Showing papers on "Conformational change published in 1989"

Hemagglutinins from Two Influenza Virus Variants Bind to Sialic Acid Derivatives with Millimolar Dissociation Constants: A 500 MHz Proton Nuclear Magnetic Resonance Study

Abstract: The equilibrium binding of influenza virus hemagglutinin to derivatives of its cell-surface ligand, sialic acid, was measured by nuclear magnetic resonance (NMR) spectroscopy. Binding was quantified by observing perturbations of sialic acid resonances in the presence of protein. The major perturbation observed was a chemical shift of the N-acetyl methyl resonance, presumably due to the proximity of the methyl group to tryptophan 153. X-31 hemagglutinin binds to the methyl alpha-glycoside of sialic acid with a dissociation constant of 2.8 mM and does not bind to the methyl beta-glycoside. Replacing the 4-hydroxyl group of sialic acid with an acetyl group has little effect, while replacing the 7-hydroxyl group with an acetyl prevents binding. Experiments with sialylated oligosaccharides confirm literature reports that mutations at amino acid 226 change the specificity of hemagglutinin for alpha(2,6) and alpha(2,3) glycosidic linkages. The NMR line broadening of sialyloligosaccharides suggests that sialic acid is the only component that contacts the protein. Saccharides containing two sialic acid residues appear to have two separate binding modes. Hemagglutinin that has undergone a low pH induced conformational change retains the ability to bind sialic acid.

The allosteric transition of glycogen phosphorylase.

Abstract: The crystal structure of R-state glycogen phosphorylase b has been determined at 2.9 A resolution. A comparison of T-state and R-state structures of the enzyme explains its cooperative behaviour on ligand binding and the allosteric regulation of its activity. Communication between catalytic sites of the dimer is provided by a change in packing geometry of two helices linking each site with the subunit interface. Activation by AMP or by phosphorylation results in a quaternary conformational change that switches these two helices into the R-state conformation.

Cytochrome c at charged interfaces. 1. Conformational and redox equilibria at the electrode/electrolyte interface probed by surface-enhanced resonance Raman spectroscopy.

Abstract: The structure and the electron-transfer properties of cytochrome c (cyt c) absorbed on a silver electrode were analyzed by surface-enhanced resonance Raman spectroscopy. It was found that the absorbed cyt c exists in various conformational states depending on the electrode potential. In state I the native structure of the heme protein is fully preserved and the redox potential (+0.02 V vs saturated calomel electrode) is close to the value for cyt c in solution. In state II the heme iron exists in a mixture of five-coordinated high-spin and six-coordinated low-spin configurations. It had been shown that these configurations form a thermal equilibrium [Hildebrandt, P., & Stockburger, M. (1986) J. Phys. Chem. 90,6017]. It is demonstrated that these equilibria strongly depend on the charge distribution within the electrical double layer of the silver electrode/electrolyte interface, indicating that the changes in the coordination shell are induced by electrostatic interactions. The structural alterations in state II are apparently restricted to the heme crevice, which assumes an open conformation compared to the close structure in state I. This leads to a strong decrease of the redox potentials, which were determined to be -0.31 and -0.41 V for the five-coordinated high-spin and six-coordinated low-spin configurations, respectively. On the other hand, gross distortions of the protein structure can be excluded since the reversible proton-induced conformational change of cyt c as found in solution at low pH also takes place in state II of the absorbed cyt c. The linkage of cyt c molecules to the surface is mediated by charged amino acid groups, and it depends on the potential which groups are thermodynamically favored to form such a molecular binding site. The conformational states I and II, which are in potential-dependent equilibrium, therefore refer to two different molecular binding sites. At potentials below zero charge (less than approximately -0.6 V) a rapid denaturation of the absorbed cyt c is noted, which is reflected by drastic and irreversible changes in the surface-enhanced resonance Raman spectrum. Our results are discussed on the background of previous electrochemical studies of cyt c at electrodes.

Epidermal growth factor binding induces a conformational change in the external domain of its receptor.

Abstract: To study the properties of the extracellular epidermal growth factor (EGF) binding domain of the human EGF receptor, we have infected insect cells with a suitably engineered baculovirus vector containing the cDNA encoding the entire ectodomain of the parent molecule. This resulted in a correctly folded, stable, 110 kd protein which possessed an EGF binding affinity of 200 nM. The protein was routinely purified in milligram amounts from 1 litre insect cell cultures using a series of three standard chromatographic steps. The properties of the ectodomain were studied before and after the addition of different EGF ligands, using both circular dichroism and fluorescence spectroscopic techniques. A secondary structural analysis of the far UV CD spectrum of the ectodomain indicated significant proportions of alpha-helix and beta-sheet in agreement with a published model of the EGF receptor. The ligand additions to the receptor showed differences in both the near- and far-UV CD spectra, and were similar for each ligand used, suggesting similar conformational differences between uncomplexed and complexed receptor. Steady-state fluorescence measurements indicated that the tryptophan residues present in the ectodomain are buried and that the solvent-accessible tryptophans in the ligands become buried on binding the receptor. The rotational correlation times measured by fluorescence anisotropy decay for the receptor-ligand complexes were decreased from 6 to 2.5 ns in each case. This may indicate a perturbation of the tryptophan environment of the receptor on ligand binding. Ultracentrifugation studies showed that no aggregation occurred on ligand addition, so this could not explain the observed differences from CD or fluorescence.(ABSTRACT TRUNCATED AT 250 WORDS)

Melittin binding causes a large calcium-dependent conformational change in calmodulin

Abstract: The interaction between calmodulin and its target protein is a key step in many calcium-regulated cellular functions. Melittin binds tightly to calmodulin in the presence of calcium and is a competitive inhibitor of calmodulin function. Using melittin as a model for the target peptide of calmodulin, we have found a large Ca2+-dependent conformational change of calmodulin in solution induced by peptide binding. Mg2+ does not substitute for Ca2+ in producing the conformation change. Small-angle x-ray scattering has shown that calmodulin exists as a dumbbell in solution, similar to that observed in the crystalline state. Our present measurements reveal that the overall structure of the Ca2+-calmodulin-melittin complex is not a dumbbell but a globular shape. Upon binding melittin, the radius of gyration decreases from 20.9 to 18.0 A and the largest dimension decreases from 60 to 47.5 A. In the absence of calcium, however, melittin has little effect on the solution structure of calmodulin.

Escherichia coli cAMP receptor protein: evidence for three protein conformational states with different promoter binding affinities.

Abstract: Cyclic AMP receptor protein (CRP) from Escherichia coli is assumed to exist in two states, namely, those represented by the free protein and that of the ligand-protein complex. To establish a quantitative structure-function relation between cAMP binding and the cAMP-induced conformational changes in the receptor, protein conformational change was quantitated as a function of cAMP concentration up to 10 mM. The protein conformation was monitored by four different methods at pH 7.8 and 23 degrees C, namely, rate of proteolytic digestion by subtilisin, rate of chemical modification of Cys-178, tryptophan fluorescence, and fluorescence of the extrinsic fluorescence probe 8-anilino-1-naphthalenesulfonic acid (ANS). Each of these techniques reveals a biphasic dependence of protein conformation on cAMP concentration. At low cAMP concentrations ranging from 0 to 200 microM, the rates of proteolytic digestion and that of Cys-178 modification increase, whereas the fluorescence intensity of the ANS-protein complex is quenched, and there is no change in the fluorescence intensity of the tryptophan residues in the protein. At higher cAMP concentrations, the rates of proteolytic and chemical modification of the protein decrease, while the fluorescence intensity of the ANS-protein complex is further quenched but there is an increase in the intensity of tryptophan fluorescence. These results show unequivocally that there are at least three conformational states of the protein. The association constants for the formation of CRP-cAMP and CRP-(cAMP)2 complexes derived from conformational studies are in good agreement with those determined by equilibrium dialysis, nonequilibrium dialysis, and ultrafiltration. Therefore, the simplest explanation would be that the protein exhibits three conformational states, free CRP and two cAMP-dependent states, which correspond to the CRP-cAMP and CRP-(cAMP)2 complexes. The binding properties of CRP-cAMP and CRP-(cAMP)2 to the lac promoter were studied by using the gel retardation technique. At a high concentration of cAMP which favors the formation of the CRP-(cAMP)2 complex, binding of the protein to DNA is decreased. This, together with conformational data, strongly suggests that only the CRP-cAMP complex is active in specific DNA binding whereas CRP and CRP-(cAMP)2 are not.

Identification of receptor-binding residues in the inflammatory complement protein C5a by site-directed mutagenesis

Abstract: C5a is an inflammatory mediator potentially involved in a number of diseases. To help define which of its 74 residues are important for receptor binding and response triggering, changes in the amino acid sequence of C5a were introduced by site-directed mutagenesis. Synthetic C5a-encoding genes incorporating point mutations were expressed in Escherichia coli, and the mutant proteins were purified to homogeneity. Modifications of the C5a molecule causing parallel reductions in binding to polymorphonuclear leukocyte membranes and in stimulation of polymorphonuclear leukocyte locomotion (chemokinesis) suggest that carboxyl-terminal residues Lys-68, Leu-72, and Arg-74 interact with the receptor. Substitutions in the disulfide-linked core of C5a revealed involvement of Arg-40 or nearby residues, because potency losses were associated with only localized conformational changes as detected by NMR. Surprisingly, a substitution at core residue Ala-26, which did not alter C5a core structure, appeared from NMR results to reduce potency by causing a long-distance conformational change centered on residue His-15. Thus, at least three discontinuous regions of the C5a molecule appear to act in concert to achieve full potency.

Crystal structure of guanosine-free ribonuclease T1, complexed with vanadate (V), suggests conformational change upon substrate binding.

Abstract: Ribonuclease T1 was crystallized in the presence of vanadate(V). The crystal structure was solved by molecular replacement and refined by least-squares methods using stereochemical restraints. The refinement was based on data between 10 and 1.8 A and converged at a crystallographic R factor of 0.137. Except for the substrate-recognition site the three-dimensional structure of ribonuclease T1 closely resembles the structure of the enzyme complexed with guanosine 2'-phosphate and its derivatives. A tetrahedral anion was found at the catalytic site and identified as H2VO4-. This is the first crystal structure of ribonuclease T1 determined in the absence of bound substrate analogue. Distinct structural differences between guanosine-free and complexed ribonuclease T1 are observed at the base-recognition site: The side chains of Tyr45 and Glu46 and the region around Asn98 changed their conformations, and the peptide bond between Asn43 and Asn44 has turned around by 140 degrees. We suggest that the structural differences seen in the crystal structures of free and complexed ribonuclease T1 are related to conformational adjustments associated with the substrate binding process.

Kinetics of the purified glucose transporter. Direct measurement of the rates of interconversion of transporter conformers.

Abstract: There is considerable evidence that the mechanism of glucose transport by the transporter of human erythrocytes is one in which the transporter oscillates between two conformations, To and Ti. Each conformer possesses a single glucose binding site that in vivo faces either the extracellular space (conformer To) or the cytoplasm (conformer Ti). In this study, the interconversions of these conformers in the absence and presence of D-glucose have been directly observed by means of the stopped-flow method with fluorescence detection. Nearly unidirectional conversion of one conformer to the other was accomplished by rapidly mixing purified transporter (a mixture of To and Ti) with either 4,6-ethylidene-D-glucose, which preferentially binds to To, or phenyl beta-D-glucoside, which preferentially binds to Ti. The values of the individual rate constants for the conversion of Ti to To and vice versa in the absence and presence of D-glucose at 10.0 degrees C have been obtained, and these show that the kinetics are consistent with the alternating conformation model for transport. Conformational change occurs much more rapidly with glucose bound to the transporter. Furthermore, the activation energy Ea for conformer interconversion is much less when glucose is bound than for unliganded transporter. For example, Ea is approximately 28 kcal/mol for Ti----To versus 17 kcal/mol for Ti + S----ToS, where S is glucose. The alpha-anomer of glucose was 37% more effective than the beta-anomer in speeding the interconversion.(ABSTRACT TRUNCATED AT 250 WORDS)

Aggregation-related conformational change of the membrane-associated coat protein of bacteriophage M13

Abstract: The state of the coat protein of bacteriophage M13, reconstituted into amphiphilic media, has been investigated. The in situ conformation of the coat protein has been determined by using circular dichroism. Minimum numbers for the protein aggregation in the system have been determined after disruption of the lipid-protein system and subsequent uptake of the protein in cholate micelles. The aggregational state and conformation of the protein were affected by (1) the method of coat protein isolation (phenol extraction vs cholate isolation), (2) the nature of amphiphiles used (variation in phospholipid headgroups and acyl chains), and (3) the ratio of amphiphiles and protein. Under all conditions, phenol-extracted coat protein was in a predominantly beta-structure and in a highly aggregated polymeric form. Cholate-isolated coat protein was initially oligomeric and contained a substantial amount of alpha-helix. Below an aggregation number of 20, this protein showed a reversible aggregation with no change in conformation. Upon further aggregation, a conformational change was observed, and aggregation was irreversible, resulting in predominantly beta-structured coat protein polymers. This effect was observed upon uptake in phospholipids at low lipid to protein molar ratios (L/P ratios) and with phosphatidylcholines (PC) and phosphatidic acids (PA) containing saturated acyl chains. After reconstitution in phospholipids with unsaturated acyl chains and with phosphatidylglycerols (PG) at high L/P ratios, the original alpha-helix-containing state of the coat protein was maintained. Cross-linking experiments demonstrated that the beta-polymers are able to form reversible superaggregates within the vesicle system. An aggregation-related conformational change mechanism for the coat protein in phospholipid systems is proposed.

Conformational-analysis Of Some Chiral Catalysts Of The Cinchona And Ephedra Family - The Alkaloid Catalyzed Addition Of Aromatic Thiols To Cyclic Alpha,beta-unsaturated Ketones

Abstract: The minimum energy conformations of quinine, quinidine, ephedrine and N-methyl-ephedrine have been calculated using molecular mechanics and semi-empirical techniques. Conformational data for quinine and quinidine in solution have been obtained by 2D-NMR. No evidence has been found for an earlier proposed conformational change of the catalysts3 in the asymmetric Michael addition between cycloalkenones and aromatic thiols. The preferred orientations between 4-methylbenzenethiol and quinine have been predicted with molecular docking calculations; they are in good agreement with NOESY NMR data. The results obtained by this combined NMR and molecular modelling approach suggest a new model for the transition state of alkaloid-catalyzed addition of aromatic thiols to 2-cyclohexenone.

Conformational changes of plasma fibronectin detected upon adsorption to solid substrates: a spin-label study.

Abstract: Changes in local environment of the free sulfhydryl groups in plasma fibronectin upon adsorption of the protein to polystyrene beads have been examined by electron spin resonance (ESR) spin-label spectroscopy. The two free sulfhydryl groups per subunit of plasma fibronectin were modified chemically with an ({sup 15}N,{sup 2}H)maleimide spin-label. For soluble fibronectin, both free sulfhydryl groups shown to be in confined environments as evidenced from the labeled protein exhibiting a strongly immobilized ESR spectrum as described previously using ({sup 14}N,{sup 1}H)maleimide spin-labels. When the labeled protein was adsorbed to the beads, half of the strongly immobilized component was found to convert into a weakly immobilized component, a result indicating that one of the two labeled sites becomes exposed and exhibits a fast tumbling motion. Experiments conducted using various spin-labeled fibronectin fragments suggest that the newly exposed labeled site is located between the DNA-binding and the cell-binding regions of the molecule. The data obtained indicate that, upon adsorption to polystyrene beads, plasma fibronectin undergoes a conformational change through which the buried free sulfhydryl group near the cell-binding region of the molecule is exposed. This observation may have important implications regarding the expression of cell adhesive properties of the fibronectin molecule.

Mechanism of protein-induced membrane fusion: fusion of phospholipid vesicles by clathrin associated with its membrane binding and conformational change.

Abstract: The clathrin-induced fusion of liposome membranes, the membrane binding of clathrin, and the conformational states of clathrin were investigated over a wide pH range using large unilamellar and multilamellar vesicles composed of phosphatidylserine (PS), phosphatidylcholine (PC), PS/PC (2:1), PS/PC (1:1), or PS/PC (1:2). The pH profiles of clathrin-induced fusion of all types of liposomes containing PS showed biphasic patterns. Their pH thresholds were found in the pH range of 5-6 and shifted to lower pH values with decrease in the PS content. Similar shifts were observed in the pH range of 5-6 and shifted to lower pH values with decrease in the PS content. Similar shifts were observed in the pH profiles of clathrin binding to these vesicles, but the pH profiles of binding were different from the biphasic fusion patterns. With PC vesicles, only small degrees of fusion and clathrin binding were observed at pH 2-4. The pH dependences of the conformation and hydrophobicity of clathrin were determined by measuring the extent of the blue shift of the fluorescence maximum of 1-anilinonaphthalene-8-sulfonate in the presence of the protein, the fluorescence intensity of N-(1-anilinonaphthyl-4)maleimide bound to the clathrin molecule, the resonance energy transfer from its tryptophan to anilinonaphthyl residues, the partitioning of the protein in Triton X-114 solution, and the hydrophobicity index of clathrin using cis-parinaric acid. These measurements indicated that conformational change and exposure of hydrophobic regions occur below pH 6 and suggested that clathrin may adopt different conformational states in the pH region where it induced membrane fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between enzyme activity and hinge-bending domain displacement in 3-phosphoglycerate kinase

Abstract: Diffuse X-ray-scattering data give evidence for large-scale structural change in pig muscle 3-phosphoglycerate kinase upon substrate binding. Simultaneous binding of 3-phosphoglycerate and MgATP either to the unmodified enzyme or to its active methylated derivative leads to about an 0.1-nm decrease in radius of gyration. These data coincide well with the previous data for yeast 3-phosphoglycerate kinase. When, instead of methylation, the two reactive thiol groups of pig muscle 3-phosphoglycerate kinase are carboxamidomethylated, the enzyme becomes inactive and the radii of gyration of its 'apo' and 'holo' forms do not differ within limits of experimental error. Thus, a correlation exists between the activity of 3-phosphoglycerate kinase and its substrate-induced large-scale conformational change. This correlation is a strong argument in favor of the functional importance of domain locking in the reaction catalyzed by 3-phosphoglycerate kinase.

13C NMR of methylated lysines of fd gene 5 protein: evidence for a conformational change involving lysine 24 upon binding of a negatively charged lanthanide chelate.

Abstract: Helical complexes formed between fd DNA and reductively methylated fd gene 5 protein were indistinguishable by electron microscopy from complexes formed with the nonmethylated protein 13C NMR spectroscopy of 13C-enriched N epsilon, N epsilon-dimethyllsyl residues of the protein showed that three of these residues (Lys-24, Lys-46, and Lys-69) were selectively perturbed by binding of the oligomer d(pA)7 These were the same lysyl residues that we previously found to be most protected from methylation by binding of the protein to poly[r(U)] [Dick, L R, Sherry, A D, Newkirk, M M, & Gray D M (1988) J Biol Chem 263, 18864-18872] Thus, these lysines are probably directly involved in the nucleic acid binding function of the protein Negatively charged chelates of lanthanide ions were used to perturb the 13C NMR resonances of labeled lysyl and amino-terminal residues of the gene 5 protein The terbium chelate was found to bind tightly (Ka approximately 10(5) M-1) to the protein with a stoichiometry of 1 chelate molecule per protein dimer 13C resonances of Lys-24, Lys-46, and Lys-69 were maximally shifted by the terbium chelate and were maximally relaxed by the gadolinium chelate Also, the terbium chelate was excluded by the oligomer d(pA)7 Computer fits of the induced chemical shifts of 13C resonances with those expected for various positions of the terbium chelate failed to yield a possible chelate binding site unless the chemical shift for Lys-24 was excluded from the fitting process(ABSTRACT TRUNCATED AT 250 WORDS)

Role of substrate binding forces in exchange-only transport systems: II. Implications for the mechanism of the anion exchanger of red cells.

Abstract: The transition-state theory of exchange-only membrane transport is applied to experimental results in the literature on the anion exchanger of red cells. Two central features of the system are in accord with the theory: (i) forming the transition state in translocation involves a carrier conformational change; (ii) substrate specificity is expressed in transport rates rather than affinities. The expression of specificity is consistent with other evidence for a conformational intermediate (not the transition state) formed in the translocation of all substrates. The theory, in conjunction with concepts derived from the chemistry of macrocyclic ion inclusion complexes, prescribes certain essential properties in the transport site. Separate subsites are required for the preferred substrates, Cl- and HCO3-, to account for tight binding in the transition state (Kdiss congruent to 1 microM). Further, the following mechanism is suggested. A substrate anion initially forms a loose surface complex at one subsite, but in the transition state the subsites converge to form an inclusion complex in which the binding forces are greatly increased through a chelation effect. The conformational change at the substrate site, which is driven by the mounting forces of binding, sets in train a wider conformational change that converts the carrier from an immobile to a mobile form. Though simple, this composite-site mechanism explains many unusual features of the system. It accounts for substrate inhibition, partially noncompetitive inhibition of one substrate by another, and "tunneling," which is net transport under conditions where exchange should prevail, according to other models. All three types of behavior result from the formation of a ternary complex in which substrate anions are bound at both subsites. The mechanism also accounts for the enormous range of substrate structures accepted by the system, for the complex inhibition by the organic sulfate NAP-taurine, and for the involvement of several cationic side chains and two different protein domains in the transport site.

Role of substrate binding forces in exchange-only transport systems: I. Transition-state theory.

Abstract: An analysis of transition-state models for exchange-only transport shows that substrate binding forces, carrier conformational changes, and coupled substrate flow are interrelated. For a system to catalyze exchange but not net transport, addition of the substrate must convert the carrier from an immobile to a mobile form. The reduction in the energy barrier to movement is necessarily paid for out of the intrinsic binding energy between the substrate and the transport site, and is dependent on the formation of two different types of complex: a loose complex initially and a tight complex in the transition state in carrier movement. Hence the site should at first be incompletely organized for optimal binding but, following a conformational change, complementary to the substrate structure in the transition state. The conformational change, which may involve the whole protein, would be induced by cooperative interactions between the substrate and several groups within the site, involving a chelate effect. The tightness of coupling, i.e., the ratio of exchange to net transport, is directly proportional to the increased binding energy in the transition state, a relationship which allows the virtual substrate dissociation constant in the transition state to be calculated from experimental rate and half-saturation constants. Because the transition state is present in minute amount, strong bonding here does not enhance the substrate's affinity, and specificity may, therefore, be expressed in maximum exchange rates alone. However, where substrates largely convert the carrier to a transport intermediate whose mobility is the same with all substrates, specificity is also expressed in affinity. Hence the expression of substrate specificity provides evidence on the translocation mechanism.

Replacement of a conserved proline and the alkaline conformational change in iso-2-cytochrome c.

Abstract: Although point mutations usually lead to minor localized changes in protein structure, replacement of conserved Pro-76 with Gly in iso-2-cytochrome c induces a major conformational change. The change in structure results from mutation-induced depression of the pK for transition to an alkaline conformation with altered heme ligation. To assess the importance of position 76 in stabilizing the native versus the alkaline structure, the equilibrium and kinetic properties of the pH-induced conformational change have been compared for normal and mutant iso-2-cytochrome c. The pKapp for the conformational change is reduced from 8.45 (normal iso-2) to 6.71 in the mutant protein (Gly-76 iso-2), suggesting that conservation of Pro-76 may be required to stabilize the native conformation at physiological pH. The kinetics of the conformational change for both the normal and mutant proteins are well-described by a single kinetic phase throughout most of the pH-induced transition zone. Over this pH range, a minimal mechanism proposed for horse cytochrome c [Davis, L. A., Schejter, A., & Hess, G. P. (1974) J. Biol. Chem. 249, 2624-2632] is consistent with the data for normal and mutant yeast iso-2-cytochromes c: NH KH----N + H+ kcf in equilibrium kcb A NH and N are native forms of cytochrome c with a 695-nm absorbance band, A is an alkaline form that lacks the 695-nm band, KH is a proton dissociation constant, and kcf and kcb are microscopic rate constants for the conformational change. The Gly-76 mutation increases kcf by almost 70-fold, but kcb and KH are unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies to human growth hormone induce an allosteric conformational change in the antigen.

Abstract: We re-investigated the properties of a monoclonal antibody (mAb), 4D11, to human growth hormone (hGH) that showed a very weak affinity, recognizing hGH only when the hormone was solubilized on a solid surface. MAb4D11 did not significantly bind 125I-hGH. It was found that three mAb directed to different hGH epitopes (mAb 3C11, 10C1 and NA71) were able to induce the binding of the soluble antigen to mAb 4D11. The co-operative effect could be demonstrated by the formation of binary complexes (Ag:Ab, 1:2) detected by high-performance liquid chromatography (HPLC) and by the increase of radioactivity found when the synergistic mAb were added to 125I-hGH incubated with mAb 4D11 immobilized on polyvinyl microplates. Other possible explanations, such as the formation of cyclic complexes or the generation of a new epitope in the Fc fragment of the first antibody (Ab), were dismissed because the Fab fragment of one of the enhancing mAb (3C11) gave the same effect as the intact Ab. The data suggest that the hGH molecule undergoes a localized conformational change after binding to mAb 3C11, NA71 or 10C1 and that mAb 4D11 binds with high affinity to the modified region of the hormone. The formation or not of ternary complexes (Ag:Ab, 1:3) was used to localize the 4D11 epitope on the surface of the Ag. It is suggested that mAb 4D11 recognizes a conformational change produced in the region defined by the AE5/AC8 epitopes, which is close to the hGH antigenic domain only expressed when the protein is immobilized on plastic surfaces.