Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “ligand binding”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5Linked to original sources
PubMed2000

Three-dimensional NMR structure of the sixth ligand-binding module of the human LDL receptor: comparison of two adjacent modules with different ligand binding specificities.

The sixth ligand-binding module of the low-density lipoprotein receptor contributes to the binding of apolipoprotein B100-containing lipoproteins. 1H NMR spectroscopy, DYANA and X-PLOR structure calculations were used to determine that this module has a well defined structure with a backbone conformation similar to other modules. Structures from calculations that simulated the presence of a calcium ion showed increased resolution without large increases in energy, increased deviations from idealised geometry or violations of experimental constraints. Investigation of the surface properties of this module indicates there are significant differences from the fifth module, which binds apolipoprotein E-containing lipoproteins in addition to apolipoprotein B100-containing lipoproteins.

Algorithms↗
PubMed2005

Crystal structure of human lectin-like, oxidized low-density lipoprotein receptor 1 ligand binding domain and its ligand recognition mode to OxLDL.

Lectin-like, oxidized low-density lipoprotein (LDL) receptor 1, LOX-1, is the major receptor for oxidized LDL (OxLDL) in endothelial cells. We have determined the crystal structure of the ligand binding domain of LOX-1, with a short stalk region connecting the domain to the membrane-spanning region, as a homodimer linked by an interchain disulfide bond. In vivo assays with LOX-1 mutants revealed that the "basic spine," consisting of linearly aligned arginine residues spanning over the dimer surface, is responsible for ligand binding. Single amino acid substitution in the dimer interface caused a severe reduction in LOX-1 binding activity, suggesting that the correct dimer arrangement is crucial for binding to OxLDL. Based on the LDL model structure, possible binding modes of LOX-1 to OxLDL are proposed.

Amino Acid Sequence↗
PubMed1993

The three-dimensional structure of the ligand-binding domain of a wild-type bacterial chemotaxis receptor. Structural comparison to the cross-linked mutant forms and conformational changes upon ligand binding.

The three-dimensional structures of the ligand-binding domain of the wild-type Salmonella typhimurium aspartate receptor have been determined in the absence (apo) and presence of bound aspartate (complex) and compared to a cross-linked mutant containing a cysteine at position 36 which does not change signaling behavior of the intact receptor. The structures of the wild-type forms were determined in order to assess the effects of cross-linking on the structure and its influence on conformational changes upon ligand binding. As in the case of the cross-linked mutant receptor, the non-cross-linked ligand-binding domain is dimeric and is composed of 4-alpha-helical bundle monomer subunits related by a crystallographic 2-fold axis in the unbound form and by a non-crystallographic axis in the aspartate-bound form. A comparative study between the non-cross-linked and cross-linked structures has led to the following observations: 1) The long N-terminal helices of the individual subunits in the cross-linked structures are bent toward each other to accommodate the disulfide bond. 2) The rest of the subunit conformation is very similar to that of the wild-type. 3) The intersubunit angle of the cross-linked apo structure is larger by about 13 degrees when compared to the wild-type apo structure. 4) The nature and magnitude of the aspartate-induced conformational changes in the non-cross-linked wild-type structures are very similar to those of the cross-linked structures.

Amino Acid Sequence↗
PubMed2004

Different structural requirements of the ligand binding domain of the aryl hydrocarbon receptor for high- and low-affinity ligand binding and receptor activation.

The aryl hydrocarbon receptor (AhR) functions as a ligand-activated transcription factor that is responsible for the regulation of several response genes, of which the best characterized is the CYP1A1 gene. The present study was undertaken to elucidate the mechanism of activation of the AhR by omeprazole (OME), 2-mercapto-5-methoxybenzimidazole (MMB), and primaquine (PRQ), compounds that have previously been reported to induce CYP1A1 expression but that are not typical AhR ligands. All compounds caused a significant increase in luciferase activity in rat H4IIE and human HepG2 hepatoma cells transfected with a Gal4-AhR construct and the corresponding Gal4-Luc reporter gene. Furthermore, MMB and PRQ, but not OME, were capable of transforming cytosolic AhR to a DNA-binding form and displacing AhR-bound [3H]TCDD in rat hepatic cytosol in vitro. By performing site-directed mutagenesis of residues in the ligand-binding domain of the Gal4-AhR, a construct containing a Y320F substitution was found to be resistant to activation by OME, MMB, and PRQ, but not by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Comparable affinities of [3H]TCDD-binding to the wild-type and the Y320F mutant Gal4-proteins, expressed in human embryonic kidney 293 cells, were obtained in the ligand-binding assay. In contrast, the competition of receptor-bound [3H]TCDD by PRQ was absent from Gal4-Y320F but not from Gal4-AhR cell extracts. The results of this study confirm that MMB and PRQ are low-affinity ligands for the AhR and suggest that high- and low-affinity ligands interact with different residues of the AhR ligand-binding pocket. In addition, the data presented here indicate that Tyr320 plays an important role in AhR activation.

Animals↗
PubMed1998

A series of function blocking antibodies against the alpha v beta 3 integrin bind allosteric to the ligand binding site and induce ligand dissociation.

The alpha v beta 3 integrin plays a critical role in bone resorption, angiogenesis, and tumor cell invasion. A blockade of this receptor has therapeutic potential in osteoporosis, vascular restenosis, and cancer. In this report, we characterize the mechanism by which six monoclonal antibodies inhibit the function of alpha v beta 3. All six antibodies interact with a common site that is partially comprised of residues 164-202 within the beta 3 subunit. This domain is physically separate from the RGD binding site, and appears to regulate ligand binding allosterically. Thus, the blocking antibodies function, in part, by inducing the dissociation of ligand from alpha v beta 3. Although this family of antibodies is able to virtually abolish alpha v beta 3-mediated cell adhesion, they only block about one-half of soluble ligand binding to the integrin. This observation is consistent with the idea that two functionally distinct populations of alpha v beta 3 are present on the cell surface. The unique mechanism of action of these antibodies provides new insight in the structure-function relationships of alpha v beta 3, and also suggest that such antibodies are likely to behave differently than RGD mimetics if used as drugs.

Allosteric Site↗
PubMed1993

Expression of the ligand-binding domain-containing region of retinoic acid receptors alpha, beta and gamma in Escherichia coli and evaluation of ligand-binding selectivity.

The complete molecule or the ligand-binding domain-containing region of each of the three subtypes of human retinoic acid receptors (hRAR alpha, hRAR beta and hRAR gamma) was expressed in Escherichia coli. The expressed recombinant RARs (rRARs: rRAR alpha/E, rRAR beta/E and rRAR gamma) showed nearly the same magnitude of binding affinity toward [3H]retinoic acid (RA) as hRARs extracted from human cells (Ka values: 6.0 x 10(9) M-1 for rRAR alpha/E and 2.7 x 10(10) M-1 for both rRAR beta/E and rRAR gamma). Therefore, the ligand-binding selectivity of the rRARs toward RA and synthetic retinoids (4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbamoyl )benzoic acid (Am80), (E)-4-[3-(3,5-di-tert-butylphenyl)-3-oxo-1-propenyl]benzoic acid (Ch55)) was examined. Am80 bound rRAR alpha/E preferentially and showed no binding activity toward rRAR gamma, which is consistent with the case of hRAR gamma. Ch55 bound all three subtypes of rRARs, preferentially rRAR beta/E. These results suggest that the intrinsic nature of the binding of each retinoid can be investigated by usage of the rRARs. However, rRARs show quantitatively different ligand-selectivity from that of hRARs: RA showed higher binding affinity toward rRARs than both Am80 and Ch55, but Ch55 binds all three subtypes of hRARs stronger than RA and Am80, which binds hRAR beta stronger than RA.

Base Sequence↗
PubMed1995

The ligand binding domain of the human retinoic acid receptor gamma is predominantly alpha-helical with a Trp residue in the ligand binding site.

Retinoic acid exerts its many biological effects by interaction with a nuclear protein, the retinoic acid receptor (RAR). The details of this interaction are unknown due mainly to the lack of sufficient quantities of pure functional receptor protein for biochemical and structural studies. We have recently subcloned the D and E domains of human RAR gamma for expression in Escherichia coli. Using nickel-chelation affinity chromatography with a polyhistidine amino-terminal tail, purification of the DE peptide with a pI of 5.18 was accomplished to greater than 98% purity. Scatchard analysis and fluorescence quenching techniques using the purified protein indicate a very high percentage of functional molecules ( > 95%) with a Kd for retinoic acid (t-RA) of 0.6 +/- 0.1 nM. Circular dichroism spectra of the purified domains predict a predominantly alpha-helical structure (approximately 56%) with little beta sheet present. No significant changes in these structural characteristics were observed upon binding of t-RA. Inspection of the amino acid sequence within these domains identified a single tryptophan residue at position 227. Modeling the amino acid sequence in this region as an alpha-helical structure indicates that this tryptophan is adjacent to alanine 234, which corresponds to alanine 225 in RAR beta that has previously been linked to the ligand binding site. Fluorescence of this tryptophan was quenched in a dose-dependent manner on the addition of t-RA, confirming that Trp-227 is within the ligand binding site. Tryptophan fluorescence quenching analysis also demonstrates that a single retinoic acid molecule is bound per receptor and suggests that receptor-ligand interactions occur within the amino-terminal portion of the predominantly alpha-helical ligand binding domain.

Base Sequence↗
PubMed2006

Conformational changes of the glucocorticoid receptor ligand binding domain induced by ligand and cofactor binding, and the location of cofactor binding sites determined by hydrogen/deuterium exchange mass spectrometry.

HXMS (hydrogen/deuterium exchange mass spectrometry) of the glucocorticoid receptor ligand-binding domain (GR LBD) complexed with the agonist dexamethasone and the antagonist RU-486 is described. Variations in the rates of exchange were observed in regions consistent with the published crystal structures of GR LBD complexed with RU-486 when compared with the GR dexamethasone complex. We also report the HXMS results for agonist-bound GR LBD with the coactivator transcriptional intermediary factor 2 (TIF2) and anatagonist-bound GR LBD with nuclear receptor corepressor (NCoR). Alterations in exchange rates observed for agonist-bound GR LBD with TIF2 present were consistent with the published crystal structural contacts for the complex. Alterations in exchange rates observed for antagonist-bound GR LBD with NCoR were a subset of those observed with TIF2 binding, suggesting a common or overlapping binding site for coactivator and corepressor.

Amino Acid Sequence↗
PubMed1991

Sites within the complement C3b/C4b receptor important for the specificity of ligand binding.

Cysteine-rich repeated units of 40-70 amino acids are building blocks of many mammalian proteins, including 12 proteins of the complement system. Human complement arranged motifs, designated short consensus repeats (SCRs), which constitute the entire extracellular portion of this protein. Klickstein et al. [Klickstein, L. B., Bartow, T. J., Miletic, V., Rabson, L. D., Smith, J. A. & Fearon, D. T. (1988) J. Exp. Med. 168, 1699-1717 (abstr.)] localized a C4b binding domain to SCR-1 and/or SCR-2 and a C3b binding domain to SCR-8 and/or SCR-9. These SCRs bind different ligands, although SCR-1 and SCR-8 are 55% homologous and SCR-2 and SCR-9 are 70% homologous. To examine if one or two SCRs are required for ligand binding and to define sites within the SCRs that determine specificity of binding, mutagenesis analysis of a truncated, secreted form of CR1, called CR1-4 by Hourcade et al. [Hourcade, D., Meisner, D. R., Atkinson, J. P. & Holers, V. M. (1988) J. Exp. Med. 168, 1255-1270], was undertaken. The latter, composed of the first eight and one-half amino-terminal SCRs of CR1, efficiently bound C4b but not iC3. SCR-1 and SCR-2 were necessary for this interaction. Analysis of the mutant CR1-4 proteins, in which amino acids in SCR-1 and SCR-2 were substituted a few at a time with the homologous amino acids of SCR-8 and SCR-9, led to the identification of one amino acid in SCR-1 and three amino acids in SCR-2 important for C4b binding. Furthermore, five amino acids at the end of SCR-9, if placed in the homologous positions of SCR-2, conferred iC3 binding and are likely essential for ligand binding activity of SCR-8 and SCR-9. This iC3 binding occurred only if SCR-1 was present, indicating that two contiguous SCRs are necessary for this interaction. These results provide identification of amino acids within SCRs that are important for ligand binding.

Amino Acid Sequence↗
PubMed1988

Characterization of the low mol. wt zinc-binding ligand from rat small intestine by comparison to the organic zinc-binding ligands.

1. 65Zn complexes of picolinate (PA), citrate (CA), L-histidine (L-his), arachidonic acid (AA) or low mol. wt zinc-binding ligand from rat intestine (LMW-ZBL) gave 65Zn eluting peak fraction numbers of 53, 53, 56, 59 and 59 respectively, in a Sephadex G-75 column chromatography. 2. The 65Zn eluting peak fraction numbers with CA, L-his, PA, prostaglandin (PG)E2, AA, no ligand, arachidonate (AT) or LMW-ZBL were 49, 50, 54, 55, 58, 64, 75 and 76 respectively in a Sephadex G-25 column chromatography. 3. In a Sephadex G-15 column chromatography, the 65Zn eluting peak fraction numbers with CA, PGE2, AA, L-his, LMW-ZBL or PA were 49, 50, 51, 52, 52 and 55 respectively. 4. The LMW-ZBL in rat small intestine appears to be an AA-like substance.

Animals↗
PubMed2005

PDB-Ligand: a ligand database based on PDB for the automated and customized classification of ligand-binding structures.

PDB-Ligand (http://www.idrtech.com/PDB-Ligand/) is a three-dimensional structure database of small molecular ligands that are bound to larger biomolecules deposited in the Protein Data Bank (PDB). It is also a database tool that allows one to browse, classify, superimpose and visualize these structures. As of May 2004, there are about 4870 types of small molecular ligands, experimentally determined as a complex with protein or DNA in the PDB. The proteins that a given ligand binds are often homologous and present the same binding structure to the ligand. However, there are also many instances wherein a given ligand binds to two or more unrelated proteins, or to the same or homologous protein in different binding environments. PDB-Ligand serves as an interactive structural analysis and clustering tool for all the ligand-binding structures in the PDB. PDB-Ligand also provides an easier way to obtain a number of different structure alignments of many related ligand-binding structures based on a simple and flexible ligand clustering method. PDB-Ligand will be a good resource for both a better interpretation of ligand-binding structures and the development of better scoring functions to be used in many drug discovery applications.

Adenosine Triphosphate↗
PubMed1994

Heterologous expression of human uteroglobin/polychlorinated biphenyl-binding protein. Determination of ligand binding parameters and mechanism of phospholipase A2 inhibition in vitro.

High level expression of a human polychlorinated biphenyl-binding protein (hPCB-BP; also termed uteroglobin or CC10) was achieved in Escherichia coli. The recombinant protein (rhPCB-BP) constituted approximately 1% of total bacterial lysate proteins as judged from in vitro ligand binding assays using 4,4'-bis([3H]methylsulfonyl)-2,2',5,5'-tetrachlorobiphenyl. rhPCB-BP was purified to homogeneity in its native dimeric form. Saturation analysis experiments indicated a Kd of approximately 69 nM for the binding of 4,4'-bis([3H]methylsulfonyl)-2,2',5,5'-tetrachlorobiphenyl to rhPCB-BP. The average number of binding sites (Bmax) calculated from such experiments on purified rhPCB-BP was 49 nmol/mg of protein and is close to the theoretical value of 1 mol of ligand associating with 1 mol of dimeric protein. Purified rhPCB-BP was also found to cause a dose-dependent inhibition of the enzyme porcine pancreatic phospholipase A2 (PLA2) in vitro. Increasing the concentrations of calcium abolished the inhibition of PLA2 by rhPCB-BP, suggesting that the protein functions in vitro by sequestering Ca2+, an essential PLA2 cofactor. This notion was further supported by direct evidence that 45Ca2+ binds to rhPCB-BP. 1 mol of dimeric protein was also found to bind 2 mol of ruthenium red, an organic dye that detects Ca(2+)-binding proteins, with a Kd of 3 microM. This binding was inhibited by Ca2+, with an IC50 of 7 mM. Finally, it was demonstrated that the addition of a high affinity ligand for the protein had no effect on its ability to inhibit PLA2 under conditions of limiting concentrations of calcium, and the addition of Ca2+ did not affect the binding characteristics of the PCB ligand, suggesting that these two properties of the protein are independent. Our results strongly support the notion that ligand binding is a conserved feature of the homologous uteroglobin/PCB-BP/cc10 proteins in different species, whereas our results question the suggested role of these proteins as specific inhibitors of PLA2.

Calcium↗
PubMed1998

Identification of specific sites involved in ligand binding by photoaffinity labeling of the receptor for the urokinase-type plasminogen activator. Residues located at equivalent positions in uPAR domains I and III participate in the assembly of a composite ligand-binding site.

Plasminogen activation by the urokinase-type plasminogen activator (uPA) is facilitated in the presence of cells expressing the glycolipid-anchored high-affinity receptor for uPA (denoted uPAR). Structures involved in the interaction between human uPAR and a decamer peptide antagonist of uPA binding (SLNFSQYLWS) were previously tagged by specific site-directed photoaffinity labeling [Ploug, M., Ostergaard, S., Hansen, L. B. L., Holm, A., and Dano, K. (1998) Biochemistry 37, 3612-3622]. Replacement of the key functional residues Phe4 and Trp9 with either benzophenone or (trifluoromethyl)aryldiazirine rendered this peptide antagonist photoactivatable, and as a consequence, it incorporated covalently upon photolysis into either uPAR domain I or domain III depending on the actual position of the photophore in the sequence. The residues of uPAR specifically targeted by photoaffinity labeling were identified by matrix-assisted laser desorption mass spectrometry, NH2-terminal sequence analysis, and amino acid composition analysis after enzymatic fragmentation and HPLC purification. According to these data, the formation of the receptor-ligand complex positions Phe4 of the peptide antagonist very close to Arg53 and Leu66 in uPAR domain I and Trp9 of the antagonist in the vicinity of His251 in uPAR domain III. The gross molecular arrangement of the deduced receptor-ligand interface provides a rational structural basis for the observed requirement for the intact multidomain state of uPAR for achieving high-affinity ligand binding, since according to this model ligand binding must rely on a close spatial proximity of uPAR domains I and III. In addition, these data suggest that the assembly of the composite ligand binding site in uPAR may resemble the homophilic interdomain dimerization of kappa-bungarotoxin, a structural homologue of the Ly-6/uPAR domain family.

Amino Acid Sequence↗
PubMed2002

Co-repressor release but not ligand binding is a prerequisite for transcription activation by human retinoid acid receptor alpha ligand-binding domain.

Nuclear hormone receptors coordinately regulate the activity of genetic networks through the recruitment of transcriptional co-regulators, including co-repressors and co-activators. Allosteric modulation of the ligand-binding domain by hormonal activators shifts the co-factor binding preference by defined structural changes in overlapping docking sites. We report here that mutations at conserved residues within the docking motif of the retinoic acid receptor alpha cause defects in dimerization, co-regulator association, and transcriptional regulation. Furthermore, although a minimal co-repressor receptor interaction domain is sufficient for receptor binding, flanking sequences appear to stabilize this interaction without interfering with ligand sensitivity. However, ligand sensitivity is changed by the K262A mutation, which requires much higher concentrations of all-trans-retinoic acid to promote co-repressor dissociation. Consequently, K262A functions as a dominant-negative mutant at low concentrations of all-trans-retinoic acid. As a result, transcriptional activation is mechanistically linked to co-repressor release.

Amino Acid Sequence↗
PubMed2006

Dimerization of protein tyrosine phosphatase sigma governs both ligand binding and isoform specificity.

Signaling through receptor protein tyrosine phosphatases (RPTPs) can influence diverse processes, including axon development, lymphocyte activation, and cell motility. The molecular regulation of these enzymes, however, is still poorly understood. In particular, it is not known if, or how, the dimerization state of RPTPs is related to the binding of extracellular ligands. Protein tyrosine phosphatase sigma (PTPsigma) is an RPTP with major isoforms that differ in their complements of fibronectin type III domains and in their ligand-binding specificities. In this study, we show that PTPsigma forms homodimers in the cell, interacting at least in part through the transmembrane region. Using this knowledge, we provide the first evidence that PTPsigma ectodomains must be presented as dimers in order to bind heterophilic ligands. We also provide evidence of how alternative use of fibronectin type III domain complements in two major isoforms of PTPsigma can alter the ligand binding specificities of PTPsigma ectodomains. The data suggest that the alternative domains function largely to change the rotational conformations of the amino-terminal ligand binding sites of the ectodomain dimers, thus imparting novel ligand binding properties. These findings have important implications for our understanding of how heterophilic ligands interact with, and potentially regulate, RPTPs.

Amino Acid Sequence↗
PubMed2002

Ligand binding and protein dynamics in neuroglobin.

Neuroglobin (Ngb) is a recently discovered protein in vertebrate brain tissue that belongs to the globin family of proteins. It has been implicated in the neuronal response to hypoxia or ischemia, although its physiological role has been hitherto unknown. Ngb is hexacoordinate in the ferrous deoxy form under physiological conditions. To bind exogenous ligands like O(2) and CO, the His E7 endogenous ligand is displaced from the sixth coordination. By using infrared spectroscopy and nanosecond time-resolved visible spectroscopy, we have investigated the ligand-binding reaction over a wide temperature range (3-353 K). Multiple, intrinsically heterogeneous distal heme pocket conformations exist in NgbCO. Photolysis at cryogenic temperatures creates a five-coordinate deoxy species with very low geminate-rebinding barriers. The photodissociated CO is observed to migrate within the distal heme pocket even at 20 K. Flash photolysis near physiological temperature (275-353 K) exhibits four sequential kinetic features: (i) geminate rebinding (t < 1 micros); (ii) extremely fast bimolecular exogenous ligand binding (10 micros < t < 1 ms) with a nontrivial temperature dependence; (iii) endogenous ligand binding (100 micros < t < 10 ms), which can be studied by using flash photolysis on deoxy Ngb; and (iv) displacement of the endogenous by the exogenous ligand (10 ms < t < 10 ks). All four processes are markedly nonexponential, suggesting that Ngb fluctuates among different conformations on surprisingly long time scales.

Animals↗
PubMed1995

A small GTP-binding protein, Rho, associates with the platelet-derived growth factor type-beta receptor upon ligand binding.

Ligand binding to the platelet-derived growth factor (PDGF) receptor initiates a complex and diverging cascade of signaling pathways. GTP-binding proteins with intrinsic GTPase activity (G-proteins) frequently link cell surface receptors to intracellular signaling pathways, but no close associations of the PDGF receptor and any small G-proteins, nor any such associations activated by ligand binding to the receptor have been previously reported. We demonstrate that a small GTP-binding protein binds specifically to the murine and human PDGF type-beta receptor. In response to PDGF-BB stimulation, there is an increase in the amount of labeled small G-protein associated with the PDGF type-beta receptor. The GTP-binding protein did not undergo ligand-induced association with a mutant receptor protein that was unable to bind ATP. Proteolytic cleavage analysis, together with two-dimensional separation techniques, identified the small G-protein specifically associating with the PDGF type-beta receptor after ligand binding as a member of the Rho family. This was confirmed by demonstration that the small G-protein coimmunoprecipitated by the anti-PDGF receptor antibody was a substrate for the ADP-ribosyltransferase C3 exoenzyme. Thus, the PDGF type-beta receptor may form a complex with one or more small G-proteins upon binding PDGF-BB, and the Rho small G-protein is likely to be an important component of the proteins making up the multimeric signaling complex of the PDGF type-beta receptor.

3T3 Cells↗
PubMed2001

Effect of hydrogen ion concentration and buffer composition on ligand binding characteristics and polymerization of cow's milk folate binding protein.

The ligand binding and aggregation behavior of cow's milk folate binding protein depends on hydrogen ion concentration and buffer composition. At pH 5.0, the protein polymerizes in Tris-HCl subsequent to ligand binding. No polymerization occurs in acetate, and binding is markedly weaker in acetate or citrate buffers as compared to Tris-HCl. Polymerization of ligand-bound protein was far more pronounced at pH 7.4 as compared to pH 5.0 regardless of buffer composition. Binding affinity increased with decreasing concentration of protein both at pH 7.4 and 5.0. At pH 5.0 this effect seemed to level off at a protein concentration of 10(-6) M which is 100-1000 fold higher than at pH 7.4. The data can be interpreted in terms of complex models for ligand binding systems polymerizing both in the absence or presence of ligand (pH 7.4) as well as only subsequent to ligand binding (pH 5.0).

Animals↗