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PubMed2005

Organization of the archaeal MCM complex on DNA and implications for the helicase mechanism.

The homomultimeric archaeal mini-chromosome maintenance (MCM) complex serves as a simple model for the analogous heterohexameric eukaryotic complex. Here we investigate the organization and orientation of the MCM complex of the hyperthermophilic archaeon Sulfolobus solfataricus (Sso) on model DNA substrates. Sso MCM binds as a hexamer and slides on the end of a 3'-extended single-stranded DNA tail of a Y-shaped substrate; binding is oriented so that the motor domain of the protein faces duplex DNA. Two candidate beta-hairpin motifs within the MCM monomer have partially redundant roles in DNA binding. Notably, however, conserved basic residues within these motifs have nonequivalent roles in the helicase activity of MCM. On the basis of these findings, we propose a model for the mechanism of the helicase activity of MCM and note parallels with SV40 T antigen.

Adenosine Triphosphatases↗
PubMed1996

XMCM7, a novel member of the Xenopus MCM family, interacts with XMCM3 and colocalizes with it throughout replication.

A minichromosome maintenance (MCM) protein complex has been implicated in restricting DNA replication to once per cell cycle in Xenopus egg extracts, based on the behavior of a single protein, XMCM3. Using a two-hybrid screen with XMCM3, we have identified a novel member of the MCM family in Xenopus that is essential for DNA replication. The protein shows strong homology to Saccharomyces cerevisiae MCM7 (CDC47) and has thus been named XMCM7. XMCM7 is present in a multiprotein complex with other MCM proteins. It binds to chromatin and is displaced from chromatin by the act of replication. XMCM7 does not preferentially colocalize with sites of DNA replication but colocalizes with XMCM3 throughout replication. Immunodepletion of the MCM complex from Xenopus egg extract by anti-XMCM7 antibodies inhibits DNA replication of sperm and permeable HeLa G2 nuclei but not permeable HeLa G1 nuclei. Replication capacity of the Xenopus egg extract immunodepleted of the MCM complex by anti-XMCM7 antibody can be rescued by MCM proteins eluted from anti-XMCM3 antibody. We conclude that both proteins are present in the same complex in Xenopus egg extract throughout the cell cycle, that they remain together after binding to chromatin and during DNA replication, and that they perform similar functions.

Animals↗
PubMed2004

Site-specific loading of an MCM protein complex in a DNA replication initiation zone upstream of the c-MYC gene in the HeLa cell cycle.

The MCM proteins participate in an orderly association, beginning with the origin recognition complex, that culminates in the initiation of chromosomal DNA replication. Among these, MCM proteins 4, 6, and 7 constitute a subcomplex that reportedly possesses DNA helicase activity. Little is known about DNA sequences initially bound by these MCM proteins or about their cell cycle distribution in the chromatin. We have determined the locations of certain MCM and associated proteins by chromatin immunoprecipitation (ChIP) in a zone of initiation of DNA replication upstream of the c-MYC gene in the HeLa cell cycle. MCM7 and its clamp-loading partner Cdc6 are highly specifically colocalized by ChIP and re-ChIP in G(1) and early S on a 198-bp segment located near the center of the initiation zone. ChIP and Re-ChIP colocalizes MCM7 and ORC1 to the same segment specifically in late G(1). MCM proteins 6 and 7 can be coimmunoprecipitated throughout the cell cycle, whereas MCM4 is reduced in the complex in late S and G(2), reappearing upon mitosis. MCM7 is not visualized by immunohistochemistry on metaphase chromosomes. MCM7 is recruited to multiple sites in chromatin in S and G(2), at which time it is not detected with ORC1. The rate of dissemination is surprisingly slow and is unlikely to be simply attributed to progression with replication forks. Results indicate sequence-specific loading of MCM proteins onto DNA in late G(1) followed by a recruitment to multiple sites in chromatin subsequent to replication.

Binding Sites↗
PubMed2005

The Croonian Lecture 2001 hunting the antisocial cancer cell: MCM proteins and their exploitation.

Replicating large eukaryotic genomes presents the challenge of distinguishing replicated regions of DNA from unreplicated DNA. A heterohexamer of minichromosome maintenance (MCM) proteins is essential for the initiation of DNA replication. MCM proteins are loaded on to unreplicated DNA before replication begins and displaced progressively during replication. Thus, bound MCM proteins license DNA for one, and only one, round of replication and this licence is reissued each time a cell divides. MCM proteins are also the best candidates for the replicative helicases that unwind DNA during replication, but interesting questions arise about how they can perform this role, particularly as they are present on only unreplicated DNA, rather than clustered at replication forks. Although MCM proteins are bound and released cyclically from DNA during the cell cycle, higher eukaryotic cells retain them in the nucleus throughout the cell cycle. In contrast, MCMs are broken down when cells exit the cycle by quiescence or differentiation. We have exploited these observations to develop screening tests for the common carcinomas, starting with an attempt to improve the sensitivity of the smear test for cervical cancer. MCM proteins emerge as exceptionally promising markers for cancer screening and early diagnosis.

Biomarkers, Tumor↗
PubMed2005

The MCM complex: its role in DNA replication and implications for cancer therapy.

The MCM complex controls the once per cell cycle DNA replication in eukaryotic cells. In a process known as DNA replication licensing, it primes chromatin for DNA replication by binding origins of DNA replication during the late M to early G1 phase of the cell cycle. Activated by S phase promoting protein kinases, the origin-bound MCM complexes unwind the double stranded DNA at the origins, recruit DNA polymerases and initiate DNA synthesis. Coupled with the initiation of DNA replication in the S phase, the MCM complexes move away from replication origins as a component of the DNA replication fork, likely serving as DNA helicases. Their departure deprives replication origins the ability to re-initiate DNA replication for the reminder of the cell cycle. Because of its vital role in genome duplication in proliferating cells, deregulation of the MCM function results in chromosomal defects that may contribute to tumorigenesis. The MCM proteins are highly expressed in malignant human cancers cells and pre-cancerous cells undergoing malignant transformation. They are not expressed in differentiated somatic cells that have been withdrawn from the cell cycle. Therefore, these proteins are ideal diagnostic markers for cancer and promising targets for anti-cancer drug development. In this article, I will overview the structures and functions of the MCM complex with an effort to integrate insights from recent biochemical and structural studies. Discussions will also cover activities and structures of the complex that may be useful for the development of drug screens.

Animals↗
PubMed2006

[Spectroscopic characterization of structure of cerium incorporated MCM-48 mesoporous molecular sieve].

Cerium incorporated MCM-48 molecular sieves have been hydrothermally synthesized using a mixed template and pH adjusting route. The samples were characterized by various physicochemical methods, including X-ray diffraction, diffuse reflectance UV-Vis spectroscopy, FTIR spectroscopy, XRF spectroscopy, and nitrogen adsorption. These results reveal that cerium is incorporated in MCM-48 and it is in the form of well-dispersed tetra-coodinated cerium ion. The proper concentration of cerium and adjusting pH can keep Ce-MCM-48 with higher specific surface area , larger unit-cell parameter, narrower pore-size distribution, and thicker pore wall than those of MCM-48. These deduce higher activity, and better selectivity, thermal stability and hydrothermal stability than those of MCM-48. The concentration of Ce incorporated in MCM-48 affects the vibration of the framewoke Si of molecular sieves.

English Abstract↗
PubMed2004

Hydrogen bonding of water confined in mesoporous silica MCM-41 and SBA-15 studied by 1H solid-state NMR.

The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling.

Hydrogen Bonding↗
PubMed1998

Pore Size Analysis of MCM-41 Type Adsorbents by Means of Nitrogen and Argon Adsorption.

Methods of nonlocal density functional theory (NLDFT), proposed recently for predictions of adsorption equilibrium and calculations of pore size distributions in micro- and mesoporous materials, were tested on reference MCM-41 materials. Five newly synthesized MCM-41 adsorbents with presumably uniform pore channels varying from 32 to 45 Å were characterized by X-ray diffraction (XRD), nitrogen adsorption at 77 K, and argon adsorption at 77 and 87 K. New sets of intermolecular interaction parameters of the NLDFT model for N2 and Ar adsorption on MCM-41 were determined. The parameters were specified to reproduce the bulk liquid-gas equilibrium densities and pressures, liquid-gas interfacial tensions, and standard adsorption isotherms on nonporous surfaces in the multilayer adsorption region. The pore size distributions calculated from the desorption branches of the experimental isotherms measured at three different temperatures were consistent with each other. Comparison of the NLDFT-calculated pore sizes with XRD data showed that the thickness of pore walls in the MCM-41 samples under consideration varied from ca. 6 to 12 Å. We found no correlation between the pore size and the pore wall thickness. The results obtained support the NLDFT model as a suitable tool for characterizing nanoporous materials and predicting adsorption equilibrium. The MCM-41 samples studied can be used as references for adsorption measurements. Copyright 1998 Academic Press.

Journal Article↗
PubMed2005

Incorporation of porphyrins into mesopores of MCM-41.

To introduce porphyrins such as the alcoholic-hydroxyl-group-appended free base porphyrin derivative (HP) of 5-[4-(3-hydroxylpropyloxycabonyl)phenyl]-10,15,20-triphenylphorphine into mesopores of MCM-41, samples were treated with 0-4.0 mmol dm-3 HP toluene solutions and the materials obtained were characterized by various means. The framework structure of MCM-41 was not altered by the treatment. With increasing HP concentration, the specific surface area and pore size decreased; in contrast, the number of HP molecules in the material increased almost linearly from 0 to 0.17 groups nm-2. These facts reveal that the HP molecules are incorporated into mesopores of MCM-41. IR results indicated that the hydroxyl group of the HP molecule reacts with surface free SiOH groups of the MCM-41 by a dehydration reaction. Diffuse reflection UV-vis spectra of the HP-introduced material were almost the same as that of pure HP molecules. The Beers plot suggested that the HP molecules in the material are dispersed at an HP concentration less than 1.0 mmol dm-3, and above that concentration, aggregation or flattening of the HP molecules on the MCM-41 surface takes place.

Journal Article↗
PubMed2005

Rate studies on the adsorption of some dyestuffs and p-nitrophenol by chitosan and monocarboxymethylated(mcm)-chitosan from aqueous solution.

In this study, the effect of temperature on the adsorption of some dyestuffs [orange II (O-II), crystal violet (CV) and reactive blue 5 (RB5)] and p-nitrophenol (PNP) by chitosan and of O-II and CV by modified chitosan [monocarboxymethylated(mcm)-chitosan] from aqueous solution was investigated. Kinetical data related to the adsorptions of each dyestuff and PNP by chitosan and of O-II and CV by mcm-chitosan were applied to Lagergren and Weber-Morris equations, and adsorption rate constants (kads) and pore diffusion rate constants (kp) were determined, respectively. In addition, Langmuir isotherm constants with experimental data related to the adsorptions of O-II and CV by chitosan and of CV by mcm-chitosan were applied to McKay et al. equation, and external mass transfer coefficients (kf) were also determined. Lastly, chitosan and mcm-chitosan were compared according to their dyestuffs and PNP uptake capabilities. It was seen that mcm-chitosan removed more O-II (99.2%) and RB5 (34.9%) but less CV (3.1%) and PNP (0.9%).

Adsorption↗
PubMed2006

Synthesis of MCM-41 from coal fly ash by a green approach: influence of synthesis pH.

The present study reports a green synthesis method for preparing pure (free of fly ash) and ordered MCM-41 materials from coal fly ash at room temperature (25 degrees C) during 24 h of reaction. It was shown that the impurities in the coal fly ash were not detrimental to the formation of MCM-41 at the tested conditions. The influence of initial synthesis pH on material properties of calcined MCM-41 samples was investigated by various techniques such as XRF, XPS, XRD, FTIR, DR-UV-vis, solid state NMR, N2 physisorption, TG-DTA, SEM and TEM. The experimental results showed that the amount of trace elements such as Al, Na, Ti and Fe incorporated into the sample increased with synthesis pH value. More aluminum species were incorporated with tetrahedral coordination in the framework under a high pH value. The particle size of the sample decreased with the synthesis pH value. Samples synthesized at high pH values had a larger pore size and were more hydrothermally stable than those at low pH values. From thermal analysis, it was observed that the synthesized MCM-41 samples showed a high thermal stability. These properties made the synthesized MCM-41 suitable for further processing into more useful materials in a wide range of applications.

Carbon↗
PubMedDate not supplied

Highly efficient "tight fit" immobilization of alpha-chymotrypsin in mesoporous MCM-41: a novel approach using precursor immobilization and activation.

The zymogen alpha-chymotrypsinogen A is bound to mesoporous silica MCM-41 with a protein loading of 170 mg/g solid (MCM-Z) by a simple stirring in aqueous tris-HCl buffer (pH 7.2). The bound zymogen is then activated with trypsin to obtain alpha-chymotrypsin immobilized on MCM-41 (MCM-E.I) that displays an effective enzyme activity corresponding to 65 mg protein/g of solid support (3250 BTEE units/g). A direct immobilization of commercially available alpha-chymotrypsin (MCM-E.II) gives lower loading (1250 BTEE units/g). Protein content of the solid support after immobilization is confirmed by thermogravimetric analysis (TGA). The enzyme is tightly bound to the support and can be used over 100 recycles over 1 week in aqueous as well as reverse micellar media. The immobilized enzyme (MCM-E.I) has been used for resolution of N-acetyl-dl-amino acid esters and racemic trans-4-methoxy-3-phenylglycidic acid (PGA) methyl ester.

Chymotrypsin↗
PubMed2005

Tailoring pore properties of MCM-48 silica for selective adsorption of CO2.

Four different types of amine-attached MCM-48 silicas were prepared and investigated for CO(2) separation from N(2). Monomeric and polymeric hindered and unhindered amines were attached to the pore surface of the MCM-48 silica and characterized with respect to their CO(2) sorption properties. The pore structures and amino group content in these modified silicas were investigated by XRD, FT-IR, TGA, N(2) adsorption/desorption at 77 K and CHN/Si analysis, which confirmed that in all cases the amino groups were attached to the pore surface of MCM-48 at 1.5-5.2 mmol/g. The N(2) adsorption/desorption analysis showed a considerable decrease of the pore volume and surface area for the MCM-48 silica containing a polymeric amine (e.g., polyethyleneimine). The CO(2) adsorption rates and capacities of the amine-attached MCM-48 samples were studied employing a sorption microbalance. The results obtained indicated that in addition to the concentration of surface-attached amino groups, specific interactions between CO(2) and the surface amino groups, and the resultant pore structure after amine group attachment have a significant impact on CO(2) adsorption properties of these promising adsorbent materials.

Journal Article↗
PubMed2005

Structure of methanol confined in MCM-41 investigated by large-angle X-ray scattering technique.

Large-angle X-ray scattering (LAXS) measurements over a temperature range from 223 to 298 K have been made on methanol confined in mesoporous silica MCM-41 with two different pore diameters, 28 A (C14) and 21 A (C10), under both monolayer and capillary-condensed adsorption conditions. To compare the structure of methanol in the MCM-41 pores with that of bulk methanol, X-ray scattering intensities for bulk methanol in the same temperature range have also been measured. The radial distribution functions (RDFs) for the monolayer methanol samples showed that methanol molecules are strongly hydrogen bonded to the silanol groups on the MCM-41 surface, resulting in no significant change in the structure of adsorbed methanol with respect to the pore size and temperature. On the other hand, the RDFs for the capillary-condensed methanol samples showed that hydrogen-bonded chains of methanol molecules are formed in both pores. However, the distance and number of hydrogen bonds estimated from the RDFs suggested that hydrogen bonds between methanol molecules in the pores are significantly distorted or partly disrupted. It has been found that the hydrogen bonds are more distorted in the smaller pores of MCM-41. With decreasing temperature, however, the hydrogen-bonded chains of methanol in the pores were gradually ordered. A comparison of the present results on methanol in MCM-41 pores with those on water in the same pores revealed that the structural change with temperature is less significant for confined methanol than for confined water.

Hydrogen Bonding↗
PubMed2006

An analytical approach to determine the pore shape and size of MCM-41 materials from X-ray diffraction data.

The pore shape and size of MCM-41 were studied analytically by comparing the observed powder X-ray diffraction intensities with that derived from the MCM-41 crystal structure models, with two different pore shapes, a hexagon and a circle. The powder diffraction patterns from the as-synthesized and the calcined MCM-41 were measured by a synchrotron radiation at SPring-8, Japan. The MCM-41 structure with circular and hexagonal pore shapes explains well for the as-synthesized and the calcined MCM-41 crystals, respectively. The pore size and boundary obtained by this approach agree with those obtained from an N2 gas adsorption measurement combined with the Fourier synthesized density map.

Journal Article↗
PubMed2006

Covalent linking of near-infrared luminescent ternary lanthanide (Er(3+), Nd(3+), Yb(3+)) complexes on functionalized mesoporous MCM-41 and SBA-15.

The near-infrared (NIR) luminescent lanthanide ions, such as Er(III), Nd(III), and Yb(III), have been paid much attention for the potential use in the optical communications or laser systems. For the first time, the NIR-luminescent Ln(dbm)(3)phen complexes have been covalently bonded to the ordered mesoporous materials MCM-41 and SBA-15 via a functionalized phen group phen-Si (phen-Si = 5-(N,N-bis-3-(triethoxysilyl)propyl)ureyl-1,10-phenanthroline; dbm = dibenzoylmethanate; Ln = Er, Nd, Yb). The synthesis parameters X = 12 and Y = 6 h (X denotes Ln(dbm)(3)(H(2)O)(2)/phen-MCM-41 molar ratio or Ln(dbm)(3)(H(2)O)(2)/phen-SBA-15 molar ratio and Y is the reaction time for the ligand exchange reaction; phen-MCM-41 and phen-SBA-15 are phen-functionalized MCM-41 and SBA-15 mesoporous materials, respectively) were selected through a systematic and comparative study. The derivative materials, denoted as Ln(dbm)(3)phen-MCM-41 and Ln(dbm)(3)phen-SBA-15 (Ln = Er, Nd, Yb), were characterized by powder X-ray diffraction, nitrogen adsorption/desorption, Fourier transform infrared (FT-IR), elemental analysis, and fluorescence spectra. Upon excitation of the ligands absorption bands, all these materials show the characteristic NIR luminescence of the corresponding lanthanide ions through the intramolecular energy transfer from the ligands to the lanthanide ions. The excellent NIR-luminescent properties enable these mesoporous materials to have potential uses in optical amplifiers (operating at 1.3 or 1.5 mum), laser systems, or medical diagnostics. In addition, the Ln(dbm)(3)phen-SBA-15 materials show an overall increase in relative luminescent intensity and lifetime compared to the Ln(dbm)(3)phen-MCM-41 materials, which was explained by the comparison of the lanthanide ion content and the pore structures of the two kinds of mesoporous materials in detail.

Journal Article↗
PubMed2004

Influence of the pore structure of MCM-41 and SBA-15 silica fibers on atomic layer chemical vapor deposition of cobalt carbonyl.

Mesoporous high surface area MCM-41 and SBA-15 type silica materials with fibrous morphology were synthesized and used as support materials for the ALCVD (atomic layer chemical vapor deposition) preparation of Co/MCM-41 and Co/SBA-15 catalysts. Co/MCM-41 and Co/SBA-15 catalysts were prepared by deposition of Co2(CO)8 from the gas phase onto the surfaces of preheated support materials in a fluidized bed reactor. For both silica materials, two different kinds of preparation methods, direct deposition and a pulse deposition method, were used. Pure silica supports as well as supported cobalt catalysts were characterized by various spectroscopic (IR) and analytical (X-ray diffraction, Brunauer-Emmett-Teller, elemental analysis) methods. MCM-41 and SBA-15 fibers showed considerable ability to adsorb Co2(CO)8 from the gas phase. For MCM-41 and SBA-15 silicas, cobalt loadings of 13.7 and 12.1 wt % were obtained using the direct deposition method. The cobalt loadings increased to 23.0 and 20.7 wt % for MCM-41 and SBA-15 silicas, respectively, when the pulse deposition method was used. The reduction behavior of silica-supported cobalt catalysts was found to depend on the catalyst preparation method and on the mesoporous structure of the support material. Almost identical reduction properties of SBA-15-supported catalysts prepared by different deposition methods are explained by the structural properties of the mesoporous support and, in particular, by the chemical structure of the inner surfaces and walls of the mesopores. Pulse O2/H2 chemisorption experiments showed catalytically promising redox properties and surface stability of the prepared MCM-41- and SBA-15-supported cobalt catalysts.

Journal Article↗
PubMed2002

Modified mesoporous MCM-41 as hosts for photochromic spirobenzopyrans.

The influence of the chemical composition and silylation of mesoporous MCM-41 materials on the photochromic behaviour of adsorbed spiropyran (BIPS) and 6-nitrospiropyran was studied. Upon incorporation, the spiropyrans underwent ring opening to form either zwitterionic merocyanine or its corresponding O-protonated form. In all silica MCM-41 or in the MCM-41 containing aluminium, the O-protonated merocyanine was predominantly formed. In the case of MCM-41 modified by silylation of the OH groups, a mixture of zwitterionic merocyanine and spiropyran was present. The photochromic response was studied by means of steady-state irradiation and by laser flash photolysis. Steady-state irradiation (lambda > 450 nm) of the solid samples gives rise in all cases to an intensity decrease of the absorption bands corresponding to either the protonated or the unprotonated merocyanine form (reverse photochromism). In contrast, laser flash photolysis at 308 nm of spiropyrans supported on silylated MCM-41 allows observation of the photochemical ring opening of residual spiropyran to the corresponding zwitterionic form (normal photochromism).

Journal Article↗