capacity. Preincubation of thyroid tissue at 50 degrees C resulted in an apparent selective loss of the high affinity component of binding measured under standard conditions. Kinetic experiments on the dissociation of bound 125I-TSH were undertaken to determine whether the non-linearity of Scatchard plots was due to two or more classes of binding sites or negative cooperativity. It was found that the experimental determinant that is presently ascribed to a negative cooperativity phenomenon regulating receptor affinity (i.e. an enhanced dilution-induced dissociation rate in the presence of excess native hormone), although apparently hormone-specific, was demonstrated under nonphysiological binding conditions and in non-target tissue. Significantly, the phenomenon was found under conditions of pH 6.0 or 20 mM Tris where a linear Scatchard plot was obtained. The evidence thus suggests that 125I-TSH binds to heterogeneous binding sites (of which the high affinity is probably the receptor for TSH) and that the enhanced dilution-induced dissociation of bound hormone by native hormone for this system, is only a characteristic of the low affinity binding site (maybe gangliosides).     

GABA, depressants and chloride ions affect the rate of dissociation of 35S-t-butylbicyclophosphorothionate binding

The dissociation of 35S-TBPS was studied from binding sites of rat cerebral cortex. Monophasic dissociation plots became polyphasic and accelerated in the presence of micromolar concentrations of GABA suggesting the involvement of low (or super-low) affinity GABA receptors. The presence of the depressants etazolate, R(-)MPPB and ethanol resulted in similarly accelerated dissociation patterns. In contrast, the convulsants S(+)MPPB and pentamethylenetetrazol did not significantly affect the dissociation of TBPS. Dissociation initiated by dilution was not affected either by an excess of picrotoxin or by varying the equilibrium occupancy of the TBPS sites. These findings rule out the possibility of a kinetic cooperativity for the binding of convulsants. The removal of chloride ions also enhanced the rate of TBPS dissociation. Kinetic heterogeneity of the TBPS binding sites can be interpreted with allosteric interactions mediated by various sites at the GABA receptor complex coupled to different states of the chloride ionophore.     

The vanadate complex of the calcium-transport ATPase of the sarcoplasmic reticulum, its formation and dissociation

Vanadate binding to different sarcoplasmic reticulum membrane preparations was determined by measuring bound vanadate colorimetrically and by phosphorylating the vanadate-free enzyme fraction with [gamma-32P] ATP. Colorimetry allowed the study of the dependence of equilibrium vanadate binding on ionized magnesium and the displacing effect of ionized calcium at vanadate concentrations greater than 0.1 mM only. At saturating magnesium concentration the enzyme binds 6-8 nmol vanadate/mg protein and half-maximum saturation is reached at 40 microM. Vanadate is displaced from the enzyme when its high-affinity calcium-binding sites are saturated and conversely calcium is solely displaced from its high-affinity binding sites by vanadate. The phosphorylation procedure allowed the measurement of equilibrium binding as well as the kinetics of vanadate binding and release at vanadate concentrations below 0.1 mM. Half-times of 30s and 3s were observed for vanadate release induced by 0.1 mM and 1 mM calcium respectively. Millimolar concentrations of ATP are required for vanadate displacement. Under equilibrium conditions the enzyme displays an affinity for vanadate of 1.6 X 10(6) M-1. The dependence on the concentration of vanadate of the rate of vanadate binding yielded an affinity of only 1 X 10(4) M-1. Closed vesicles bind vanadate much more slowly than calcium-permeable preparations. The initial rate of calcium-induced vanadate dissociation is accelerated considerably when the vesicles are made calcium permeable. The rate of vanadate dissociation from calcium-permeable vesicles reaches half-maximum values at 1-2 mM calcium indicating that the internal low-affinity calcium-binding sites must first be occupied in order to release bound vanadate. The results suggest that vanadate binding leads to a transition of the external high to internal low-affinity calcium-binding sites.     

Chondrodysplasia in transgenic mice harboring a 15-amino acid deletion in the triple helical domain of pro alpha 1(II) collagen chain

This report describes analysis of factors which regulate the binding of EGF to EGF receptor, receptor internalization, and receptor recycling. Three different methods were used to inhibit high-affinity EGF binding as measured at equilibrium: treatment of cells with an active phorbol ester (PMA), binding of a mAb directed against the EGF receptor (mAb108), and truncation of most of the cytoplasmic domain of the receptor. These treatments reduced the rate at which low concentrations of EGF bound to cells, but did not affect the rate of EGF dissociation. We conclude that high-affinity EGF binding on living cells results from a difference in the apparent on rate of EGF binding. We then used these conditions and cell lines to test for the rate of EGF internalization at different concentrations of EGF. We demonstrate that internalization of the EGF receptor is stimulated roughly 50-fold at saturating concentrations of EGF, but is stimulated an additional two- to threefold at low concentrations (less than 1 nM). Four treatments reduce the rate of internalization of low concentrations of EGF to the rate seen at saturating EGF concentrations. Phorbol ester treatment and mAb108 binding to "wild type" receptor reduce this rate (and reduce high-affinity binding). Point mutation at Lys721 (kinase negative EGF receptor) and point mutation at Thr654 (removing a major site of protein kinase C phosphorylation) reduce the internalization rate, without affecting high-affinity binding. We suggest that while EGF stimulates endocytosis for all receptors, high-affinity receptors bind and are internalized more quickly than low-affinity receptors. Tyrosine kinase activity and the Thr654 region appear necessary for this response.     

Interaction of human monomeric C3b with its receptor (complement receptor type 1, CR1) on neutrophils. Evidence for negative cooperativity

The binding of highly purified monomeric 125I-C3b to its receptor (CR1) on resting human polymorphonuclear neutrophils (PMN) was analyzed under equilibrium conditions, at 4 degrees C and low ionic strength. Scatchard analysis of specific binding data yielded curvilinear concave upward plots, which resulted from the presence of site-site interactions of the negative type among PMN C3b-receptors (negative cooperativity), as shown by dissociation kinetic experiments. Indeed, the dissociation rate of 125I-C3b from PMN was markedly increased in the presence of an excess of unlabeled C3b in the dilution medium and was directly dependent on the degree of initial receptor occupancy with the radioligand. These interactions occurred when 2% of the receptors were occupied with 125I-C3b and resulted in a 4-fold decrease in CR1 affinity when the receptor went from its "empty" to its "filled" conformation. In a disease associated with a continuous production of C3b (factor I deficiency), CR1 on in vivo circulating PMN was found to be in a "low affinity" and "high dissociating" state similar to that of normal CR1 at high occupancy. Finally, negative cooperativity among CR1 sites disappeared after PMN activation with chemotactic peptides.     

Kinetic basis for insensitivity to tetrodotoxin and saxitoxin in sodium channels of canine heart and denervated rat skeletal muscle

The single-channel blocking kinetics of tetrodotoxin (TTX), saxitoxin (STX), and several STX derivatives were measured for various Na-channel subtypes incorporated into planar lipid bilayers in the presence of batrachotoxin. The subtypes studied include Na channels from rat skeletal muscle and rat brain, which have high affinity for TTX/STX, and Na channels from denervated rat skeletal muscle and canine heart, which have about 20-60-fold lower affinity for these toxins at 22 degrees C. The equilibrium dissociation constant of toxin binding is an exponential function of voltage (e-fold per 40 mV) in the range of -60 to +60 mV. This voltage dependence is similar for all channel subtypes and toxins, indicating that this property is a conserved feature of channel function for batrachotoxin-activated channels. The decrease in binding affinity for TTX and STX in low-affinity subtypes is due to a 3-9-fold decrease in the association rate constant and a 4-8-fold increase in the dissociation rate constant. For a series of STX derivatives, the association rate constant for toxin binding is approximately an exponential function of net toxin charge in membranes of neutral lipids, implying that there is a negative surface potential due to fixed negative charges in the vicinity of the toxin receptor. The magnitude of this surface potential (-35 to -43 mV at 0.2 M NaCl) is similar for both high- and low-affinity subtypes, suggesting that the lower association rate of toxin binding to toxin-insensitive subtypes is not due to decreased surface charge but rather to a slower protein conformational step. The increased rates of toxin dissociation from insensitive subtypes can be attributed to the loss of a few specific bonding interactions in the binding site such as loss of a hydrogen bond with the N-1 hydroxyl group of neosaxitoxin, which contributes about 1 kcal/mol of intrinsic binding energy.     

Comparison of the effects of chemical and isotopic dilution on the dissociation of bound labeled ligands

The dissociation of a labeled ligand from a binding structure to which it is reversibly attached can be promoted either by dilution or by chase. The kinetics of the dissociation brought up by dilution can be modified or not by the presence of various concentrations of cold ligand, according to the molecular mechanism of interaction. Analog computer simulation leads to the following results: (i) no cooperativity, monomolecular dissociation: no modification; (ii) positive or negative cooperativity (sequential models): acceleration, no modification, or slowing down (according to the kinetic constants); (iii) positive cooperativity (concerted model): no modification; (iv) two-step interaction: no modification if both interaction steps take place in the same phase, otherwise acceleration; and, (v) bimolecular association and dissociation: acceleration. This methodology could be used in order to characterize the molecular properties of various binding structures in the field of drug and hormone receptors as well as in enzymology.     

Benzodiazepine ligands can act as allosteric modulators of the Type 1 cholecystokinin receptor

The cholecystokinin (CCK₁) receptor is a G protein-coupled receptor important for nutrient homeostasis. The molecular basis of CCK-receptor binding has been debated, with one prominent model suggesting occupation of the same region of the intramembranous helical bundle as benzodiazepines. Here, we used a specific assay of allosteric ligand interaction to probe the mode of binding of devazepide, a prototypic benzodiazepine ligand. Devazepide elicited marked slowing of dissociation of pre-bound CCK, only possible through binding to a topographically distinct allosteric site. This effect was disrupted by chemical modification of a cysteine in the benzodiazepine-binding pocket. Application of an allosteric model to the equilibrium interaction between a series of benzodiazepine ligands and CCK yielded quantitative estimates of each modulator’s affinity for the allosteric site, as well as the degree of negative cooperativity for the interaction between occupied orthosteric and allosteric sites. The allosteric nature of benzodiazepine binding to the CCK₁ receptor provides new opportunities for small molecule drug development.     

Highly sequential binding of protein kinase C and related proteins to membranes

Protein kinase C belongs to a class of proteins that displays simultaneous interaction with calcium and phospholipids. Other members of this class include two proteins (Mr 64K and 32K) isolated from bovine brain. The association of these proteins with membranes exhibited highly unusual properties that were not consistent with a simple equilibrium. Titration of protein-phospholipid binding as a function of calcium showed an apparently normal curve with a low degree of cooperativity. The binding was rapid and quickly adjusted to changes in the calcium concentration. Calcium was readily exchanged from the protein-phospholipid complex. However, at each calcium concentration, membrane-bound protein was not in rapid equilibrium with free protein in solution; the half-time for dissociation exceeded 24 h. Titration of phospholipid vesicles with proteins showed different saturation levels of bound protein at different calcium concentrations. The amount of protein bound was almost entirely determined by the concentration of calcium and was virtually unaffected by the free protein concentration. These properties suggested that protein-phospholipid binding involved a sequence of steps that were each irreversible upon completion. These binding properties were consistent with high-affinity interaction between protein and phospholipid, high cooperativity with respect to calcium (N greater than or equal to 10), clustering of acidic phospholipids, and negative cooperativity with respect to protein density on the membrane. A major apparent problem with the complete titration of PKC-membrane interaction was a requirement for calcium in excess of intracellular levels. However, a highly sequential binding process showed that a number of protein-binding sites on the membrane would be saturated with calcium at physiological levels.(ABSTRACT TRUNCATED AT 250 WORDS)