Ceruloplasmin: Macromolecular Assemblies with Iron-Containing Acute Phase Proteins

Copper-containing ferroxidase ceruloplasmin (Cp) forms binary and ternary complexes with cationic proteins lactoferrin (Lf) and myeloperoxidase (Mpo) during inflammation. We present an X-ray crystal structure of a 2Cp-Mpo complex at 4.7 Å resolution. This structure allows one to identify major protein–protein interaction areas and provides an explanation for a competitive inhibition of Mpo by Cp and for the activation of p-phenylenediamine oxidation by Mpo. Small angle X-ray scattering was employed to construct low-resolution models of the Cp-Lf complex and, for the first time, of the ternary 2Cp-2Lf-Mpo complex in solution. The SAXS-based model of Cp-Lf supports the predicted 1∶1 stoichiometry of the complex and demonstrates that both lobes of Lf contact domains 1 and 6 of Cp. The 2Cp-2Lf-Mpo SAXS model reveals the absence of interaction between Mpo and Lf in the ternary complex, so Cp can serve as a mediator of protein interactions in complex architecture. Mpo protects antioxidant properties of Cp by isolating its sensitive loop from proteases. The latter is important for incorporation of Fe³⁺ into Lf, which activates ferroxidase activity of Cp and precludes oxidation of Cp substrates. Our models provide the structural basis for possible regulatory role of these complexes in preventing iron-induced oxidative damage.     

Synthesis and characterization of glucosyl-curcuminoids as Fe3+ suppliers in the treatment of iron deficiency

The Fe³⁺ chelating ability of some curcumin glucosyl derivatives (Glc-H; Glc-OH; Glc-OCH₃) is tested by means of UV and NMR study. The pK _a values of the ligands and the overall stability constants of Fe³⁺ and Ga³⁺ complexes are evaluated from UV spectra. The only metal binding site of the ligand is the β-diketo moiety in the keto-enolic form; the glucosyl moiety does not interact with metal ion but it contributes to the stability of metal/ligand 1:2 complexes by means of hydrophilic interactions. These glucosyl derivatives are able to bind Fe³⁺ in a wide pH rage, forming complex species thermodynamically more stable than those of other ligands commonly used in the treatment of iron deficiency. In addition they demonstrate to have a poor affinity for competitive biological metal ions such as Ca²⁺. All ligands and their iron complexes have a good lypophilicity (log P > −0.7) suggesting an efficient gastrointestinal absorption in view of their possible use as iron supplements in oral therapy. The ligand molecules are also tested for their antioxidant properties in “ex vivo” biological system.     

Characterization of a recombinant aryl β-glucosidase from Neosartorya fischeri NRRL181

An isolated gene from Neosartorya fischeri NRRL181 encoding a β-glucosidase (BGL) was cloned, and its nucleotide sequence was determined. DNA sequence analysis revealed an open reading frame of 1,467 bp, capable of encoding a polypeptide of 488 amino acid residues. The gene was over-expressed in Escherichia coli, and the protein was purified using nickel-nitrilotriacetic acid chromatography. The purified recombinant BGL showed a high level of catalytic activity, with V _max of 886 μmol min⁻¹ mg-protein⁻¹ and a K _m of 68 mM for p-nitrophenyl-β-d-glucopyranoside (pNPG). The optimal temperature for enzyme activity was about 40°C, and the optimal pH was about 6.0. A homology model of N. fischeri BGL1 was constructed based on the X-ray crystal structure of Phanerochaete chrysosporium BGLA. Molecular dynamics simulation studies of the enzyme with the pNPG and cellobiose shed light on the unique substrate specificity of N. fischeri BGL1 only towards pNPG.     

Cloning,purification, and characterization of β-galactosidase from <Emphasis Type="Italic">Bacillus licheniformis</Emphasis> DSM 13

The gene encoding homodimeric β-galactosidase (lacA) from Bacillus licheniformis DSM 13 was cloned and overexpressed in Escherichia coli, and the resulting recombinant enzyme was characterized in detail. The optimum temperature and pH of the enzyme, for both o-nitrophenyl-β-d-galactoside (oNPG) and lactose hydrolysis, were 50°C and 6.5, respectively. The recombinant enzyme is stable in the range of pH 5 to 9 at 37°C and over a wide range of temperatures (4–42°C) at pH 6.5 for up to 1 month. The K _m values of LacA for lactose and oNPG are 169 and 13.7 mM, respectively, and it is strongly inhibited by the hydrolysis products, i.e., glucose and galactose. The monovalent ions Na⁺ and K⁺ in the concentration range of 1–100 mM as well as the divalent metal cations Mg²⁺, Mn²⁺, and Ca²⁺ at a concentration of 1 mM slightly activate enzyme activity. This enzyme can be beneficial for application in lactose hydrolysis especially at elevated temperatures due to its pronounced temperature stability; however, the transgalactosylation potential of this enzyme for the production of galacto-oligosaccharides (GOS) from lactose was low, with only 12% GOS (w/w) of total sugars obtained when the initial lactose concentration was 200 g/L.     

Interaction of Synthetic Dipeptide Bestim with Mouse Thymocytes and Macrophages

Tritium-labeled dipeptide bestim (γ-D-Glu-L-Trp) with a specific activity of 45 Ci/mmol was obtained by the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. [³H]bestim was found to bind with high affinity to mouse peritoneal macrophages (K _d 2.1 ± 0.1 nM) and thymocytes (K _d 3.1 ± 0.2 nM) and also plasma membranes isolated from these cells (K _d 18.6 ± 0.2 and 16.7 ± 0.3 nM respectively). The specific bonding of [³H]bestim with macrophages and thymocytes was inhibited by unlabeled dipeptide thymogen (L-Glu-L-Trp) (K _i 0.9 ± 0.1 and 1.1 ± 0.1 nM respectively). Treatment of the macrophages and thymocytes with trypsin led to their loss of capacity to bind [³H]bestim. Bestim at concentrations range of 0.1–1000 nМ reduced the adenylate cyclase activity in macrophage and thymocyte membranes.     

d-Aspartate binding sites in rat Harderian gland

Radioligand binding of d-[³H]aspartic and l-[³H]glutamic acids to plasma membranes from rat Harderian gland was evaluated. Binding was optimal under physiological conditions of pH and temperature, and equilibrium was reached within 50 min. Specific binding for d-Asp and l-Glu was saturable, and Eadie–Hofstee analysis revealed interaction with a single population of binding sites (for d-Asp K _d = 860 ± 28 nM, B _max = 27.2 ± 0.5 pmol/mg protein; for l-Glu, K _d = 580 ± 15 nM and B _max = 51.3 ± 0.8 pmol/mg protein). l-[³H]glutamate had higher affinity and a greater percentage of specific binding than did d-[³H]aspartate. The pharmacological binding specificity of l-[³H]glutamate indicated an interaction with NMDA-type receptors. Specifically, the order of potency of the displacing compound tested was l-Glu > d-Asp > NMDA > MK801 > d-AP5 > glycine. For d-[³H]aspartate, the data revealed an interaction of d-Asp with either NMDA-type receptors or putative specific binding sites.     

Ethidium bromide binding to native and denatured poly(dA)poly(dT)

Isotherms of the EtBr adsorption on native and denatured poly(dA)poly(dT) in the temperature interval 20–70°C were obtained. The EtBr binding constants and the number of binding sites were determined. The thermodynamic parameters of the EtBr intercalation complex upon changes of solution temperature 20–48°C were calculated: 1.0·10⁶ M⁻¹≤K≤1.4·10⁶ M⁻¹, free energy ΔG ^o=−8.7±0.3 kcal/mol, enthalpy ΔH ^o≅0, and entropy ΔS ^o=28±0.5 cal/(mol deg). UV melting has shown that the melting temperature (T _m) of EtBr-poly(dA)poly(dT) complexes (μ=0.022,4.16·10⁻⁵ M EtBr) increased by 17°C as compared with the ΔT _m of free homopolymer, whereas the half-width of the transition (T _m) is not changed. It was shown for the first time that EtBr forms complexes of two types on single-stranded regions of poly(dA)poly(dT) denatured at 70°C: strong (K ₁=1.7·10⁵ M⁻¹; ΔG ^o=−8.10±0.03 kcal/mol) and weak (K ₂=2.9·10³ M⁻¹; ΔG ^o=−6.0±0.3 kcal/mol).The ΔG ^o of the strong and weak complexes was independent of the solution ionic strength, 0.0022≤μ≤0.022. A model of EtBr binding with single-stranded regions of poly(dA)poly(dT) is discussed.     

Synthesis and evaluation of phosphopeptides containing iminodiacetate groups as binding ligands of the Src SH2 domain

Phosphopeptide pTyr-Glu-Glu-Ile (pYEEI) has been introduced as an optimal Src SH2 domain ligand. Peptides, Ac-K(IDA)pYEEIEK(IDA) (1), Ac-KpYEEIEK (2), Ac-K(IDA)pYEEIEK (3), and Ac-KpYEEIEK(IDA) (4), containing 0–2 iminodiacetate (IDA) groups at the N- and C-terminal lysine residues were synthesized and evaluated as the Src SH2 domain binding ligands. Fluorescence polarization assays showed that peptide 1 had a higher binding affinity (K_d = 0.6 μM) to the Src SH2 domain when compared with Ac-pYEEI (K_d = 1.7 μM), an optimal Src SH2 domain ligand, and peptides 2–4 (K_d = 2.9–52.7 μM). The binding affinity of peptide 1 to the SH2 domain was reduced by more than 2-fold (K_d = 1.6 μM) upon addition of Ni²⁺ (300 μM), possibly due to modest structural effect of Ni²⁺ on the protein as shown by circular dichroism experimental results. The binding affinity of 1 was restored in the presence of EDTA (300 μM) (K_d = 0.79 μM). These studies suggest that peptides containing IDA groups may be used for designing novel SH2 domain binding ligands.     

Adhesion forces in the self-recognition of oligosaccharide epitopes of the proteoglycan aggregation factor of the marine sponge <Emphasis Type="Italic">Microciona prolifera</Emphasis>

Cell aggregation in the marine sponge Microciona prolifera is mediated by a multimillion molecular-mass aggregation factor, termed MAF. Earlier investigations revealed that the cell aggregation activity of MAF depends on two functional domains: (i) a Ca²⁺-independent cell-binding domain and (ii) a Ca²⁺-dependent proteoglycan self-interaction domain. Structural analysis of involved carbohydrate fragments of the proteoglycan in the self-association established a sulfated disaccharide β-d-GlcpNAc3S-(1→3)-α-l-Fucp and a pyruvated trisaccharide β-d-Galp4,6(R)Pyr-(1→4)-β-d-GlcpNAc-(1→3)-α-l-Fucp. Recent UV, SPR, and TEM studies, using BSA conjugates and gold nanoparticles of the synthetic sulfated disaccharide, clearly demonstrated self-recognition on the disaccharide level in the presence of Ca²⁺-ions. To determine binding forces of the carbohydrate–carbohydrate interactions for both synthetic MAF oligosaccharides, atomic force microscopy (AFM) studies were carried out. It turned out that, in the presence of Ca²⁺-ions, the force required to separate the tip and sample coated with a self-assembling monolayer of thiol-spacer-containing β-d-GlcpNAc-(1→3)-α-l-Fucp-(1→O)(CH₂)₃S(CH₂)₆S- was found to be quantized in integer multiples of 30 ± 6 pN. No binding was observed between the two monolayers in the absence of Ca²⁺-ions. Cd²⁺-ions could partially induce the self-interaction. In contrast, similar AFM experiments with thiol-spacer-containing β-d-Galp4,6(R)Pyr-(1→4)-β-d-GlcpNAc-(1→3)-α-l-Fucp-(1→O)(CH₂)₃S(CH₂)₆S- did not show a binding in the presence of Ca²⁺-ions. Also TEM experiments of gold nanoparticles coated with the pyruvated trisaccharide could not make visible aggregation in the presence of Ca²⁺-ions. It is suggested that the self-interaction between the sulfated disaccharide fragments is stronger than that between the pyruvated trisaccharide.     

Galacto-oligosaccharide production using microbial β-galactosidase: current state and perspectives

A recombinant β-galactosidase from Caldicellulosiruptor saccharolyticus was purified with a specific activity of 211 U mg⁻¹ by using heat treatment and His-trap affinity chromatography. The native enzyme was an 80-kDa trimer with a molecular mass of 240 kDa. Maximum activity was observed at pH 6.0 and 80oC, and the half-life at 70oC was 48 h. The enzyme exhibited hydrolytic activity for p-nitrophenyl-β-d-galactopyranoside (pNPGal), oNPGal, or lactose, whereas no activity for p-nitrophenyl-β-d-glucopyranoside (pNPGlu), oNPGlu, or cellobiose. The catalytic residues E150 and E311 of β-galactosidase from C. saccharolyticus were completely conserved in all aligned glycoside hydrolase family 42 β-galactosidases. The results indicated that the enzyme was a β-galactosidase. Galactose uncompetitively inhibited the enzyme. Glucose inhibition of the enzyme was the lowest among β-galactosidases. When 50 g l⁻¹ galactose was added, the enzyme activity for pNPGal was reduced to 26%. When 400 g l⁻¹ glucose instead of galactose was added, the activity was reduced to 82%. When adding galactose (200 g l⁻¹), only 14% of the lactose was hydrolyzed after 180 min. In contrast, the addition of glucose (400 g l⁻¹) did not affect lactose hydrolysis, and more than 99% of the lactose was hydrolyzed after 120 min.     

Kinetics of ferrous iron oxidation by batch and continuous cultures of thermoacidophilic Archaea at extremely low pH of 1.1–1.3

The extreme acid conditions required for scorodite (FeAsO₄·2H₂O) biomineralization (pH below 1.3) are suboptimal for growth of most thermoacidophilic Archaea. With the objective to develop a continuous process suitable for biomineral production, this research focuses on growth kinetics of thermoacidophilic Archaea at low pH conditions. Ferrous iron oxidation rates were determined in batch-cultures at pH 1.3 and a temperature of 75°C for Acidianus sulfidivorans, Metallosphaera prunea and a mixed Sulfolobus culture. Ferrous iron and CO₂ in air were added as sole energy and carbon source. The highest growth rate (0.066 h⁻¹) was found with the mixed Sulfolobus culture. Therefore, this culture was selected for further experiments. Growth was not stimulated by increase of the CO₂ concentration or by addition of sulphur as an additional energy source. In a CSTR operated at the suboptimal pH of 1.1, the maximum specific growth rate of the mixed culture was 0.022 h⁻¹, with ferrous iron oxidation rates of 1.5 g L⁻¹ d⁻¹. Compared to pH 1.3, growth rates were strongly reduced but the ferrous iron oxidation rate remained unaffected. Influent ferrous iron concentrations above 6 g L⁻¹ caused instability of Fe²⁺ oxidation, probably due to product (Fe³⁺) inhibition. Ferric-containing, nano-sized precipitates of K-jarosite were found on the cell surface. Continuous cultivation stimulated the formation of an exopolysaccharide-like substance. This indicates that biofilm formation may provide a means of biomass retention. Our findings showed that stable continuous cultivation of a mixed iron-oxidizing culture is feasible at the extreme conditions required for continuous biomineral formation.     

Characterization of a glucose 3-dehydrogenase from the cultivated mushroom (Agaricus bisporus )

An extracellular enzyme with glucose dehydrogenase activity was purified from liquid cultures of the basidiomycete Agaricus bisporus after growth with d-cellobiose or d-glucose as carbon source. The molecular mass was measured as 57 kDa by gel filtration and 55 kDa by sodiumdodecyl sulphate/polyacrylamide gel electrophoresis, while the isoelectric point was at pH 3.6. By analysis of ¹H-NMR spectra in D₂O, the product of d-glucose oxidation was identified as 3-ketoglucose. The substrates oxidized included d-cellobiose, l-arabinose, d-xylose and sucrose, but the specificity parameter (k _cat/K _m) was highest for d-glucose. Two electron acceptors were identified, namely 2,6-dichloroindophenol and p-benzoquinone, but reduction of dioxygen, ferricyanide or cytochrome c was not detectable. The selective C-3 oxidation of d-glucose is well-characterized for Agrobacterium and Flavobacterium, but this is the first report for a fungus. Received: 19 June 1998 / Received revision: 15 September 1998 / Accepted: 17 September 1998     

Characterisation of acidic sites of Pseudomonas biomass capable of binding protons and cadmium and removal of cadmium via biosorption

Summary The ability of Pseudomonas aeruginosa to accumulate Cd(II) ions from wastewater industries was experimentally investigated and mathematically modelled. From the potentiometric titration and non-ideal competitive analysis (NICA) model, it was found that the biomass contains three acidic sites. The values of proton binding (pK _i =1.66±3.26×10⁻³, 1.92±1.63×10⁻⁴ and 2.16±3.79×10⁻⁴) and binding constant of cadmium metal ions (pK _M1=1.99±2.45×10⁻³ and pK _M2=1.67±4.08×10⁻³) on the whole surface of biomass showed that protonated functional groups and biosorption of Cd(II) ions could be attributed to a monodentate binding to one acidic site, mainly the carboxylic group. From the isothermal sorption experimental data and Langmuir model, it was also found that the value of Langmuir equilibrium (pK _f) constant is 2.04±2.1×10⁻⁵ suggesting that the carboxyl group is the main active binding site. In addition, results showed that the maximum cadmium capacity (q _max) and affinity of biomass towards cadmium metal ions (b) at pH 5.1 and 20 min were 96.5±0.06 mg/g and 3.40×10⁻³± 2.10×10⁻³, respectively. Finally, interfering metal ions such as Pb(II), Cu(II), Cr(III), Zn(II), Fe(II), Mn(II), Ca(II) and Mg(II) inhibited Cd(II) uptake. Comparing the biosorption of Cd(II) by various Pseudomonas isolates from contaminated environment samples (soil and sewage treatment plant) showed that maximum capacities and equilibrium times were different, indicating that there was a discrepancy in the chemical composition between biomasses of different strains.     

Grape skins (Vitis vinifera L.) catalyze the in?vitro enzymatic hydroxylation of p-coumaric acid to caffeic acid

The ability of grape skins to catalyze in vitro conversion of p-coumaric acid to the more potent antioxidant caffeic acid was studied. Addition of different concentrations of p-coumaric to red grape skins (Cabernet Sauvignon) resulted in formation of caffeic acid. This caffeic acid formation (Y) correlated positively and linearly to p-coumaric acid consumption (X): Y = 0.5 X + 9.5; R ² = 0.96, P < 0.0001. The kinetics of caffeic acid formation with time in response to initial p-coumaric acid levels and at different grape skin concentrations, indicated that the grape skins harboured an o-hydroxylation activity, proposedly a monophenol- or a flavonoid 3′-monooxygenase activity (EC 1.14.18.1 or EC 1.14.13.21). The K _m of this crude o-hydroxylation activity in the red grape skin was 0.5 mM with p-coumaric acid.     

Engineering lower inhibitor affinities in β-<Emphasis Type="SmallCaps">d</Emphasis>-xylosidase of <Emphasis Type="Italic">Selenomonas ruminantium</Emphasis> by site-directed mutagenesis of Trp145

β-d-Xylosidase/α-l-arabinofuranosidase from Selenomonas ruminantium is the most active enzyme reported for catalyzing hydrolysis of 1,4-β-d-xylooligosaccharides to d-xylose. One property that could use improvement is its relatively high affinities for d-glucose and d-xylose (K _i ~ 10 mM), which would impede its performance as a catalyst in the saccharification of lignocellulosic biomass for the production of biofuels and other value-added products. Previously, we discovered that the W145G variant expresses K _i ^d-glucose and K _i ^d-xylose twofold and threefold those of the wild-type enzyme. However, in comparison to the wild type, the variant expresses 11% lower k _cat ^d-xylobiose and much lower stabilities to temperature and pH. Here, we performed saturation mutagenesis of W145 and discovered that the variants express K _i values that are 1.5–2.7-fold (d-glucose) and 1.9–4.6-fold (d-xylose) those of wild-type enzyme. W145F, W145L, and W145Y express good stability and, respectively, 11, 6, and 1% higher k _cat ^d-xylobiose than that of the wild type. At 0.1 M d-xylobiose and 0.1 M d-xylose, kinetic parameters indicate that W145F, W145L, and W145Y catalytic activities are respectively 46, 71, and 48% greater than that of the wild-type enzyme.     

Characterization of a novel ginsenoside-hydrolyzing α-l-arabinofuranosidase, AbfA, from Rhodanobacter ginsenosidimutans Gsoil 3054T

The gene encoding an α-l-arabinofuranosidase that could biotransform ginsenoside Rc {3-O-[β-d-glucopyranosyl-(1–2)-β-d-glucopyranosyl]-20-O-[α-l-arabinofuranosyl-(1–6)-β-d-glucopyranosyl]-20(S)-protopanaxadiol} to ginsenoside Rd {3-O-[β-d-glucopyranosyl-(1–2)-β-d-glucopyranosyl]-20-O-β-d-glucopyranosyl-20(S)-protopanaxadiol} was cloned from a soil bacterium, Rhodanobacter ginsenosidimutans strain Gsoil 3054^T, and the recombinant enzyme was characterized. The enzyme (AbfA) hydrolyzed the arabinofuranosyl moiety from ginsenoside Rc and was classified as a family 51 glycoside hydrolase based on amino acid sequence analysis. Recombinant AbfA expressed in Escherichia coli hydrolyzed non-reducing arabinofuranoside moieties with apparent K _m values of 0.53 ± 0.07 and 0.30 ± 0.07 mM and V _max values of 27.1 ± 1.7 and 49.6 ± 4.1 μmol min⁻¹ mg⁻¹ of protein for p-nitrophenyl-α-l-arabinofuranoside and ginsenoside Rc, respectively. The enzyme exhibited preferential substrate specificity of the exo-type mode of action towards polyarabinosides or oligoarabinosides. AbfA demonstrated substrate-specific activity for the bioconversion of ginsenosides, as it hydrolyzed only arabinofuranoside moieties from ginsenoside Rc and its derivatives, and not other sugar groups. These results are the first report of a glycoside hydrolase family 51 α-l-arabinofuranosidase that can transform ginsenoside Rc to Rd.     

Binding of Heparin to Metals

Heparin is a major prophylactic and treatment agent for thrombosis. Structurally, this anticoagulant is a polydisperse, highly negatively charged polysaccharide mixture that contains a variable density of sulfate group substituents per molecule. Previous study has shown that heparin molecules have a high affinity for a wide range of metal ions with varying oxidation states. However, reports in literature on binding of heparin to metals have investigated only a small sampling of heparin–metal ion interactions. Since interaction of heparin with fluid phase and cell surface macromolecules in vivo is dependent on the heparin structure when bound in a metal ion complex, a survey of the physical parameters for heparin binding to metals is imperative. Atomic absorption and spectrophotometry experiments were performed for metal quantification, and in this study, the relative values for affinity constants and number of binding sites for heparin binding to several alkaline, alkaline earth, main group, and transition metals in their most common oxidation states are reported. We found an overall trend for heparin–metal affinity to be Mn²⁺ > Cu²⁺ > Ca²⁺ > Zn²⁺ > Co²⁺ > Na⁺ > Mg²⁺ > Fe³⁺ > Ni²⁺ > Al³⁺> Sr²⁺, with the trend in N _b being opposite compared with the K _a.     

Reversible two-step unfolding of heme–human serum albumin: a 1H-NMR relaxometric and circular dichroism study

Human serum albumin (HSA) participates in heme scavenging, the bound heme turning out to be a reactivity center and a powerful spectroscopic probe. Here, the reversible unfolding of heme–HSA has been investigated by ¹H-NMR relaxometry, circular dichroism, and absorption spectroscopy. In the presence of 6 equiv of myristate (thus fully saturating all available fatty acid binding sites in serum heme–albumin), 1.0 M guanidinium chloride induces some unfolding of heme–HSA, leading to the formation of a folding intermediate; this species is characterized by increased relaxivity and enhanced dichroism signal in the Soret region, suggesting a more compact heme pocket conformation. Heme binds to the folding intermediate with K _d = (1.2 ± 0.1) × 10⁻⁶ M. In the absence of myristate, the conformation of the folding intermediate state is destabilized and heme binding is weakened [K _d = (3.4 ± 0.1) × 10⁻⁵ M]. Further addition of guanidinium chloride (up to 5 M) brings about the usual denaturation process. In conclusion, myristate protects HSA from unfolding, stabilizing a folding intermediate state in equilibrium with the native and the fully unfolded protein, envisaging a two-step unfolding pathway for heme–HSA in the presence of myristate.     

Hormone activity of a synthetic decapeptide with the adrenocorticotropin-like sequence of human immunoglobulin G1

The antiproliferative and immunosuppressivein vitro effects ofimmunocortin, a synthetic adrenocorticotropin-like (ACTH-like) decapeptide H-Val-Lys-Lys-Pro-Gly-Ser-Ser-Val-Lys-Val-OH, whose sequence corresponds to segment 11–20 of the variable part of the human IgG1 heavy chain, were studied. At concentrations of 10⁻¹¹−10⁻⁷ M, immunocortin was found to inhibit the growth of the human MT-4 T-lymphoblastoid cell line, to suppress the blast transformation of thymocytes, and to decrease the spontaneous mobility of peritoneal macrophages and their bactericidal action toward the virulent strainSalmonella typhimurium 415. By using a¹²⁵I-labeled “addressing” fragment of ACTH {[¹²⁵I]ACTH (13–24)}, we showed that MT-4 cells express specific receptors for ACTH (K _d 97 pM). Immunocortin and human ACTH (but not the heavy chain of IgG1) competitively inhibited the binding of [¹²⁵I]ACTH-(13–24) to these receptors withK _i1 of 0.38 andK _i2 of 0.34 nM, respectively. Specific receptors for ACTH (K _d 5.8 nM) on mouse thymocytes were detected and characterized. The unlabeled immunocortin was shown to compete with labeled ACTH-(13–24) for binding to these receptors (K _i=1.8 nM), and this binding of immunocortin to receptors on thymocytes activates adenylate cyclase from these cells and increases the intracellular concentration of cAMP.     

Iron–sulfur cluster biosynthesis: characterization of IscU–IscS complex formation and a structural model for sulfide delivery to the [2Fe–2S] assembly site

Recent work on the bacterial iron–sulfur cluster (isc) family of gene products, and eukaryotic homologs, has advanced the molecular understanding of cellular mechanisms of iron–sulfur cluster biosynthesis. Members of the IscS family are pyridoxyl-5′-phosophate dependent proteins that deliver inorganic sulfide during assembly of the [2Fe–2S] cluster on the IscU scaffold protein. Herein it is demonstrated through calorimetry, fluorescence, and protein stability measurements that Thermotoga maritima IscS forms a 1:1 complex with IscU in a concentration-dependent manner (K _D varying from 6 to 34 μM, over an IscS concentration range of approximately 2–50 μM). Docking simulations of representative IscU and IscS proteins reveal critical contact surfaces at the N-terminal helix of IscU and a C-terminal loop comprising a chaperone binding domain. Consistent with the isothermal titration calorimetry results described here, an overall dominant contribution of charged surfaces with a change in the molar heat capacity of binding, ΔC _p ~ 199.8 kcal K⁻¹ mol⁻¹, is observed that accounts for approximately 10% of the total accessible surface area at the binding interface. Both apo and holo IscUs and homologs were found to bind to IscS in an enthalpically driven reaction with comparable K _D values. Both helix and loop regions are highly conserved among phylogenetically diverse organisms from a pool of archael, bacterial, fungal, and mammalian representatives.