Studies of the association and conformational properties of metal-free insulin in alkaline sodium chloride solutions by one- and two-dimensional 1H NMR

Navigate Flying Molecular Elephants Safely to the Ground: Mass-Selective Soft Landing up to the Mega-Dalton Range by Electrospray Controlled Ion-Beam Deposition

The prototype of a highly versatile and efficient preparative mass spectrometry system used for the deposition of molecules in ultrahigh vacuum (UHV) is presented, along with encouraging performance data obtained using four model species that are thermolabile or not sublimable. The test panel comprises two small organic compounds, a small and very large protein, and a large DNA species covering a 4-log mass range up to 1.7 MDa as part of a broad spectrum of analyte species evaluated to date. Three designs of innovative ion guides, a novel digital mass-selective quadrupole (dQMF), and a standard electrospray ionization (ESI) source are combined to an integrated device, abbreviated electrospray controlled ion-beam deposition (ES-CIBD). Full control is achieved by (i) the square-wave-driven radiofrequency (RF) ion guides with steadily tunable frequencies, including a dQMF allowing for investigation, purification, and deposition of a virtually unlimited m/z range, (ii) the adjustable landing energy of ions down to ∼2 eV/z enabling integrity-preserving soft landing, (iii) the deposition in UHV with high ion beam intensity (up to 3 nA) limiting contaminations and deposition time, and (iv) direct coverage control via the deposited charge. The maximum resolution of R = 650 and overall efficiency up to T_total = 4.4% calculated from the solution to UHV deposition are advantageous, whereby the latter can be further enhanced by optimizing ionization performance. In the setup presented, a scanning tunneling microscope (STM) is attached for in situ UHV investigations of deposited species, demonstrating a selective, structure-preserving process and atomically clean layers.

Metal-dependent hormone function: the emerging interdisciplinary field of metalloendocrinology

For over 100 years, there has been an incredible amount of knowledge amassed concerning hormones in the endocrine system and their central role in human health. Hormones represent a diverse group of biomolecules that are released by glands, communicate signals to their target tissue, and are regulated by feedback loops to maintain organism health. Many disease states, such as diabetes and reproductive disorders, stem from misregulation or dysfunction of hormones. Increasing research is illuminating the intricate roles of metal ions in the endocrine system where they may act advantageously in concert with hormones or deleteriously catalyze hormone-associated disease states. As the critical role of metal ions in the endocrine system becomes more apparent, it is increasingly important to untangle the complex mechanisms underlying the connections between inorganic biochemistry and hormone function to understand and control endocrinological phenomena. This tutorial review harmonizes the interdisciplinary fields of endocrinology and inorganic chemistry in the newly-termed field of "metalloendocrinology". We describe examples linking metals to both normal and aberrant hormone function with a focus on highlighting insight to molecular mechanisms. Hormone activities related to both essential metal micronutrients, such as copper, iron, zinc, and calcium, and disruptive nonessential metals, such as lead and cadmium are discussed.

Structural meta-analysis of regular human insulin in pharmaceutical formulations

We have studied regular acting, wild-type human insulin at potency of 100 U/mL from four different pharmaceutical products directly from their final finished formulation by the combined use of mass spectrometry (MS), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), and single-crystal protein crystallography (PX). All products showed similar oligomeric assembly in solution as judged by DLS and SAXS measurements. The NMR spectra were compatible with well folded proteins, showing close conformational identity for the human insulin in the four products. Crystallographic assays conducted with the final formulated products resulted in all insulin crystals belonging to the R3 space group with two a dimer in the asymmetric unit, both with the B-chain in the T configuration. Meta-analysis of the 24 crystal structures solved from the four distinct insulin products revealed close similarity between them regardless of variables such as biological origin, product batch, country origin of the product, and analytical approach, revealing a low conformational variability for the converging insulin structural ensemble. We propose the use of MS, SAXS, NMR fingerprint, and PX as a precise chemical and structural proof of folding identity of regular insulin in the final, formulated product.

Structure of human insulin monomer in water/acetonitrile solution

Here we present evidence that in water/acetonitrile solvent detailed structural and dynamic information can be obtained for important proteins that are naturally present as oligomers under native conditions. An NMR-derived human insulin monomer structure in H2O/CD3CN, 65/35 vol%, pH 3.6 is presented and compared with the available X-ray structure of a monomer that forms part of a hexamer (Acta Crystallogr. 2003 Sec. D59, 474) and with NMR structures in water and organic cosolvent. Detailed analysis using PFGSE NMR, temperature-dependent NMR, dilution experiments and CSI proves that the structure is monomeric in the concentration and temperature ranges 0.1-3 mM and 10-30 degrees C, respectively. The presence of long-range interstrand NOEs, as found in the crystal structure of the monomer, provides the evidence for conservation of the tertiary structure. Starting from structures calculated by the program CYANA, two different molecular dynamics simulated annealing refinement protocols were applied, either using the program AMBER in vacuum (AMBER_VC), or including a generalized Born solvent model (AMBER_GB).

Zinc-ligand interactions modulate assembly and stability of the insulin hexamer -- a review

Zinc and calcium ions play important roles in the biosynthesis and storage of insulin. Insulin biosynthesis occurs within the beta-cells of the pancreas via preproinsulin and proinsulin precursors. In the golgi apparatus, proinsulin is sequestered within Zn(2+)- and Ca(2+)-rich storage/secretory vesicles and assembled into a Zn(2+) and Ca(2+) containing hexameric species, (Zn(2+))(2)(Ca(2+))(Proin)(6). In the vesicle, (Zn(2+))(2)(Ca(2+))(Proin)(6) is converted to the insulin hexamer, (Zn(2+))(2)(Ca(2+))(In)(6), by excision of the C-peptide through the action of proteolytic enzymes. The conversion of (Zn(2+))(2)(Ca(2+))(Proin)(6)to (Zn(2+))(2)(Ca(2+))(In)(6) significantly lowers the solubility of the hexamer, causing crystallization within the vesicle. The (Zn(2+))(2)(Ca(2+))(In)(6) hexamer is an allosteric protein that undergoes ligand-mediated interconversion among three global conformation states designated T(6), T(3)R(3) and R(6). Two classes of allosteric sites have been identified; hydrophobic pockets (3 in T(3)R(3) and 6 in R(6)) that bind phenolic ligands, and anion sites (1 in T(3)R(3) and 2 in R(6)) that bind monovalent anions. The allosteric states differ widely with respect to the physical and chemical stability of the insulin subunits. Fusion of the vesicle with the plasma membrane results in the expulsion of the insulin crystals into the intercellular fluid. Dissolution of the crystals, dissociation of the hexamers to monomer and transport of monomers to the liver and other tissues then occurs via the blood stream. Insulin action then follows binding to the insulin receptors. The role of Zn(2+) in the assembly, structure, allosteric properties, and dynamic behavior of the insulin hexamer will be discussed in relation to biological function.

Intracellular retention of newly synthesized insulin in yeast is caused by endoproteolytic processing in the Golgi complex

An insulin-containing fusion protein (ICFP, encoding the yeast prepro-alpha factor leader peptide fused via a lysine-arginine cleavage site to a single chain insulin) has been expressed in Saccharomyces cerevisiae where it is inefficiently secreted. Single gene disruptions have been identified that cause enhanced immunoreactive insulin secretion (eis). Five out of six eis mutants prove to be vacuolar protein sorting (vps)8, vps35, vps13, vps4, and vps36, which affect Golgi<-->endosome trafficking. Indeed, in wild-type yeast insulin is ultimately delivered to the vacuole, whereas vps mutants secrete primarily unprocessed ICFP. Disruption of KEX2, which blocks intracellular processing to insulin, quantitatively reroutes ICFP to the cell surface, whereas loss of the Vps10p sorting receptor is without effect. Secretion of unprocessed ICFP is not based on a dominant secretion signal in the alpha-leader peptide. Although insulin sorting mediated by Kex2p is saturable, Kex2p functions not as a sorting receptor but as a protease: replacement of Kex2p by truncated secretory Kex2p (which travels from Golgi to cell surface) still causes endoproteolytic processing and intracellular insulin retention. Endoproteolysis promotes a change in insulin's biophysical properties. B5His residues normally participate in multimeric insulin packing; a point mutation at this position permits ICFP processing but causes the majority of processed insulin to be secreted. The data argue that multimeric assembly consequent to endoproteolytic maturation regulates insulin sorting in the secretory pathway.

Green fluorescent protein photobleaching: a model for protein damage by endogenous and exogenous singlet oxygen

Characterization of protein damage during photosensitization of chlorin e6-treated cells was performed using the green fluorescent protein (GFP). The GFP-chromophore damage caused by singlet oxygen was studied in COS 7 kidney cells and E. coli bacteria following light irradiation. Electron spin resonance (ESR) revealed the generation of endogenous singlet oxygen (1O2) by photoactivated GFP, an effect similar to that produced by the exogenous photosensitizer chlorin e6. A light dose-dependent photobleaching effect of GFP was pronounced at low pH or upon photosensitization with chlorin e6. However, the 1O2 quenchers beta-carotene and sodium azide minimized GFP photo-bleaching. Gel electrophoresis of photosensitized GFP followed by fluorescence multi-pixel spectral imaging revealed the binding of chlorin e6 to GFP, affecting the photobleaching efficacy. Fluorescence multi-pixel spectral imaging of GFP-transfected COS 7 cells demonstrated the presence of GFP in the cytoplasm and nucleus, while chlorin e6 was found to be concentrated in the perinuclear vesicles. Exposure of the cells to light induced GFP photobleaching in the close vicinity of chlorin e6 vesicles. We conclude that photoactivated GFP generates endogenous 1O2, inducing chromophore damage, which can be enhanced by the cooperation of exogenous chlorin e6.

Effects of altered tonicity by sodium chloride on L-tryptophan binding to hepatic nuclei

This study was concerned with the effects of NaCl administered in vivo or added in vitro to isolated nuclei on [(3)H]tryptophan binding to rat hepatic nuclei assayed in vitro. Hypertonic (10.7%) NaCl administered in vivo to rats caused at 10 min a marked decrease in in vitro binding (total and specific) of [(3)H]tryptophan to hepatic nuclei. In vitro incubation of isolated hepatic nuclei, but not of isolated nuclear envelopes, with added NaCl (particularly at 0.125 x 10(-4) M and 0.25 x 10(-4) M) revealed significant inhibition of [(3)H]tryptophan binding. However, isolated hepatic nuclear envelopes prepared after in vitro incubation of isolated nuclei with added NaCl did show inhibition of [(3)H]tryptophan binding (total and specific) compared with controls. Other salts (KCl, MgCl(2), NaHCO(3), NaC(2)H(3)O(2), NaF, or Na(2)SO(4)), at similar concentrations to that of NaCl except for MgCl(2), when added to isolated nuclei did not appreciably inhibit nuclear tryptophan binding. Kinetic studies of in vitro nuclear [(3)H]tryptophan binding in the presence of 0.125 x 10(-4) M NaCl revealed that binding decreased at 0.5 h and continued to 2 h compared with nuclear [(3)H]tryptophan binding with controls (without NaCl addition). The results obtained in vivo in rats and those obtained in vitro with isolated hepatic nuclei revealed NaCl-induced inhibitory effects on [(3)H]tryptophan binding to hepatic nuclei. Although the inhibitory effects were similar under the two different experimental conditions, the mechanism for each may be different in that the NaCl concentration in hepatic cells after administration of NaCl in vivo was appreciably higher than the low levels added in vitro to the isolated hepatic nuclei.

Role of metal ions in the T- to R-allosteric transition in the insulin hexamer

The role of metal ions in the T- to R-allosteric transition is ascertained from the investigation of the T- to R-allosteric transition of transition metal ions substituted-insulin hexamers, as well as from the kinetics of their dissociation. These studies establish that ligand field stabilization energy (LFSE), coordination geometry preference, and the Lewis acidity of the metal ion in the zinc sites modulate the T- to R-state transition. (1)H NMR, (113)Cd NMR, and UV-vis measurements demonstrate that, under suitable conditions, Fe2+/3+, Ni2+, and Cd2+ bind insulin to form stable hexamers, which are allosteric species. (1)H NMR R-state signatures are elicited by addition of phenol alone in the case of Ni(II)- and Cd(II)-substituted insulin hexamers. The Fe(II)-substituted insulin hexamer is converted to the ferric analogue upon addition of phenol. For the Fe(III)-substituted insulin hexamer, appearance of (1)H NMR R-state signatures requires, additionally to phenol, ligands containing a nitrogen that can donate a lone pair of electrons. This is consistent with stabilization of the R-state by heterotropic interactions between the phenol-binding pocket and ligand binding to Fe(III) in the zinc site. UV-vis measurements indicate that the (1)H NMR detected changes in the conformation of the Fe(III)-insulin hexamer are accompanied by a change in the electronic structure of the iron site. Kinetic measurements of the dissociation of the hexamers provide evidence for the modulation of the stability of the hexamer by ligand field stabilization effects. These kinetic studies also demonstrate that the T- to R-state transition in the insulin hexamer is governed by coordination geometry preference of the metal ion in the zinc site and the compatibility between Lewis acidity of the metal ion in the zinc site and the Lewis basicity of the exogenous ligands. Evidence for the alteration of the calcium site has been obtained from (113)Cd NMR measurements. This finding adds to the number of known conformational changes that occur during the T- to R-transition and is an important consideration in the formulation of allosteric mechanisms of the insulin hexamer.

NMR triple-quantum filtered relaxation analysis of 17O-water in insulin solutions: an insight into the aggregation of insulin and the properties of its bound water

Transverse triple-quantum filtered NMR spectroscopy (TTQF) of 17O-water was used to study the properties of water in insulin solutions at different Zn2+ concentrations and pH values. It was established that strongly bound water molecules are already present in Zn-free insulin. On the assumption that the effective correlation time of a strongly bound water molecule, tau sb, is 10 ns, the apparent number of strongly bound water molecules was approximately 3 to 4 per insulin monomer. Addition of Zn2+ equivalent to approximately 2 g-atoms per hexamer did not produce substantial increases in the overall 17O-water TTQF signal intensity and apparent fraction of bound water. The dramatic enhancement of the TTQF signals observed for samples with a Zn2+/hexamer ratio greater than approximately 2:1 could be attributed to the increase in correlation time of the strongly bound water, due to the formation of higher-order oligomers of the protein.

Solution structure of an engineered insulin monomer at neutral pH

Insulin circulates in the bloodstream and binds to its specific cell-surface receptor as a 5808 Da monomeric species. However, studies of the monomer structure and dynamics in solution are severely limited by insulin self-association into dimers and higher oligomers. In the present work we use site-directed mutagenesis of the dimer- and hexamer-forming surfaces to yield the first insulin species amenable for structure determination at neutral pH by nuclear magnetic resonance (NMR) spectroscopy. The preferred insulin mutant, i.e., (B1, B10, B16, B27) Glu, des-B30 insulin retains 47% biological potency and remains monomeric at millimolar concentrations in aqueous solution at pH 6.5-7.5 as judged by NMR and near-UV circular dichroism (CD) spectroscopy. From a series of 2D 1H-NMR spectra collected at pH 6.5 and 34 degrees C, the majority of the resonances are assigned to specific residues in the sequence, and nuclear Overhauser enhancement (NOE) cross-peaks are identified. NOE-derived distance restraints in conjunction with torsion restraints based on measured coupling constants, 3JHNH alpha, are used for structure calculations using the hybrid method of distance geometry and simulated annealing. The calculated structures show that the major part of the insulin mutant is structurally well defined with an average root mean square (rms) deviation between the 25 calculated structures and the mean coordinates of 0.66 A for backbone atoms (A2-A19 and B4-B26) and 1.31 A for all backbone atoms. The A-chain consists of two antiparallel helices, A2-A7 and A12-A19, connected by a loop. The B-chain contains a loop region (B1-B8), an alpha-helix (B9-B19), and a type I turn (B20-B23) and terminates as an extended strand (B24-B29). The B1-B4 and B27-B29 regions are disordered in solution. The structure is generally similar to crystal structures and resembles a crystalline T-state more than an R-state in the sense that the B-chain helix is confined to residues B9-B19.

Reactivity of singlet oxygen toward large peptides

The reactions of singlet oxygen, 1O2, with amino acids and their derivatives have been studied previously. It was found that only five amino acid residues interact readily with 1O2. Here we describe its reactions with the large peptides melittin, neuropeptide Y (NPY) and insulin in their native and in their denatured forms. The singlet oxygen quenching by a polypeptide was compared with that of a solution at the same concentration as those of its constituent amino acids, which are known to react efficiently with 1O2. It was found that the quenching rate by such a mixture exceeded that of the polypeptides in their native form. The ratio of the rate constants for NPY to that of the corresponding amino acid mixture in solution was 0.75. For melittin in its monomeric form it was 0.83 and for a tetramer of melittin (at high ionic strength) it was 0.70. For native insulin the ratio of the rate constants was 0.55. For oxidized insulin with its -S-S- bridges opened the figure became 0.80. However, the quenching by all the polypeptides in their fully denatured form (in the presence of 6 M urea) equalled that of the corresponding amino acid mixtures. Although polypeptides are generally supposed not to possess a stable secondary structure in solution the effects are explained by shielding of some of the reactive amino acid residues in the chain by temporary folding or incipient secondary structures of the native polypeptide.(ABSTRACT TRUNCATED AT 250 WORDS)

High-resolution structure of an engineered biologically potent insulin monomer, B16 Tyr-->His, as determined by nuclear magnetic resonance spectroscopy

Site-directed mutagenesis is used in conjunction with 1H nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy in order to find an insulin species amenable for structure determination in aqueous solution by NMR spectroscopy. A successful candidate in this respect, i.e., B16 Tyr-->His mutant insulin, is identified and selected for detailed characterization by two-dimensional 1H NMR. This mutant species retains 43% biological potency and native folding stability, but in contrast to human insulin it remains monomeric at millimolar concentration in aqueous solution at pH 2.4. The resulting homogeneous sample allows high-quality 2D NMR spectra to be recorded. The NMR studies result in an almost complete assignment of the 1H resonance signals as well as identification of NOE cross peaks. NOE-derived distance restraints in conjunction with torsion restraints based on measured coupling constants, 3JHNH alpha, are used for structure calculations using the hybrid method of distance geometry and simulated annealing. The calculated structures show that the major part of the insulin monomer is structurally well-defined with an average rms deviation between the 20 calculated structures and the mean coordinates of 0.89 A for all backbone atoms, 0.46 A for backbone atoms (A2-A19 and B4-B28), and 1.30 A for all heavy atoms. The structure of the A-chain is composed of two helices from A2 to A7 and from A12 to A19 connected by a short extended strand. The B-chain consists of a loop, B1-B8, an alpha-helix, B9-B19, a beta-turn, B20-B23, and an extended strand from B24 to B30.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of ionic strength and pH on the aggregation properties of zinc-free insulin studied by static and dynamic laser light scattering

The aggregation properties of zinc-free insulin have been studied using static and dynamic light scattering. The aggregation has been investigated as a function of three parameters, the concentration of sodium chloride (in the range 10-100 mM), the pH value (in the range pH 7.5-10.5), and the insulin concentration (1.8-13.4 mg/mL). The measured homodyne autocorrelation function was used to determine the apparent mean hydrodynamic diameter as well as the apparent weight-averaged molar mass of the insulin species in solution. A method of data analysis was employed, which allows the separation of light scattering contributions from the insulin oligomers and from irrelevant macromolecules and possible impurities present in the sample solutions. Also, a simple phenomenological equilibrium model describing the association of oligomers of insulin is presented. One aspect of this model is that it makes it possible to determine weight average molar masses corrected for virial effects on the Rayleigh ratio. This was necessary because virial effects cannot be isolated and corrected for by dilution since this would change the equilibrium distribution of oligomers. The basis of the model is a positive contribution to Gibbs free energy from charge repulsion depending on the protein charge and the number of monomers in the oligomers, and an assumed constant negative contribution to Gibbs free energy arising from either an entropic gain or hydrogen bonding upon association. The equilibrium model gives a good description of both the apparent weight average molar masses and the apparent hydrodynamic diameters, when the effect of the insulin concentration is taken into account by including virial effects arising from charge-charge repulsion (Donnan effect). The result shows that the association of insulin as a function of pH and ionic strength can be described by an effective charge equal to the charge derived from proton titration reduced by the number of sodium ions binding to insulin. At the lowest pH and highest salt concentration (pH 7.5, 100 mM NaCl, 12 mg/mL insulin), the weight average molar mass is close to that of the hexamer, and at the highest pH and lowest salt concentration (pH 10.5, 10 mM NaCl, 1.9 mg/mL), the weight average molar mass is close to that of the monomer. In all cases, however, a distribution of oligomers is present with a relative Gaussian width of about 30%.(ABSTRACT TRUNCATED AT 400 WORDS)