Hexameric Units Research Articles

Endogenous and exogenous modulation of gap junctional intercellular communication: Toxicological and pharmacological implications

During the evolution of single-celled organisms to multicellular metazoans, a family of highly conserved genes coding for proteins (connexins), which as hexameric units (connexons), has evolved to form intercellular channels (gap junctions). These gap junctions allow ions and small molecular weight molecules to flow between coupled cells, thereby facilitating synchronization of electrotonic or metabolic cooperation. Control of cell proliferation, cell differentiation and adaptive responses of differentiated cells have been speculated to be biological roles of gap junctions. The regulation of these gap junctions can occur at the transcriptional, translational and posttranslational levels. Transient downregulation by endogenous or exogenous chemicals can bring about adaptive or maladaptive consequences depending or circumstances. Stable abnormal regulation of gap junction function has been associated with the activation of several oncogenes. Several tumor suppressor genes have also been associated with the up-regulation of gap junction function. Since gap junctions exist in all organs of the multi-cellular organisms, the dysfunction of these gap junctions by various toxic chemicals which have cell type/tissue/organ specificity could bring about very distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the CNS system, hyperplasia of the skin, and tumor promotion of initiated tissue. Understanding how many non-mutagenic chemicals might alter. normal gap junction function should form the basis of “epigenetic” toxicology. On the other hand, restoring normal gap junction function to cells which have dysfunctional intercellular communication could be the basis for a new approach for therapeutic pharmaceuticals.

Structure of crystalline VDAC, the voltage-gated channel in the mitochondrial outer membrane

The voltage-gated, mitochondrial channel, VDAC, is formed by a 31-kDa outer-membrane polypeptide. Crystalline arrays of this channel, produced by phospholipase A2 treatment of Neurospora crassa mitochondrial outer membranes, consist of groups of 6 channels repeated on a parallelogram (“oblique”) lattice (a=13.3nm, b=11.5nm, γ = 109°). These membrane crystals are polymorphic, i.e. lateral contraction is triggered by a polyanion which also decreases VDAC’s gating potential. Projection images of unstained, frozen-hydrated VDAC arrays indicate that lattice contraction is accompanied by changes in the distribution of protein away from the hexameric repeat unit.

Chemical stability of insulin. 2. Formation of higher molecular weight transformation products during storage of pharmaceutical preparations.

Formation of covalent, higher molecular weight transformation (HMWT) products during storage of insulin preparations at 4-45 degrees C was studied by size exclusion chromatography. The main products are covalent insulin dimers (CID), but in protamine-containing preparations the concurrent formation of covalent insulin-protamine (CIP) products takes place. At temperatures greater than or equal to 25 degrees C parallel or consecutive formation of covalent oligo- and polymers can also be observed. Rate of HMWT is only slightly influenced by species of insulin but varies with composition and formulation, and for isophane (NPH) preparations, also with the strength of preparation. Temperature has a pronounced effect on CID, CIP, and, especially, covalent oligo- and polymer formation. The CIDs are apparently formed between molecules within the hexameric unit common for all types of preparations and rate of formation is generally faster in glycerol-containing preparations. Compared with insulin hydrolysis reactions (see the preceding paper), HMWT is one order of magnitude slower, except for NPH preparations.

Hexamers of subunit II from Limulus hemocyanin (a 48‐mer) have the same quaternary structure as whole Panulirus hemocyanin molecules

Hemocyanins are copper-containing proteins that transport oxygen in a variety of invertebrates. Considerable evidence has accumulated that arthropodan hemocyanins are multimers of a fundamental hexameric unit. X-Ray crystallographic structure determination has revealed that the hemocyanin molecule from the spiny lobster Panulirus interruptus is a single hexamer having 32 point group symmetry. Using crystals of subunit II, one of 8 polypeptide types comprising the octahexameric hemocyanin of the horseshoe crab Limulus polyphemus, and the molecular replacement method for crystallographic phase determination we show that subunit II forms assemblies with the same hexameric quaternary structure as the whole Panulirus hemocyanin molecule. Observation of the same hexameric motif in two widely separated species provides strong additional evidence that this quaternary structural unit is a universal building block of arthropodan hemocyanins.

Protein-protein interactions in an alphavirus membrane

Using homobifunctional chemical cross-linkers with various span distances, we have determined the near-neighbor associations and planar organization of the E1 and E2 envelope glycoproteins which compose the icosahedral surface of Sindbis virus. We have found that E1-E2 heterodimers, which form the virus protomeric units, exist in two conformationally distinct forms, reflecting their nonequivalent positions in the icosahedron. Three of these heterodimers form the trimeric morphologic units (capsomeres) which are held together by central E1-E1 interactions. In addition, we present data which suggest that E2-E2 interactions organize the capsomeres into pentameric and hexameric geometric units and that E1-E1 interactions between capsomeres maintain the icosahedral lattice in mature virions.

The cell envelope of the hyperthermophilic archaebacterium Pyrobaculum organotrophum consists of two regularly arrayed protein layers: three‐dimensional structure of the outer layer

The cell envelope of the hyperthermophilic sulphur-reducing archaebacterium Pyrobaculum organotrophum H10 was found to be composed of two distinct hexagonally arranged crystalline protein arrays. Electron microscopic analysis of freeze-etched cells and isolated envelopes in conjunction with image processing showed that the inner layer (lattice centre-to-centre spacing 27.9 nm) is essentially identical to the protein array of Pyrobaculum islandicum GEO3, a complex, rigid structure implicated in the maintenance of cell shape. The outer layer has clear p6 symmetry and a lattice spacing of 20.6 nm. Its three-dimensional structure was reconstructed from a negative stain tilt series of an intact double-layered envelope using Fourier filtration to separate the desired information from the other lattices present. The outer layer is a unique, porous network of blocklike dimers disposed around six-fold axes, and exhibits minimal asymmetry between its inner and outer faces. It appears to be rather loosely associated with the outer surface of the inner layer. In most H10 envelopes, the inner layer is orientated with one base vector exactly perpendicular to the long axis of the cell, so that the cylindrical portion is composed of a series of parallel cell-girdling hoops of hexameric morphological units. All the other known Pyrobaculum strains were found to have a GEO3-type envelope structure, consisting of a single rigid protein array and a fibrous capsule. Although H10 does not possess a capsule, fibrils appear to be sandwiched between the two protein layers.

Komplexkatalyse: XXXI. Kristallstruktur des hexameren Mono-2,4-dimethyl-pentadienyldichlorenoedymiun(III)·0.33Tetrahydrofuran. [Nd 6(2,4-C 7H 11) 6Cl 12(THF) 2] eines Präkatalysators für die 1,4- cis-Polymerisation des Butadiens

Nd(2,4-C 7H 11)Cl 2 · 0.33THF was synthesized by reactin of Nd(2,4-C 7H 11) 3 with NdCl 3 · 2THF. The crystal structure study confirms the exitence of a hexameric unit Nd 6Cl 12(2,4-C 7H 11) 6 · 2THF which consists of two Nd 3Cl 5 entities connected via two chloro bridges. The structure of the Nd 3Cl 5 entity contains an unusual polyhedron, namely a distorted hexagonal bipyramid. The 2,4-C 7H 11 anion is connected tot he neodymium atoms in a pentahapto mode of bonding.

Recent aspects of the subunit organization and dissociation of hemocyanins

1. The hemocyanins of the arthropod phylum are built of multiples of hexamers consisting of 1,2,4,6 and 8 of such basic assemblies. Their molecular weights range from about 0.45 x 10(6) to 3.9 x 10(6) daltons. The basic hexameric unit consists of bean-shaped monomers organized in the form of two layers of trimers placed on top of one another. The subunits are heterogeneous, in most cases consisting of four or more electrophoretically different polypeptide chains. 2. Molluscan hemocyanins have an entirely different structure and pattern of assembly from the arthropodan hemocyanins. The basic assembly of the molluscan hemocyanins are decamers organized in the form of right-handed cylinders approximately 300 A in diameter and 140-190 A in height. Different species have one, two and sometimes more than two such assemblies forming correspondingly longer cylindrical particles with molecular weights ranging from about 3.3 x 10(6) to 13 x 10(6) daltons. Cephalopod and chiton hemocyanins consist of single decameric particles, while gastropods have hemocyanins organized of di-decamers or higher assemblies. The subunits of these hemocyanins are elongated protein chains with seven or eight folded globular domains, each housing a binuclear copper center capable of binding and delivering oxygen. 3. The dissociation behavior of the arthropod hemocyanin hexamers and di-hexamers with the hydrophobic urea series of reagents suggest polar and ionic interactions as the main sources of stabilization of the hexamers and the hexamer to hexamer contacts within the di-hexamers. 4. Dissociation studies with the same urea probes with the molluscan hemocyanins, however, suggest a different pattern of stabilization. The stabilization of the decamer to decamer contacts within the gastropod di-decamers appear to be predominantly polar and ionic with relatively few hydrophobic interaction sites. The dimer contacts within the decamers and the monomer to monomer contacts within the dimers observed in the octopus and chiton hemocyanins appear to be predominantly hydrophobic in nature. 5. The urea and the pH dissociation profiles of the single decameric assemblies of some of the octopus and chiton hemocyanins investigated by light-scattering molecular weight methods, have been fitted using either a two-species, decamer to dimer and decamer to monomer scheme of subunit dissociation or a three-species, decamer to dimer to monomer scheme of dissociation.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and reactivity of N,N-dialkylcarbamato complexes of manganese(II). Crystal and molecular structure of [Mn6(O2CNEt2)12], a hexamer with four five-co-ordinated manganese(II) atoms

The compounds [{Mn(O2CNR2)2}n](R = Me, Et, or Pri) have been prepared by treating [Mn(cp)2](cp =η5-C5H5) with CO2–NHR2 in organic solvents. These reactions may possibly involve the addition compound [Mn(cp)2]·NHR2. In the case of R = Et, the adduct has been isolated and characterized, and the space group, crystal data, and metal connectivity established. The structure of the N,N-diethylcarbamato complex has been solved by X-ray diffraction. Crystal data: monoclinic, space group C2/c, a= 18.546(3), b= 19.287(3), c= 24.877(3)A, β= 95.38(2)°, R= 0.078. It consists of hexameric units [Mn6(O2CNEt2)12], containing four five-co-ordinated Mn atoms, joined by bridging carbamato ligands. It was found to be unreactive towards nucleophiles, while by reaction with [{Ti(cp)2Cl}2] the new complex [Ti(cp)2(O2CNEt2)] has been obtained. Organic electrophiles react regioselectively at the oxygen or at the nitrogen atom of the carbamato ligand of the manganese complexes, and in the former case the CO2 fragment is retained in the reaction products.

Synthesis and structure of hexakis (p-hydroxyphenyloxy)benzene: A versatile analogue of the hydrogen-bonded hexameric unit of ?-hydroquinone

Synthesis and structure of hexakis (p-hydroxyphenyloxy)benzene: A versatile analogue of the hydrogen-bonded hexameric unit of ?-hydroquinone

X-ray crystal structure of trimethylsilylmethyllithium

The solid state structure of trimethylsilylmethyllithium has been determined by single crystal X-ray diffraction techniques. The compound crystallizes in the monoclinic system, space group P2 1/ n. Cell dimensions were determined as follows: a 10.931(3), b 18.397(6), c 21.490(8) Å, β 96.0(2)°, V 4298(2) Å 3, Z = 4, and a final R f 5.1% based on 2203 data with σ( I) ≥ 2.5σ( I). The compound is formed by hexameric units, {LiCH 2Si(CH 3) 3} 6, with two distinct classes of LiLi distances of 2.46 and 3.18 Å. There are also two LiC distances av 2.20 and 2.27 Å. The LiH distances to the methylene H atoms have been determined and are short varying between 2.0 and 2.3 Å to the closest lithium atom. The structure, including possible LiH interactions, is discussed and compared with the other known hexameric aggregates.

Picosecond study of energy-transfer kinetics in phycobilisomes of Synechococcus 6301 and the mutant AN 112

Excitation-energy-transfer kinetics in isolated phycobilisomes from the cyanobacterium Synechococcus 6301 ( Anacystis nidulans) and the mutant AN 112 (rods containing one hexameric C-phycocyanin unit only) was investigated by picosecond absorption and fluorescence techniques. The different chromophores in the phycobilisomes were selectively excited. A lifetime component of about 10 ps was found for both C-phycocyanin and allophycocyanin in both types of phycobilisomes. We assign these signals to a transfer of excitation energy from sensitizing (‘s’) to fluorescing (‘f’) chromophores within C-phycocyanin and allophycocyanin units. A 10 ps component was also observed in the anisotropy relaxation measurements. The anisotropy decay is attributed mainly to differently oriented transition dipole moments of ‘s’- and ‘f’-chromophores and partially to ‘f’ → ‘f’ transfer. An absorption recovery signal of τ ≈ 90 ps at λ ≤ 630 nm in phycobilisomes of Synechococcus 6301 is reduced to 40–50 ps in AN 112 phycobilisomes. This is rationalized in terms of a decreased rod → core transfer time in the shorter rods of AN 112. The 40–50 ps lifetime of fluorescence and absorption recovery in AN 112 phycobilisomes is assigned mainly to a rate-limiting transfer step between C-phycocyanin and the allophycocyanin core. A decay component of allophycocyanin τ ≈ 50 ps was observed both in absorption recovery measurements and in fluorescence decay. It is assigned to energy transfer to the terminal chromophores. The final emitter(s) of the phycobilisomes from AN 112 have fluorescence lifetimes of 1.9 and 1.3 ns. We find a good correlation in the fluorescence kinetics between the decay times of phycocyanin and allophycocyanin and the fluorescence risetimes of the terminal emitters.

The crystal structure of [tris(trimethylsilyl)methyl]silanetriol. A novel and remarkably stable hydrogen-bonded cage structure

The compound (Me3Si)3CSi(OH)3 crystallizes as discrete hexameric hydrogen-bonded cage units of remarkable stability.

ChemInform Abstract: (S‐BUTYLDITHIOCARBANILATO)COPPER(I), CU(BUSC(S)NPH). PREPARATION AND CRYSTAL STRUCTURE

AbstractDie 1:1‐molare Umsetzung von Cu(I)‐chlorid mit S‐Butyldithiocarbanilsäure in Acetonitril in Gegenwart eines Äquivalents Triethylamin führt zu dem hexameren, diamagnetischen Titelcluster (I), dessen Molekül‐ und Kristallstrukturdaten (RG R3‐, Z=6) röntgenographisch ermittelt werden.

Polarized absorption picosecond kinetics as a probe of energy transfer in phycobilisomes of Synechococcus 6301

The energy transfer between C-phycocyanin chromophores in intact phycobilisomes of Synechococcus 6301 is shown to lead to an anisotropy relaxation with a lifetime of 10 ± 2 ps. However, due to the molecular order within the hexameric units of C-phycocyanin the anisotropy does not decay to zero. The Förster dipole-dipole mechanism of energy transfer can qualitatively explain these data provided that there is no back transfer of excitation energy and that the chromophore distribution is non-random. The rate of energy transfer in phycobilisomes between C-phycocyanin and allophycocyanin can best be described by a double exponential with lifetimes of 12 ± 3 and 84 ± 8 ps.

(S‐Butyldithiocarbanilato)copper(I), Cu[BuSC(S)NPh]. Preparation and crystal structure

AbstractThe preparation and the crystal structure of a copper(I) complex with the deprotoned S‐Butyldithiocarbanilic ester is described. The crystals of Cu[BuSC(S)NPh] are rhombohedral [space group R3, Z = 6, a = 14.684(3) Å, α = 112.96(4)°] with one hexameric unit in the unit cell.

Crystal structure analysis of the inclusion compound of 2?-hydroxy-2,4,4,7,4?-pentamethylflavan with 1,4-dioxan and water

The 2: 1: 2 complex of 2′-hydroxy-2,4,4,7,4′-pentamethylflavan (1) with 1,4-dioxan and water is triclinic, space groupP\(\bar 1\), witha=8.164(1),b=8.960(1),c=14.644(1) A, α=91.56(1), β=97.19(1), γ=103.37(1)°; there are two flavan host, two water molecules and a single 1,4-dioxan guest molecule in the unit cell. The dioxan molecule is linked,via a hydrogen-bonded water molecule, to the phenolic OH group. The second hydrogen of this water molecule is also involved in hydrogen bonding, in this case to the ring oxygen of a second host molecule. Thus a hydrogen-bonded hexameric unit, a common feature of many clathrates of phenolic hosts, is not found in the present structure.

Thioureido complexes of copper(I), copper(II) and copper(III). Crystal structure of the copper(I) complex [Cu[(C6H5)NC(S)N[(C2H4)2O]]]6

AbstractDeprotonated N‐phenyl,N′,N′‐dialkylthiourea (PhtuR2) complexes of Cu(I) and Cu(II) have been isolated and the existence of a Cu(III) complex inferred from electrochemical measurements. The crystal structure of Cu(I)[(C6H5)NC(S)N[(C2H4)2O]] (space group Pl̄, Z = 24, a = 10.487(9), b = 28.961(8), c = 29.579(12) Å, α = 120.72(3), β = 103.69(5), γ = 83.14(4)°) revealed 4 hexameric units per unit cell. The metals form a distorted octahedron; the ligands are situated outside six of the faces of this octahedron. The copper coordination is threefold planar; a N atom is linked to one Cu and the S atoms to two Cu atoms. The Cu–Cu distances exclude strong metal‐metal interactions. ESR measurements on Cu(II)(PhtuBu2)2 suggest a planar coordination around Cu with the N atoms in trans‐position. Cyclic voltamograms and differential pulse polarograms show the one‐electron oxidation, Cu(II)(PhtuR2)2 → Cu(III)(PhtuR2)2 + + e. The Cu(III) complex is only stable on the electrochemical time scale (scan rate 100 mV s−1).

Molecular packing and intermolecular contacts of sickling deer type III hemoglobin

The X-ray structure of sickling deer type III hemoglobin, solved by the molecular replacement method and refined to an R value of ~25%, has been used to determine the mode of molecular packing and the residues involved in the intermolecular contacts between the hemoglobin tetramers in the crystalline state. The molecules pack in linear arrays (“fibrils”), with adjacent fibrils displaced ~27 Å from one another along the long axis of the arrays. A view down this axis shows an hexagonal network of six fibrils surrounding a central solvent cavity (each hexameric unit is termed a fiber) with adjacent fibers sharing a common wall of fibrils. Contacts less than 5 Å are observed between the following subunits of different molecules: α 1 α′ 1, α 1 α′ 2, α 1 β′ 1, α 1 β′ 2, α 2 β′ 1, α 2 β′ 2, β 2 β′ 2, in which the primes refer to adjacent molecules.

Binding Studies of NADPH to NADP‐Specific l‐Glutamate Dehydrogenase from Saccharomyces cerevisiae

Optical characteristics of enzyme-reduced coenzyme complexes of yeast NADP-specific glutamate dehydrogenase have been investigated in the presence and absence of product (L-glutamate) and in the presence or absence of phosphate. The phosphate effect, pointed out in a previous work, is found again: inorganic phosphate (Pi) destabilizes the binary complex (E - NADPH), the dissociation constant of which is equal to 14 muM, a value much higher than that determined in Tris-HCl buffer: Kd = 0.9 muM. Concerning the role of phosphate some assumptions are drawn up with respect to a similar behaviour of Pi toward yeast glutamate dehydrogenase and ADP toward the beef liver enzyme. In the same way, L-glutamate induces a stabilization of the binary complex; this latter effect is unchanged in the presence of phosphate, yet it is less marked than in the case of beef liver glutamate dehydrogenase. Protein fluorescence, nucleotide fluorescence and circular dichroism measurements allowed the determination of three identical and independent NADPH binding sites per hexameric active unit. In analogy with beef liver enzyme, it seems that yeast glutamate dehydrogenase is a good model to study anticooperativity in ligand binding.