Calcium in biological systems

Effective binding of Tb3+ and La3+ cations on the donor side of Mn-depleted photosystem II

The interaction of Tb³⁺ and La³⁺ cations with different photosystem II (PSII) membranes (intact PSII, Ca-depleted PSII (PSII[-Ca]) and Mn-depleted PSII (PSII[-Mn]) membranes) was studied. Although both lanthanide cations (Ln³⁺) interact only with Ca²⁺-binding site of oxygen-evolving complex (OEC) in PSII and PSII(-Ca) membranes, we found that in PSII(-Mn) membranes both Ln³⁺ ions tightly bind to another site localized on the oxidizing side of PSII. Binding of Ln³⁺ cations to this site is not protected by Ca²⁺ and is accompanied by very effective inhibition of Mn²⁺ oxidation at the high-affinity (HA) Mn-binding site ([Mn²⁺  + H₂O₂] couple was used as a donor of electrons). The values of the constant for inhibition of electron transport K_i are equal to 2.10 ± 0.03 µM for Tb³⁺ and 8.3 ± 0.4 µM for La³⁺, whereas OEC inhibition constant in the native PSII membranes is 323 ± 7 µM for Tb³⁺. The value of K_i for Tb³⁺ corresponds to K_i for Mn²⁺ cations in the reaction of diphenylcarbazide oxidation via HA site (1.5 µM) presented in the literature. Our results suggest that Ln³⁺ cations bind to the HA Mn-binding site in PSII(-Mn) membranes like Mn²⁺ or Fe²⁺ cations. Taking into account the fact that Mn²⁺ and Fe²⁺ cations bind to the HA site as trivalent cations after light-induced oxidation and the fact that Mn cation bound to the HA site (Mn4) is also in trivalent state, we can suggest that valency may be important for the interaction of Ln³⁺ with the HA site.

The PSII calcium site revisited

Oxidation of H2O by photosystem II is a unique redox reaction in that it requires Ca2+ as well as Cl- as obligatory activators/cofactors of the reaction, which is catalyzed by Mn atoms. The properties of the binding site for Ca2+ in this reaction resemble those of other Ca2+ binding proteins, and recent X-ray structural data confirm that the metal is in fact ligated at least in part by amino acid side chain oxo anions. Removal of Ca2+ blocks water oxidation chemistry at an early stage in the cycle of redox reactions that result in O-O bond formation, and the intimate involvement of Ca2+ in this reaction that is implied by this result is confirmed by an ever-improving set of crystal structures of the cyanobacterial enzyme. Here, we revisit the photosystem II Ca2+ site, in part to discuss the additional information that has appeared since our earlier review of this subject (van Gorkom HJ, Yocum CF In: Wydrzynski TJ, Satoh K (eds) Photosystem II: the light-driven water:plastoquinone oxidoreductase), and also to reexamine earlier data, which lead us to conclude that all S-state transitions require Ca2+.

Potential influence of Asp in the Ca2+ coordination position 5 of parvalbumin on the calcium-binding affinity: A computational study

Parvalbumins (PV) are calcium-binding proteins, all sharing the common helix-loop-helix (EF-hand) motif. This motif contains a central twelve-residue Ca(2+)-binding loop with the flanking helices positioned roughly perpendicular to each other. The precise role of these coordination residues has been the subject of intense studies. In this work, we focus on the coordination position 5 in the CD Ca(2+)-binding site of silver hake parvalbumin isoform B (SHPV-B). The most common residue at site 5 of calcium-binding loop in canonical EF-hands is Asp [B.J. Marsden, G.S. Shaw, B.D. Sykes, Biochem. Cell Biol. 68 (1990) 587-601], but in the CD site of PV, this position is almost always serine (Ser). The substitution of Ser with Asp will add the 5th carboxylate residue in the CD coordination sphere. However, as predicted by the acid pair hypothesis, the Ca(2+)-binding affinity would be maximized in an EF-hand motif that has four carboxylate ligands paired along the +/-x, and +/-z-axes [R.E. Reid, R.S. Hodges, J. Theor. Biol. 84 (1980) 401-444]. Molecular dynamics simulations and free energy calculations were employed to investigate the influence of Ser to Asp mutation at position 5 on calcium-binding affinity. We found that the Asp variant exhibited remarkable stability during the entire molecular dynamics simulation, with not only the retention of the Ca(2+)-binding site, but also increased compactness in the coordination sphere. The S55D fragment also accommodated Ca(2+) well. We conclude that the reason why Asp which is the most common residue at site 5 of calcium-binding loop in canonical EF-hands has never been identified at this position experimentally for PVs might be related to its physiological functions.

Identification of commercial hake and grenadier species by proteomic analysis of the parvalbumin fraction

Analysis of parvalbumin fractions through proteomic methodologies allowed the differential classification of ten commercial, closely related species of the family Merlucciidae. Muscle extracts from nine hake species of the genus Merluccius including two subspecies of Merluccius australis (australis and polylepsis) and one grenadier species Macruronus novaezelandiae with two populations (novaezelandiae and magellanicus) were evaluated by 2-DE and MALDI-TOF MS. 2-DE demonstrated that the species tested displayed a low intra-specific degree of polymorphism and the isoform patterns were noticeably species-specific. MALDI-TOF mass fingerprints showed clear differences in the pattern of peptides produced by tryptic digestion between the Merluccius and the Macruronus, making the genus differentiation possible. In addition, a selective peptide mass present in the spectra from certain hakes allowed its classification in two groups: Euro-African and American hakes. Besides, some specific masses allowed a clearly individual identification for M. bilinearis, M. australis polylepsis, M. australis australis, M. productus, M. paradoxus and M. polli, while the rest of the hake species can be grouped in two clusters, comprising M. hubbsi and M. gayi in one and M. merluccius and M. capensis in the other.

Calcium EXAFS establishes the Mn-Ca cluster in the oxygen-evolving complex of photosystem II

The proximity of Ca to the Mn cluster of the photosynthetic water-oxidation complex is demonstrated by X-ray absorption spectroscopy. We have collected EXAFS data at the Ca K-edge using active PS II membrane samples that contain approximately 2 Ca per 4 Mn. These samples are much less perturbed than previously investigated Sr-substituted samples, which were prepared after Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 A, a distance that agrees with that surmised from previously recorded Mn EXAFS. This result is also consistent with earlier related experiments at the Sr K-edge, using samples that contained functional Sr, that show Mn is approximately 3.5 A distant from Sr. The totality of the evidence clearly advances the notion that the catalytic center of oxygen evolution is a Mn-Ca heteronuclear cluster.

Molecular dynamics study of Ca(2+) binding loop variants of silver hake parvalbumin with aspartic acid at the "gateway" position

The helix-loop-helix (i.e., EF-hand) Ca(2+) ion binding motif is characteristic of a large family of high-affinity calcium ion binding proteins, including the parvalbumins, oncomodulins and calmodulins. In this work we describe a set of molecular dynamics computations on the major parvalbumin from the silver hake (SHPV-B) and on functional fragments of this protein, consisting of the first four helical regions (the ABCD fragment), and the internal helix-loop- helix region (the CD fragment). In both whole protein and protein fragments (i.e., ABCD and CD fragments), the 9th loop residue in the calcium ion binding site in the CD helix-loop-helix region (the so-called "gateway" position) has been mutated from glutamic acid to aspartic acid. Aspartic acid is one of the most common residues found at the gateway position in other (non-parvalbumin) EF- hand proteins, but has never been found at the gateway position of any parvalbumin. (Interestingly, aspartic acid does occur at the gateway position in the closely related rat and human oncomodulins.) Consistent with experimental observations, the results of our molecular dynamics simulations show that incorporation of aspartic acid at the gateway position is very disruptive to the structural integrity of the calcium ion coordination site in the whole protein. The aspartic acid mutation is somewhat less disruptive to the calcium ion coordination sites in the two parvalbumin fragments (i.e., the ABCD and CD fragments), presumably due to the higher degree of motional freedom allowable in these protein fragments. One problem associated with the E59D whole protein variant is a prohibitively close approach of the aspartate carboxyl group to the CD calcium ion observed in the energy-minimized (pre-molecular dynamics) structure. This steric situation does not emerge during energy-minimization of the wild-type protein. The damage to the structural integrity of the calcium ion coordination site in the whole protein E59D variant is not relieved during the molecular dynamics simulation. In fact, during the course of the 300 picosecond simulation, all of the calcium ion ligands leave the primary coordination sphere. In addition, the conserved hydrogen- bonds (in the short beta-sheet structure) that links the CD site to the symmetry-related EF site (in the non-mutated whole protein) is also somewhat disrupted in the E59D whole protein variant. These results suggest that the Ca(2+) ion binding deficiencies in the CD loop are related, at least in part, to the unique interaction that exists between the paired CD and EF hands in the whole protein. Our theoretical results correlate well with previous studies on engineered EF-hand proteins and with all of our experimental evidence on whole silver hake parvalbumin and enzymatically-generated parvalbumin fragments.

Calcium depletion modifies the structure of the photosystem II O(2)-evolving complex

A 5 min exposure of photosystem II to a pH 3 citric acid solution is a simple method for selective removal of Ca(2+) from the O(2)-evolving complex. The resulting preparation retains the 23 and 17 kDa extrinsic polypeptides, but the activity of this material is only 10-20% of that of an untreated control sample. Biochemical characterization of citrate-treated photosystem II reveals that some reaction centers lose the extrinsic proteins during citrate treatment. Furthermore, a comparison of photosystem II preparations treated with citrate, or depleted of 23 and 17 kDa extrinsic polypeptides by high-salt treatment, shows that low concentrations of a small reductant, NH(2)OH, which has little effect on the activity of intact photosystem II, can reduce and inhibit the Mn cluster in both types of preparations. In contrast, a large reductant, hydroquinone, cannot access the majority of O(2)-evolving centers in citrate-treated preparations, while 23 and 17 kDa-depleted material is rapidly inactivated by the reductant. Incubation of the citrate-treated samples in high ( approximately 60 mM) concentrations of CaCl(2) restores 50% of the lost activity; this Ca(2+)-reconstituted activity is chelator-insensitive, indicating that rebinding of Ca(2+) restores the structural integrity of the O(2)-evolving complex. A characterization of Ca(2+) and Cl(-) affinities in steady-state activity assays shows that citrate-treated preparations exhibit a Cl(-) requirement similar to that of polypeptide-depleted photosystem II, while Ca(2+) reactivation of O(2) evolution appears to occur at two structurally distinct sites. One site exhibits a high Ca(2+) affinity, similar to that found in polypeptide-depleted samples, but a second, lower-affinity site also exists, with a K(M) that is approximately 10 times greater than that of the high-affinity site, which is associated with centers that retain the extrinsic polypeptides. These data indicate that citrate-induced Ca(2+) depletion causes release of the 23 and 17 kDa extrinsic polypeptides from some photosystem II reaction centers, and also modifies the structure of the polypeptide-retaining O(2)-evolving centers so that the Mn cluster is exposed to small, but not large, reductants. This change may be due to subtle modifications to the structure of the photosystem II extrinsic proteins that produces a new pathway between the solvent and the Mn cluster or, alternatively, to the opening of an existing channel in the intrinsic lumenal polypeptide domain, between the solvent and the Mn cluster, that is normally occluded by a bound Ca(2+) atom.

Molecular dynamics study of Ca(2+) binding loop variants of parvalbumin with modifications at the "gateway" position

The helix-loop-helix (i.e. EF-hand) Ca(2+) ion binding motif is characteristic of a large family of high-affinity Ca(2+) ion binding proteins, including the parvalbumins and calmodulins. In this paper we describe a set of molecular dynamics computations on the major parvalbumin from the silver hake (SHPV-B). In all variants examined, both whole protein and fragments thereof, the ninth loop residue in the Ca(2+) binding coordination site in the CD helix-loop-helix region (the so-called "gateway" residue) has been mutated. The three gateway mutations examined are arginine, which has never been found at the gateway position of any EF-hand protein, cysteine, which is the residue observed least in natural EF-hand sites, and serine, which is the most common (by far) non-acidic residue substitution at this position in EF-hand proteins in general, but never in parvalbumins. Results of the molecular dynamics simulations indicate that all three modifications are disruptive to the integrity of the mutated Ca(2+) binding site in the whole parvalbumin protein. In contrast, only the arginine and cysteine mutations are disruptive to the integrity of the mutated Ca(2+) binding site in the CD fragment of the parvalbumin protein. Surprisingly, the serine variant of the CD helix-loop-helix fragment exhibited remarkable stability during the entire molecular dynamics simulation, with retention of the Ca(2+) binding site. These results indicate that there are no inherent problems (for Ca(2+) ion binding) associated with the sequence of the CD helix-loop-helix fragment that precludes the incorporation of serine at the gateway position. Since the CD site is totally disrupted in the whole protein serine variant, this indicates that the Ca(2+) ion binding deficiencies are most likely related to the unique interaction that exists between the paired EF-hands in the whole protein. Our theoretical results correlate well with previous studies on engineered EF-hand proteins and with all of our experimental evidence on the silver hake parvalbumin.

X-Ray crystal structure and molecular dynamics simulations of silver hake parvalbumin (Isoform B)

Parvalbumins constitute a class of calcium-binding proteins characterized by the presence of several helix-loop-helix (EF-hand) motifs. In a previous study (Revett SP, King G, Shabanowitz J, Hunt DF, Hartman KL, Laue TM, Nelson DJ, 1997, Protein Sci 7:2397-2408), we presented the sequence of the major parvalbumin isoform from the silver hake (Merluccius bilinearis) and presented spectroscopic and structural information on the excised "EF-hand" portion of the protein. In this study, the X-ray crystal structure of the silver hake major parvalbumin has been determined to high resolution, in the frozen state, using the molecular replacement method with the carp parvalbumin structure as a starting model. The crystals are orthorhombic, space group C2221, with a = 75.7 A, b = 80.7 A, and c = 42.1 A. Data were collected from a single crystal grown in 15% glycerol, which served as a cryoprotectant for flash freezing at -188 degrees C. The structure refined to a conventional R-value of 21% (free R 25%) for observed reflections in the range 8 to 1.65 A [1 > 2sigma(I)]. The refined model includes an acetylated amino terminus, 108 residues (characteristic of a beta parvalbumin lineage), 2 calcium ions, and 114 water molecules per protein molecule. The resulting structure was used in molecular dynamics (MD) simulations focused primarily on the dynamics of the ligands coordinating the Ca2+ ions in the CD and EF sites. MD simulations were performed on both the fully Ca2+ loaded protein and on a Ca2+ deficient variant, with Ca2+ only in the CD site. There was substantial agreement between the MD and X-ray results in addressing the issue of mobility of key residues in the calcium-binding sites, especially with regard to the side chain of Ser55 in the CD site and Asp92 in the EF site.

Strontium EXAFS Reveals the Proximity of Calcium to the Manganese Cluster of Oxygen-Evolving Photosystem II

The oxygen-evolving complex of Photosystem II (PS II) in green plants and algae contains a cluster of four manganese atoms in the active site, which catalyzes the photoinduced oxidation of water to dioxygen. Along with Mn, calcium and chloride ions are necessary cofactors for proper functioning of the complex. A key unresolved question is whether Ca is close to the Mn cluster, within about 3.5 Å. To further test and verify this finding, we substituted strontium for Ca and probed from the Sr point-of-view for any nearby Mn. Sr has been shown to replace Ca and still maintain enzyme activity (about 40% of normal rate). The extended X-ray absorption fine structure (EXAFS) of Sr-PS II probes the local environment around the Sr cofactor to detect any nearby Mn. We focused on the functional Sr by removing nonessential, loosely bound Sr in the protein environment. For comparison, an inactive sample was prepared by treating the intact PS II with hydroxylamine to disrupt the Mn cluster and to produce nonfunctional enzyme. Sr EXAFS results indicate major differences in the phase and amplitude between the functional (intact) and nonfunctional (NH2OH-treated) samples. In intact samples, the Fourier transform of the Sr EXAFS shows a peak that is missing in inactive samples. This Fourier peak II is best simulated by two Mn neighbors at a distance of 3.5 Å. Thus, with X-ray absorption studies on Sr-reconstituted PS II, we confirm the proximity of Ca (Sr) cofactor to the Mn cluster and show that the active site is a Mn-Ca heteronuclear cluster.

Characterization of a helix-loop-helix (EF hand) motif of silver hake parvalbumin isoform B

Parvalbumins are a class of calcium-binding proteins characterized by the presence of several helix-loop-helix (EF-hand) motifs. It is suspected that these proteins evolved via intragene duplication from a single EF-hand. Silver hake parvalbumin (SHPV) consists of three EF-type helix-loop-helix regions, two of which have the ability to bind calcium. The three helix-loop-helix motifs are designated AB, CD, and EF, respectively. In this study, native silver hake parvalbumin isoform B (SHPV-B) has been sequenced by mass spectrometry. The sequence indicates that this parvalbumin is a beta-lineage parvalbumin. SHPV-B was cleaved into two major fragments, consisting of the ABCD and EF regions of the native protein. The 33-amino acid EF fragment (residues 76-108), containing one of the calcium ion binding sites in native SHPV-B, has been isolated and studied for its structural characteristics, ability to bind divalent and trivalent cations, and for its propensity to undergo metal ion-induced self-association. The presence of Ca2+ does not induce significant secondary structure in the EF fragment. However, NMR and CD results indicate significant secondary structure promotion in the EF fragment in the presence of the higher charge-density trivalent cations. Sedimentation equilibrium analysis results show that the EF fragment exists in a monomer-dimer equilibrium when complexed with La3+.

The isolation of parvalbumin isoforms from the tail muscle of the American alligator (Alligator mississipiensis)

Multiple parvalbumin isoforms have been detected in the tail (skeletal) muscle of the American alligator (Alligator mississipiensis). One of these isoforms (APV-1) has been highly purified and partially characterized. Protein purification involved mainly gel filtration and anion exchange chromatography, and characterization included gel electrophoresis, amino acid composition analysis, metal ion analysis, MALDI-TOF and ESI mass spectrometry, ultraviolet and fluorescence spectroscopy, and one- and two-dimensional 500 MHz proton NMR spectroscopy. The alligator isoforms are rich in phenylalanine and deficient in the other aromatic residues as is typical for parvalbumins. In fact, the one highly purified isoform that forms the basis of this study has only phenyl-alanine as an aromatic residue. Ion exchange chromatography further indicates that this isoform has a relatively high isoelectric point (pl approximately 5.0), indicating that it is an alpha-lineage parvalbumin. This alligator parvalbumin isoform is unusual in that it has an atypically high Ca2+ content (almost 3.0 mole of Ca2+ per mole of protein) following purification, a fact supported by terbium fluorescence titration experiments. Preliminary comparative analysis of the highly purified alligator parvalbumin isoform (in the Ca2-loaded state) by two-dimensional 1H-NMR (2D 1H TOCSY and 2D 1H NOESY) indicates that there is considerable similarity in structure between the alligator protein and a homologous protein obtained from the silver hake (a saltwater fish species).

Macromolecules in mollusc shells and their functions in biomineralization

Mollusc shells are used as a model for studying ‘organic-matrix -mediated’ biomineralization, in which crystals are nucleated and grow in a pre-formed structural framework composed of proteins and polysaccharides. In particular, the possibility that the organic matrix functions as a template for crystal formation by epitaxial growth, is examined. In general, individual organic matrix sheets are composed of a thin layer of β-chitin sandwiched between layers of proteins adopting the antiparallel β-sheet conformation. The protein polypeptide chains are oriented perpendicular to the chitin fibrils. The matrix surfaces contain acidic proteins and polysaccharides. X -ray and electron diffraction patterns of matrices and mineral crystals from the nacreous layers of a bivalve, a gastropod and the cephalopod, Nautilus , show that the chitin fibres and the protein polypeptide chains are aligned with the a and b aragonite crystallographic axes, respectively. This strongly suggests that the mineral formed epitaxially upon the matrix surface. However, as the degree of orientation of the organic constituents is much less than the mineral constituents, it is postulated that the site of nucleation of the mineral crystals comprises only a small part of the matrix structure and is itself composed of well oriented macromolecules, probably acidic proteins. Two acidic proteins, which may constitute part of the nucleation site for calcite in a bivalve, were identified by comparing all the acidic proteins in the calcite layer with those in the aragonite layer. The two proteins were present in the calcite layer only, and in addition were found to have an aspartic-acid -containing amino acid sequence, not present in any of the other soluble matrix proteins. The concept of a matrix composed of a structural core coated with acidic macromolecules, some of which constitute a nucleation site, may well be applicable to other mineralized tissues.

Possible role of tissue-bound calcium ions in citrate-mediated high-temperature antigen retrieval

High-temperature preheating of sections in the presence of a salt (e.g., citrate) or a protein denaturant (e.g., urea) solution has been shown recently to provide a reliable alternative to tissue proteolysis for antigen retrieval from formaldehyde-fixed, paraffin-embedded tissues. However, the underlying mechanism of action of this form of pretreatment remains highly speculative. In this study, we show that calcium chelating agents such EDTA and EGTA are more effective than citrate in the retrieval of a citrate-sensitive nuclear antigen, Ki-67. Also, sodium carbonate and another calcium precipitating agent are both able to effect antigen retrieval at high temperatures. The overall data therefore suggest that either the chelation or the precipitation of tissue-bound calcium ions, and perhaps also other divalent metal cations, is a critical step in salt-mediated antigen retrieval. As a corollary, it is suggested that tight complexing of calcium ions or other divalent metal cations with proteins during formaldehyde tissue fixation is responsible for the masking of certain antigens.

Activation and inhibition of protein kinase C isozymes alpha and beta by Gd3+

Gd3+ was evaluated as a probe for Ca2+ sites on protein kinase C (PKC) by studying its ability to replace Ca2+ in activation of PKC isozymes II (beta) and III (alpha) in the lipid systems phosphatidylserine/1,2-dioleoyl-sn-glycerol (PS/DO) and diheptanoylphosphatidylcholine (PC7)/DO. PKC beta was stimulated by Ca2+ or Gd3+ in PS/DO whereas activity in PC7/DO was independent of these metals. Thus, it is suggested that Gd3+ replaces Ca2+ at a site involving metal-lipid interactions. High concentrations of Ca2+ or Gd3+ inhibited activity in both lipid systems. Analysis of the Gd3+ inhibition in the PC7/DO system suggests that it is due to formation of GdATP, which competes at the MgATP site. Activity of PKC alpha was dependent on low concentrations of Ca2+ in both lipid systems. The ability of Gd3+ to substitute for Ca2+ could not be evaluated in the PS system due to the inability to completely remove contaminating Ca2+ without chelating buffers. Successful reduction of contaminating Ca2+ was achieved in the PC7 system but Gd3+ failed to substitute for Ca2+ in activating PKC alpha and only caused inhibition. This is consistent with binding of Gd3+ to a Ca2+ site at or near the active site of the enzyme rather than to a site on the lipid. These results indicate that interactions between PKC and Gd3+ are complex, involving occupation of more than one class of sites. Conditions for separately evaluating the individual sites can be manipulated by selection of isozyme and lipid system.

Thermodynamics of Ca2+ binding to calmodulin and its tryptic fragments

The binding of Ca2+ to calmodulin and its two tryptic fragments has been studied using microcalorimetry. The binding process is accompanied by the uptake or release of protons, depending on the ionic strength. With no added salt, the total enthalpy change for the binding of four calcium ions to calmodulin is -41 kJ mol-1 but in the presence of 0.15 mM KCl delta Htot is +17 kJ mol-1. The mode of binding of Ca2+ is also completely different with and without added salt. It is also shown that for the C-terminal fragment of calmodulin, TR2C, the drastic reduction in delta Gtot for the binding process on increasing the ionic strength is largely an enthalpic effect. Domain interactions in calmodulin are indicated by the fact that the sum of the enthalpies of calcium binding to the two tryptic fragments is not the same as the total binding enthalpy to calmodulin itself. The binding of Ca2+ to calmodulin has also been studied calorimetrically at different temperatures in the range 21-37 degrees C. delta Cp is large and negative in this interval.

Sr and Ba transients in isolated snail neurones studied with fura-2. The recovery from depolarization induced load and modulation of Ca release from intracellular stores

The fluorescent Ca-sensitive probe fura-2 was used in isolated non-identified neurons of the snail, Helix pomatia, to study the recovery from Sr and Ba loading and the effect of these cations on Ca-induced Ca release. Sr was shown to behave similarly to Ca, whereas Ba ions were not sequestered within the cell. Intracellular Ba blocked both the voltage-gated channels in the plasma membrane and the Ca release channels in the membrane of intracellular stores.

Some aspects of calcium regulation in cell biology

The roles of intracellular calcium in the regulation of cell metabolism and cell membrane permeability are highlighted with examples taken from recent studies.

Interactions of spin-labeled calmodulin with trifluoperazine and phosphodiesterase in the presence of Ca(II), Cd(II), La(III), Tb(III), and Lu(III)

Bovine calmodulin analogues, spin-labeled at methionine and tyrosine residues, have been utilized in electron paramagnetic resonance (EPR) studies designed to investigate calmodulin interactions with the antipsychotic drug trifluoperazine and the calmodulin-binding protein 3',5'-cyclic nucleotide phosphodiesterase. Trifluoperazine titrations of spin-labeled calmodulin analogues were carried out in the presence of Ca(II), Cd(II), and Tb(III). Similar experiments were performed with the phosphodiesterase in the presence of Ca(II), Cd(II), La(III), Tb(III), and Lu(III). EPR signals from the methionine-directed probe proved to be more sensitive to the binding of target molecules than signals from the tyrosine-directed probe, perhaps indicating that the spin-labeled methionine is at a site close to the target molecule binding site. While the binding of TFP, as monitored by EPR spectral changes in the methionine spin-labeled calmodulin, was in evidence with Ca(II), Cd(II), and all the lanthanides examined, no binding of phosphodiesterase to calmodulin could be detected in the presence of the lanthanide ions, perhaps due to inactivation of the phosphodiesterase by lanthanide ion binding. The abilities of the spin-labeled calmodulins to activate phosphodiesterase were also investigated. The spin-labeled tyrosine calmodulin was able to activate phosphodiesterase as well as native calmodulin, while a lower degree of activation was found when the spin-labeled methionine analogue was used.

Ca2+-induced phosphatidylcholine vesicle aggregation in the presence of ferricyanide

The titration of sonicated vesicles of egg phosphatidylcholine with ferricyanide in the presence of Ca2+ results in the formation of aggregates. The turbidity increase caused by these aggregates cannot be reversed by EDTA treatment. In addition, no rearrangement of the bilayer structure has been found in this process, either measuring leakage of vesicle content or exchange of lipids among the bilayers themselves. The aggregation is dependent on the Ca2+ content of the vesicles, the outer Ca2+ and Fe(CN)3-(6) concentration and the order of addition of Ca2+ and ferricyanide. The results can be explained by a specific adsorption of Fe(CN)3-(6) to bilayers of sonicated vesicles, in contrast to other multivalent anions. In contrast to the stability found with sonicated vesicles, the aggregation causes a leakage of the internal solution when multilamellar liposomes are titrated with Fe(CN)3-(6).

Channels produced by spider venoms in bilayer lipid membrane: mechanisms of ion transport and toxic action

The selectivity of ion channels produced by latrotoxin obtained from a black widow spider venom and by venom from the spider Steatoda paykulliana in bilayer phospholipid membrane was studied. Experimental current-voltage curves of these channels were used for the estimation of parameters of a two barrier model of their energy profiles. Selectivities of both types of channels are similar. Alkaline earth cations are permeable, the permeability increasing in the order Mg2+ less than Ca2+ less than Sr2+ less than Ba2+. In contrast transition metal cations block the channel, their efficiency decreases in the order: Cd2+ greater than or equal to Ni2+ greater than Zn2+ greater than Co2+ greater than Mn2+ (Steatoda paykulliana spider venom) and Cd2+ greater than Co2+ greater than Ni2+ greater than Zn2+ greater than Mn2+ (latrotoxin). Amplitudes of current carried by corresponding ions are mainly determined by the depth of the potential well for this ion, i.e., by its affinity to the cation binding site in the channel. The channels are also permeable to monovalent cations but they do not bind them. Selectivity for monovalent cations depends on Ca2+ concentration at the cis-side of membrane in the micromolar range. However, the addition of Ca2+ to the trans-side up to 10 mM does not affect currents carried by monovalent ions. It is suggested that venom-induced calcium channels have two conformational states with different selectivities which interconvert upon binding one calcium ion. Possible general schemes for the organisation of calcium channels in excitable membranes are also discussed. Finally, using a mathematical model of synaptic transmission, possible mechanisms of toxic action of spider venoms are considered.

1H NMR spectroscopic studies of calcium-binding proteins. 1. Stepwise proteolysis of the C-terminal alpha-helix of a helix-loop-helix metal-binding domain

A series of modified parvalbumins, differing only in length of alpha-helix F at the C-terminus, was prepared by carboxypeptidase-mediated digestions of the beta-lineage parvalbumin (pI = 4.25) from carp (N; 108 residues). Removal of Ala-108 to form the N-1 derivative (des-Ala108,Lys107-parvalbumin) only slightly alters the protein's ability to chelate Ca(II) or lanthanides(III). Analysis of the kinetics of their Yb(III) off-rates by optical stopped-flow techniques, determination of their Lu(III)-binding constants by high-resolution 1H NMR methods, and inspection of their solution structures by Yb(III)-shifted 1H NMR techniques indicate N-1 and N-2 are very similar to N (0.1-0.2 M KCl; pH 6-7; 23-55 degrees C). However, removal of the next one or two residues, Val-106 or Val-106/Leu-105, to generate the N-3 and N-4 derivatives severely alters the metal ion binding characteristics of the protein. Although two Yb(III) off-rates are observed for N-3, both are faster than that for the unmodified protein: kCD by a factor of 2 and kEF by a factor of 2200. Removal of Ala-104 and Ala-104/Thr-103 to give a mixture of N-5 and N-6 derivatives eliminates the slow-release site altogether, the single observable koff being 20-30 times faster than release of Yb(III) from the CD site of native parvalbumin. Removal of the C-terminal alpha-helix by digestion through Phe-102 to give N-7 destabilizes the entire protein structure as judged both by the random-coil appearance of its 1H NMR spectrum and by its aberrant kinetics. Although one abnormally fast koff is still observed at micromolar concentrations, Ln(III) chelation tends to precipitate N-7 at higher parvalbumin concentrations (1-3 mM). In contrast to the critical instability of the N-3 through N-7 derivatives, the remarkable stability of the N-1 and N-2 forms of carp parvalbumin may be attributed to the maintainance of two key structural features: an ion pair bond between the negatively charged C-terminal carboxyl function and the protonated epsilon-NH3+ of Lys-27 and hydrophobic interactions of the inner side of helix F with residues in the protein's core.

The thermodynamics of calcium binding to thermolysin

Calcium binding isotherms were determined for thermolysin in the range pH 5.6-10.5, and from 5 to 45 degrees C. An extensive statistical analysis of the binding data suggests that at least two of the four binding sites bind Ca2+ with complete positive cooperativity and independently of the other two. Nonlinear regression analysis of the binding data was used to calculate cooperative (K1) and independent (K2) binding constants for the four calcium sites. Thermodynamic parameters obtained from a van't Hoff analysis indicate that calcium binding to both cooperative and independent sites is an entropy-driven process. At pH 7.0, delta H1 = 90.4 kJ/mol; delta H2 = 97.5 kJ/mol; delta S1 = 456 J K-1 mol-1; delta S2 = 262 J K-1 mol-1. These results are compared to those obtained for other calcium-binding proteins. An analysis of the pH dependence of the calcium binding constants indicates that the binding of four protons at the cooperative site and one to two protons at the independent sites, modulates the calcium affinity. This confirms an earlier structural assignment of the double-site as the locus of the two cooperatively binding Ca2+. Calcium binding to thermolysin is enhanced in the presence of an active site directed inhibitor, suggesting that there may be positive cooperativity between substrate and calcium binding.

The role of calcium, magnesium, and zinc in carcinogenesis

The effects of calcium, magnesium, and zinc supplementation and of magnesium depletion on carcinogenesis are comprehensively reviewed, including epidemiologic and experimental investigations. Some data on the effects of neoplasia on the homeostasis of these metals are also presented. Despite many conflicting results, this review reveals that calcium supplementation is more likely to enhance than inhibit chemical carcinogenesis; magnesium or zinc supplementation tends to inhibit carcinogenesis; magnesium deficiency increases the incidence of neoplasia in humans and animals; parenteral administration of magnesium along with a carcinogen produces local anticarcinogenic effects, while zinc's activity tends to be systemic; and there is a simple correlation between the inhibition of carcinogenesis by the magnesium and zinc supplementation and the reduction of carcinogen binding to cells and DNA. The mechanisms of these effects are not clear. They may involve molecular interactions between metal and carcinogen at different enzymatic and regulatory sites of target cells undergoing neoplastic transformation, as well as stimulation of the host immune system.