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PubMed2001

Predicted location and limited accessibility of protein kinase A phosphorylation site on Na-K-ATPase.

Regulation of Na-K-ATPase by cAMP-dependent protein kinase occurs in a variety of tissues. Phosphorylation of the enzyme's catalytic subunit at a classical phosphorylation consensus motif has been observed with purified enzyme. Demonstration of phosphorylation at the same site in normal living cells or tissues has been more difficult, however, making it uncertain that the Na-K-ATPase is a direct physiological substrate of the kinase. Recently, the structure of the homologous sarco(endo)plasmic reticulum Ca-ATPase (SERCA1a) has been determined at 2.6 A resolution (Toyoshima C, Nakasako M, Nomura H, and Ogawa H. Nature 405: 647-655, 2000.), and the Na-K- ATPase should have the same fold. Here, the Na-K-ATPase sequence has been aligned with the Ca-ATPase structure to examine the predicted disposition of the phosphorylation site. The location is close to the membrane and partially buried by adjacent loops, and the site is unlikely to be accessible to the kinase in this conformation. Conditions that may expose the site or further bury it are discussed to highlight the issues facing future research on regulation of Na-K-ATPase by cAMP-dependent pathways.

Amino Acid Sequence↗
PubMed2005

General practitioners and schizophrenia: results from a Swiss survey.

BACKGROUND: General practitioners (GPs) have an important role in the care of patients with chronic schizophrenia and of those in the early phases of this disorder. AIMS: To obtain information about the number of patients in the early and chronic stages of schizophrenia seen in general practice; the needs and attitudes of GPs, their diagnostic knowledge concerning early phases of schizophrenia and their knowledge and practice concerning treatment of patients with first-episode and multi-episode schizophrenia. METHOD: A postal survey was conducted among randomly selected GPs in Switzerland. RESULTS: A total of 1089 GPs responded to the survey. Early psychosis had a low prevalence in general practice, and GPs expressed a wish for specialised, low-threshold referral services. Diagnostic and treatment knowledge showed inconsistencies. Most GPs said they would treat first-episode schizophrenia with antipsychotics, but only a third recommended maintenance treatment after a first episode of schizophrenia that would conform with international recommendations. CONCLUSIONS: Easily accessible, low-threshold referral services are pivotal in supporting GPs in the management and treatment of emerging schizophrenia in primary healthcare patients.

Antipsychotic Agents↗
PubMed1999

Alpha-Gal oligosaccharides: chemistry and potential biomedical application.

This article focuses on the most recent research efforts by the Wang group in the field of alpha-Gal oligosaccharides. alpha-Gal oligosaccharides are carbohydrate structures bearing a Gal( alpha)1-3Gal(beta) terminus. This class of compounds are believed to act as xenoactive antigens that instigate the hyperacute rejection in xenotransplantation. Enzymatic methods using recombinant (alpha)1-3 galactosyltransferase were employed to synthesize several alpha-Gal oligosaccharides. In addition, a chemical synthetic scheme was devised in order to produce readily accessible amounts of alpha-Gal. Conformational analysis was done using both NMR techniques and molecular modeling protocols. These studies provide important information in the structure-function relationship of alpha-Gal and anti- alpha-Gal antibodies.

Animals↗
PubMed1992

Antibodies to histones in systemic lupus erythematosus and drug-induced lupus syndromes.

Antihistone antibodies are some of the most frequent autoantibodies in rheumatic diseases and can represent useful diagnostic markers in some autoimmune syndromes. Histone epitopes are often located in accessible regions of chromatin or are conformational determinants resulting from the association of several components. These observations and studies with murine models of lupus strongly support the view that histones play a direct stimulatory role in triggering autoantibody production.

Antibodies↗
PubMed1990

[Phosphorylation of tau protein].

In aged human brain and particularly in Alzheimer's disease brain, paired helical filaments (PHFs) accumulate in the neuronal cell. Recently, it has been found that the highly phosphorylated tau protein, one of the microtubule-associated proteins (MAPs), is a component of PHF. The authors attempted to clarify the mechanism underlying the accumulation of PHF from the following two aspects; 1) What is the mechanism of phosphorylation of tau protein? 2) Is the highly phosphorylated tau protein capable of forming PHFs? From rat or bovine microtubule proteins we partially purified and characterized a novel protein kinase that specifically phosphorylated tau and MAP2 among many proteins in the brain extract, and which formed a PHF epitope on the phosphorylated human tau. This enzyme was one of the protein serine/threonine kinases and was independent of known second messengers. The phosphorylation of tau by this enzyme was stimulated by tubulin under the condition of microtubule formation, suggesting that the phosphorylation of tau could occur concomitantly with microtubule formation in the brain. Since this kinase was usually bound to tau but not directly to tubulin, the enzyme was associated with microtubules through tau. From these properties related to tau, this kinase is designated as tau protein kinase. The tau that been phosphorylated with this kinase using [gamma-32P]ATP as a phosphate donor, was digested by endoprotinase Lys-C to produce three labeled fragments, K1, K2 and K3. These three fragments were sequenced and the phosphorylation sites on tau by this kinase were identified. The K2 fragment overlapped with the tau-1 site known to be one of the phosphorylation site in PHF. This result strengthens the possibility that tau protein phosphorylated by tau protein kinase is incorporated into PHF. Tubulin binding sites on tau were located between K1 and K3 fragments, while K2 fragment was located in the neighboring to N-terminus of K1. No phosphorylated sites were found on the tubulin-binding domain of tau, leading us to the idea that the interaction of tau with tubulin could induce conformational changes on tau making it accessible to effects of the kinase. We detected -SP- as a sequence common to three major phosphorylation sites on K1, K2 and K3 fragments. Neurofilament-specific kinase and growth-associated histone H1 kinase are known to recognize the consensus sequence including -SP-. These enzymes exhibit certain properties similar to tau protein kinase and seem to play a crucial role in the regulation of neurite outgrowth or cell growth, through the phosphorylation of a specific substrate, neurofilaments or histone H1, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Alzheimer Disease↗
PubMed1985

[Thyroglobulin and the biosynthesis of thyroid hormones].

Thyroglobulin (mol. wt. 660 kDa) is the specific protein of the thyroid gland in which are synthesized and stored the thyroid hormones (thyroxine and 3,5,3'-triiodothyronine). It is formed of equal-sized subunits (330 kDa) containing each identical polypeptide chains to which are associated two types of oligosaccharide units representing 8 to 10% by weight of the protein. The studies reported in this paper describe the presence in thyroglobulin of discrete hormonogenic sites. After chemical (CNBr) and enzymatic (trypsin and protease V8 of S. aureus) treatments of the protein, four different hormone-containing peptide segments have been isolated, purified and sequenced. They correspond to the hormonogenic tyrosine-containing sites of the protein. One tyrosine is located at 4 amino acid residues from the N-terminal asparagine of the chain and is a major site for thyroxine synthesis. Another one which represents the triiodothyronine site is situated 2 amino acids before the C-terminal lysine. Finally, two other sites, one of low affinity and the other of high affinity for iodine and thyroxine formation, are equally located in the C-terminal part of the chain. The hormone-forming regions localized at the opposite far ends of the thyroglobulin chain(s) likely represent zones more accessible to iodination and with a conformation suited for the coupling of iodotyrosine into iodothyronine residues and ultimately protease attack to release the free hormones into the circulation. The presence of hormonogenic sites of different affinities for iodine allows thyroglobulin to modulate adaptively its hormonogenic capacity to external iodine supply. The molecular mechanism of this process is still unknown.

Amino Acid Sequence↗
PubMed1994

Zero length conformation-dependent cross-linking of phosphorylase kinase subunits by transglutaminase.

Transglutaminase, a zero length cross-linker that catalyzes the formation of isopeptide bonds between proximal Gln and Lys side chains, was used as a structural and conformational probe of the hexadecameric phosphorylase kinase molecule (alpha beta gamma delta)4. Brief cross-linking of nonactivated kinase caused formation of alpha-beta dimers, with no cross-linking involving the gamma- and delta-subunits. When the kinase was first activated by autophosphorylation, significant amounts of alpha-alpha dimers were also observed in addition to the alpha-beta, demonstrating the occurrence of a conformational change in the alpha-subunits concomitant with activation. Both dimers resulted from intramolecular cross-linking. Because the COOH-terminal regions of the alpha-subunits are at the lobe tips of this bilobal kinase (Wilkinson D. A. Marion, T. N., Tillman, D. M., Norcum, M. T., Hainfeld, J. F., Seyer, J. M., and Carlson, G. M. (1994) J. Mol. Biol. 235, 974-982), the formation of zero length cross-linked alpha-alpha dimers indicates that the polypeptide backbones of these subunits must stretch from the lobe tips to a more central location where they abut each other. Excess putrescine, as the amine substrate in place of endogenous Lys, was incorporated by transglutaminase predominately into the alpha-subunits of the kinase, with only slight modification of the beta- and gamma-subunits. Exogenous calmodulin (delta'), an activator of the kinase with a binding site on the alpha-subunits (James, P., Cohen, P., and Carafoli, E. (1991) J. Biol. Chem. 266, 7087-7091), was a potent inhibitor of cross-linking. It also inhibited incorporation of putrescine into the alpha-subunits but stimulated incorporation into the beta- and gamma-subunits. Heparin, another activator of the kinase, had the same effects as exogenous calmodulin on cross-linking and putrescine incorporation, suggesting a commonality in the mechanism through which these two effectors activate the holoenzyme, including promoting a conformational change that increases the surface accessibility of target Gln residues on the catalytic gamma-subunit.

Animals↗
PubMed1997

[Glycosphingolipids as tumor markers].

Interest in research in the sphere of tumours has concentrated for many years on finding tumour specific antigenic structures which can be used in immunodiagnosis and immunotherapy. During the last 30 years a number of monoclonal antibodies against tumours was prepared by means of which evidence was provided that antigens associated with tumourous processes have very often the nature of carbohydrates, in particular those linked to lipids, i.e. glycolipids. Carbohydrate structures with which anti-tumour antibodies react are usually found (at least in small concentrations) also in normal tissues. In the strict sense, from the chemical aspect specific tumour antigens do not exist but rather antigens associated with the tumourous process-tumour markers. Factors which influence the presence of a glycolipid of a tumour marker may be a) a simplification of the glycolipid spectrum by incomplete biosynthesis with concurrent deficiency of higher complex glycolipids and cumulation of the glycolipid precursor, b) activation of new pathways of biosynthesis, c) changes in the ceramide portion, d) lactonisation in the carbohydrate portion of the molecule, e) changes in the accessibility of the glycolipid molecule, their conformation and density on the cell surface. The author reviews the main glycolipid tumour markers which can be used in immunodiagnostics.

Antigens, Neoplasm↗
PubMed1997

Water accessibility to the tryptophan indole N-H sites of gramicidin A transmembrane channel: detection of positional shifts of tryptophans 11 and 13 along the channel axis upon cation binding.

Gramicidin A analogues, in which one of four Trp residues was selectively substituted by carbon-deuterated Trp, were incorporated into phospholipid liposomes and their Raman spectra were recorded in the absence and presence of Na+. Detailed analyses of the Raman spectra have revealed the conformation, strength of hydrophobic interaction, and water accessibility of each individual Trp residue of the gramicidin transmembrane channel. The absolute value of the torsion angle chi2,1 about the CalphaCbeta-C3C2 linkage is found to be 94 degrees +/- 6 degrees in both the cation-free and Na+-bound states. The Trp side chains are generally involved in strong hydrophobic interactions with the lipid acyl chains of the membrane and/or with another Trp residue. The water accessibility to the indole N1H site is in the order Trp-15 > Trp-13 >> Trp-11 > Trp-9 in the cation-free state. In the Na+-bound state, however, the water accessibility significantly decreases for Trp-13 and increases for Trp-11 without change for Trp-15 and -9. The site-specific changes of water accessibility are explained by a combination of positional shifts of Trp-13 and -11 toward the channel center and the channel mouth, respectively. Model building has shown that such positional shifts of Trp indole rings can be linked with deflections of amide C&dbd;O bonds toward the channel pore, suggesting a cation-induced conformational transition of the channel backbone structure.

Cations↗
PubMed2002

Conformational changes of the nucleotide site of the plasma membrane Ca2+-ATPase probed by fluorescence quenching.

Fluorescence quenching by the water-soluble ions I(-) and Cs(+) was used to probe solvent accessibility and polarity of the nucleotide/fluorescein isothiocyanate binding pocket of the purified soluble Ca(2+)-ATPase from plasma membranes. The E(1).Ca.CaM conformer was the least accessible state studied, presenting the lowest suppression constant (K(q)) for both I(-) (K(q) = 6.7 M(-)(1)) and Cs(+) (K(q) = 0.7 M(-)(1)). Accessibility to I(-) was similar for the E(2).VO(4) and E(1).Ca states (K(q) = 7.13 and 7.5 M(-)(1), respectively), whereas E(2) was slightly more accessible (K(q) = 9.1 M(-)(1)). The phosphorylated state E(2)-P presented the highest accessibility, with a K(q) of 16.5 M(-)(1), very near the K(q) of 20.3 M(-)(1) for free FITC. I(-) was unequivocally a better fluorescence quencher, being usually nearly 3-fold as efficient as Cs(+), as indicated by the K(q)(I(-))/K(q)(Cs(+)) ratio (R(q)). The advent of a positive charge cluster on the nucleotide/fluorescein binding pocket in different states was suggested by the increase in R(q), which reached a value as high as 9.5 for the E(1).Ca.CaM conformer. These results indicate (i) a very high water accessibility of the nucleotide/fluorescein pocket for E(2)-P that (ii) is more restricted on the free E(2) state and (iii) becomes rather lower for the E(1).Ca states. Additionally, a positive charge effect of amino acids on the nucleotide site, possibly related to ATP binding and phosphoryl transfer, appears in these E(1).Ca states, being absent in the phosphorylated and nonphosphorylated E(2) states.

Aniline Compounds↗
PubMed1983

Molecular dynamics of native protein. II. Analysis and nature of motion.

The 132 picosecond simulation of atomic motion in bovine pancreatic trypsin inhibitor protein generated in the accompanying paper is analysed here using a variety of different methods. Together, these techniques, many of which have been used before in analyses of protein co-ordinate refinement, give a complete and comprehensible description of the trajectory. Some highlights of the simulation are as follows. (1) The atoms vibrate about a time-averaged conformation that is close to the X-ray structure (within 1.1 A root-mean-square deviation for the main-chain of all residues except the first and last two). The vibration amplitude is least for main-chain atoms in alpha-helix or beta-sheet secondary structure and most for side-chain atoms in the charged polar side-chains (Asp, Glu, Lys and Arg). The overall extent and distribution of atomic motion is in agreement with the temperature factors derived from the X-ray refinement: the reorientation of bond vectors is much less than observed by nuclear magnetic resonance. (2) The protein explores four distinct regions of conformational space in the 132 picoseconds simulated. The conformational change from region III to IV and back again lasts 40 picoseconds and is of particular interest as it is reversible and involves an increase in the hydrogen bond energy. (3) The changes in main-chain torsion angles show the expected cooperativity of phi i + 1 and psi i; side-chains that are close in space also change their conformational angles in unison. (4) Hydrogen bonds are variable and many break and reform again in the 132 picoseconds. Certain hydrogen bonds are much less stable than others; with particular variability seen in the alpha-helices and at the ends of the beta-hairpin. Most noticeable are the co-operative changes of hydrogen bonds at both ends of the beta-hairpin that occur in going from region III to IV of the conformational space. (5) The overall solvent-accessible area remains close to that of the X-ray structure but polar charged residues become less exposed while non-polar hydrophobic residues become more exposed. Together these results give a conceptual model for protein dynamics in which the molecule vibrates about a particular conformation but then suddenly changes conformation, jumping over an energy barrier into a new region of conformational space.

Animals↗
PubMed1979

Mechanism of carbamoyl-phosphate synthetase. Binding of ATP by the rat-liver mitochondrial enzyme.

This paper demonstrates, by pulse-chase techniques, the binding to rat liver mitochondrial carbamoyl phosphate synthetase of the ATP molecule (ATPB) which transfers its gamma-phosphoryl group to carbamoyl phosphate. This bound APTB can react with NH3, HCO-3 and ATP (see below) to produce carbamoyl phosphate before it exchanges with free ATP. Mg2+ and N-acetylglutamate, but not NH3 or HCO-3, are required for this binding; the amount bound depends on the concentration of ATP (Kapp = 10--30 microns ATP) and the amount of enzyme. At saturation at least one ATPB molecule binds per enzyme dimer. Binding of ATPB follows a slow exponential time course (t1/2 8--16 s, 22 degrees C), independent of ATP concentration and little affected by NH3, NCO-3 or by incubation of the enzyme with unlabelled ATP prior to the pulse of [gamma-32P]ATP. Formation of carbamoyl phosphate from traces of NH3 and HCO-3 when the enzyme is incubated with ATP follows the kinetics expected if it were generated from the bound ATPB, indicating that the latter is a precursor of carbamoyl phosphate ('Cbm-P precursor') in the normal enzyme reaction. This indicates that the site for ATPB is usually inaccessible to ATP in solution but becomes accessible when the enzyme undergoes a periodical conformational change. Bound ATP becomes Cbm-P precursor when the enzyme reverts to the inaccessible conformation. Pulse-chase experiments in the absence of NH3 and HCO-3 (less than 0.2 mM) also demonstrate binding of ATPA (the molecule which yields Pi in the normal enzyme reaction), as shown by a 'burst' in 32Pi production. Therefore, (in accordance with our previous findings) both ATPA and ATPB can bind simultaneously to the enzyme and react with NH3 and HCO-3 in the chase solution before they can exchange with free ATP. However, at low ATP concentration (18 micron) in the pulse incubation, only ATPB binds since ATP is required in the chase (see above). Despite the presence of two ATP binding sites, the bifunctional inhibitor adenosine(5')pentaphospho(5')adenosine(Ap5A) fails to inhibit the enzyme significantly. A more detailed modification of the scheme previously published [Rubio, V. & Grisolia, S. (1977) Biochemistry, 16, 321--329] is proposed; it is suggested that ATPB gains access to the active centre when the products leave the enzyme and the active centre is in an accessible configuration. The transformation from accessible to inaccessible configuration appears to be part of the normal enzyme reaction and may represent to conformational change postulated by others from steady-state kinetics. The properties of the intermediates also indicate that hydrolysis of ATPA must be largely responsible for the HCO-3-dependent ATPase activity of the enzyme. The lack of inhibition of the enzyme by Ap5A indicates substantial differences between the Escherichia coli and the rat liver synthetase.

Adenosine Triphosphate↗
PubMed1990

Molecular mechanisms of local anesthesia: a review.

Impulse block by LA occurs through the inhibition of voltage-gated Na+ channels. Both protonated and neutral LAs can inhibit Na+ channels though interference with the conformational changes that underly the activation process (the sequence of events that occurs as channels progress from the closed resting state to the open conducting state). The occlusion of open channels contributes little to the overall inhibition. Local anesthetic inhibition of Na+ currents increases with repetitive depolarizations in a process called phasic block. Phasic block represents increased LA binding, either because more channels become accessible during depolarization or because the channel conformations favored by depolarization bind LA with higher affinity. The details of phasic block are dependent on LA chemistry: certain LAs bind and dissociate quite rapidly, others act more slowly; some LAs interact effectively with closed states that occur intermediately between resting and open states, others favor the open channel, and still others have a higher affinity for inactivated states. Channel activation accelerates LA binding, and LAs may bind more tightly to activated and inactivated than to resting channels. In this regard, both the modulated receptor and the guarded receptor hypotheses are valid. In binding to activated and inactivated channels, LAs prevent the conformational changes of activation and antagonize the binding of activator agents that poise channels in activated, open states. These reciprocal actions are one aspect of the concerted conformational rearrangements that occur throughout Na+ channels during gating. The LA binding site may exist in the channel's pore, at the membrane-protein interface, or within the protein subunits of the channel. Judging from its susceptibility to intracellular proteases and its accessibility to LAs with limited membrane permeability (i.e., quaternary LAs in the cytoplasm), the site lies nearer to the cytoplasmic than the external surface of the membrane. Nevertheless, protons in the external medium influence the dissociation of LA from the closed channel. Binding of LAs at the inhibitory site is weak and loose. If one accounts for the membrane-concentrating effects of LA hydrophobicity that are expressed as membrane: buffer partition coefficients equal to 10(2)-10(4), then the apparent LA affinities are low. The equilibrium dissociation constants calculated on the basis of free drug in the membrane are 1-10 mM, with a correspondingly weak binding to the inhibitory LA site. The stereospecificity of LA action is also relatively nonselective, suggesting a loose fit between ligand and binding site.(ABSTRACT TRUNCATED AT 400 WORDS)

Anesthesia, Epidural↗
PubMed2000

Studies on the solution conformation and dynamics of a polysaccharide from Sinorhizobium fredii HH103 and its monosaccharide repeating unit.

The conformational behavior of the homopolysaccharide isolated from Sinorhizobium fredii HH103 and its monosaccharide repeating unit (5-acetamido-3,5,7,9-tetradeoxy-7-(3-hydroxybutyramido)-L-glycero- L- manno-nonulosonic acid) was analyzed by nuclear magnetic resonance (NMR) spectroscopy and extensive molecular dynamics simulations (MD). The results indicate that the glycosidic linkages and lateral chains may adopt a variety of conformations. MD simulations using the generalized Born solvent-accessible surface area (GB/SA) continuum solvent model for water and the MM3* force field provide a population distribution of conformers that satisfactorily agrees with the experimental NMR data for the torsional degrees of freedom of the molecule.

Carbohydrate Conformation↗
PubMed2004

Is esophageal pH monitoring used appropriately in an open-access system? A prospective multicenter study.

OBJECTIVES: To evaluate the referral patterns and indications for esophageal pH monitoring in an open-access system and to determine whether these indications conform to practice guidelines of the American Gastroenterological Association (AGA). METHODS: A total of 851 consecutive patients referred for ambulatory pH monitoring to nine open-access gastrointestinal units over a 12-month period received a structured interview. The indication for the examination was decided by the physician performing the procedure, on the basis of the patient's clinical history and main complaint. RESULTS: Three hundred and twenty-three (38%) examinations were for indications in accordance with the guidelines. The proportion of appropriate requests in each center ranged from 34% to 47%. This figure was not significantly different in larger gastrointestinal units (more than 150 examinations per year) and smaller ones (35% and 40%; p= 0.14). The proportion of appropriate requests was 45% for gastroenterologists, 38% for surgeons, 32% for other specialists, and 24% for primary care physicians (PCPs) (p < 0.001). The percentage of appropriateness was significantly different between gastrointestinal specialists and PCPs (p < 0.001 vs gastroenterologists, p= 0.015 vs GI surgeons), and between gastroenterologists and other specialists (p= 0.006). The underuse of an empirical trial of acid-suppression therapy in patients with suspected reflux disease and the overuse of this test to confirm a diagnosis in patients with erosive esophagitis and in endoscopy-negative cases with typical symptoms responding completely to antisecretory therapy accounted for most of the referrals, which was not in accordance with the guidelines. CONCLUSIONS: In an open-access system, a high proportion of esophageal pH studies are done for indications not consistent with published guidelines, particularly among the examinations not requested by gastrointestinal specialists. Further education is still needed on the appropriate use of esophageal pH monitoring and management of gastroesophageal reflux disease (GERD).

Adolescent↗
PubMed2006

The C-terminus of complement regulator Factor H mediates target recognition: evidence for a compact conformation of the native protein.

The complement inhibitor Factor H has three distinct binding sites for C3b and for heparin, but in solution uses specifically the most C-terminal domain, i.e. short consensus repeats (SCR) 20 for ligand interaction. Two novel monoclonal antibodies (mABs C14 and C18) that bind to the most C-terminal domain SCR 20 completely blocked interaction of Factor H with the ligands C3b, C3d, heparin and binding to endothelial cells. In contrast, several mAbs that bind to the N-terminus and to the middle regions of the molecule showed no or minor inhibitory effects when assayed by enzyme-linked immunosorbent assay (ELISA) and ligand interaction assays. This paradox between a single functional binding site identified for native Factor H versus multiple interaction sites reported for deletion constructs is explained by a compact conformation of the fluid phase protein with one accessible binding site. On zymosan particles mAbs C14 and C18 blocked alternative pathway activation completely. Thus demonstrating that native Factor H makes the first and initial contact with the C terminus, which is followed by N terminally mediated complement regulation. These results are explained by a conformational hypothetical model: the native Factor H protein has a compact structure and only one binding site accessible. Upon the first contact the protein unfolds and exposes the additional binding sites. This model does explain how Factor H mediates recognition functions during complement control and the clustering of disease associated mutations in patients with haemolytic uraemic syndrome that have been reported in the C-terminal recognition domain of Factor H.

Antibodies, Monoclonal↗
PubMed1997

Calcium-binding properties of human erythrocyte calpain.

The results presented provide more information on the sequential mechanism that promotes the Ca2+-induced activation of human erythrocyte mu-calpain under physiological conditions. The primary event in this process corresponds to the binding of Ca2+ to eight interacting sites, of which there are four in each of the two calpain subunits. Progressive binding of this metal ion is linearly correlated with the dissociation of the proteinase, which reaches completion when all eight binding sites are occupied. The affinity for Ca2+ in the native heterodimeric calpain is increased 2-fold in the isolated 80 kDa catalytic subunit, but it reaches a Kd consistent with the physiological concentration of Ca2+ only in the active autoproteolytically derived 75 kDa form. Binding of Ca2+ in physiological conditions, and thus the formation of the 75 kDa subunit, can occur only in the presence of positive modulators. These are represented by the natural activator protein, found to be a Ca2+-binding protein, and by highly digestible substrates. The former produces a very large increase in the affinity of calpain for Ca2+, and the latter a smaller but still consistent decrease in the Kd of the proteinase for the metal ion. As a result, both dissociation into the constituent subunits and the autoproteolytic conversion of the native 80 kDa subunit into the active 75 kDa form can occur within the physiological fluctuations in Ca2+ concentration. The delay in the expression of the proteolytic activity with respect to Ca2+ binding to native calpain, no longer detectable in the 75 kDa form, can be attributed to a Ca2+-induced functional conformational change, which is correlated with the accessibility of the active site of the enzyme.

Animals↗
PubMed1989

The heparin and pentosan polysulfate binding sites of human antithrombin overlap but are not identical.

Four sulfated polysaccharides (unfractioned heparin, low-molecular-mass heparin, heparan sulfate and pentosan polysulfate) were investigated for their abilities (a) to bind antithrombin, (b) to induce conformational change of the inhibitor and (c) to potentiate antithrombin inhibition of thrombin. The binding capacity was reflected by the shielding of the heparin binding site. This was characterized by the extent to which a polysaccharide could protect chemical modification of Lys-125 and Lys-136, two lysyl residues of antithrombin which have been implicated in heparin binding. The conformational change was measured by fluorescence enhancement and the increased accessibility of Lys-236 to chemical modification. Our results reveal that the events of polysaccharide binding, conformational change and the enhancement of inhibitory activity are not quantitatively interlinked. Compared to the unfractionated heparin on an equal mass basis, the low-molecular-mass heparin (molecular mass 4-6 kDa) binds more strongly to antithrombin, induces a greater conformational change (about twofold), but is less potent in accelerating the inhibitory activity. Both heparin and heparan sulfate shield Lys-125 and Lys-136 and induce a conformational change that leads to exposure of Lys-236 and an increased fluorescence. On the other hand, pentosan polysulfate protects only Lys-125 and causes no appreciable conformational change, although it is also capable of enhancing the antithrombin-thrombin interaction. These data clearly demonstrate that the heparin and pentosan polysulfate binding sites of antithrombin overlap (at Lys-125) but are not identical.

Antithrombins↗