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Query: UMLS:C0027960 (mole)
 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Selenium (Se) is an essential trace element for animals and humans. Its biological role was established following the discovery that Se is a structural component of the active center of the enzyme glutathione peroxidase (GSH-Px). During the last decade remarkable progress has been made in the recognition of the structure and function of several selenoproteins. Cellular GSH-Px was the first enzyme recognized as a selenoprotein. In it Se was found in the form of selenocysteine. The enzyme is a tetrameric protein and is composed of four apparently identical subunits each containing one gram atom of Se. Plasma GSH-Px also has a tetrameric form with identical subunits and with one atom of Se per subunit. It is, however, a glycosylated protein, and is distinct from cellular enzyme. Both enzymes catalyze the reduction of hydrogen peroxide and a variety of organic hydroperoxides by glutathione. A third GSH-Px, called phospholipid hydroperoxide glutathione peroxidase (PHGSH-Px), is a monomeric, membrane-associated enzyme containing one atom of Se per mole of protein. This enzyme destroys esterified lipid hydroperoxides. The fourth known mammalian selenoenzyme is a type I iodothyronine 5'-deiodinase that catalyzes the deiodination of L-thyroxine to the biologically active hormone 3,3',5-triiodothyronine. It is a monomeric enzyme and contains one atom of Se per mole of protein. Selenoprotein P, a fifth known selenoprotein, is a glycosylated, monomeric protein containing ten atoms of Se per molecule. The function of this protein is not known, but it may play a role in Se transport or be connected with a protective activity against free radicals. In all these selenoproteins the Se is incorporated into the protein molecule via the selenocysteinyl-tRNA which recognizes the specific UGA codons in mRNAs to insert selenocysteine into the primary structure of selenoproteins.
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PMID:Mammalian selenoproteins. 148 33

We have elaborated a method for the isolation of ribosomal subunits from fresh unfrozen human placenta containing intact rRNA and a complete set of ribosomal proteins. Activity of 80S ribosomes obtained by reassociation of 40S and 60S subunits in nonenzymatic poly(U)-dependent binding of Phe-tRNA(Phe) was equal to 80% (above 1.5 mol [14C]Phe-tRNA(Phe) is coupled to 1 mol of ribosomes). The activity of 80S ribosomes in poly(U)-directed synthesis of polyphenylalanine was tested in a polysome-free protein-synthesizing system from rabbit reticulocytes. About 100 mol of phenylalanine residue was polymerized by a mole of ribosomes at a rate of 0.83 residues per minute in this system (2 h, 37 degrees C).
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PMID:Isolation of ribosomal subunits containing intact rRNA from human placenta: estimation of functional activity of 80S ribosomes. 179 4

Photoaffinity labeling of E. coli ribosomes within the 70S initiation complex was studied by using photoreactive derivatives of fMet-tRNAfMet bearing arylazidogroups scattered statistically over guanosine residues. It is shown that fMet-azido-tRNAfMet-II bearing 2 moles of the reagent residues per mole of tRNA (modified in the conditions of stability of tRNA tertiary structure) is fully active in aminoacylation and in the factor-dependent binding with ribosomes to form the 70S initiation complex. Functional activity of fMet-azido-tRNAfMet-I bearing also 2 moles of the reagent residues per mole of tRNA (but modified in conditions of lability of tRNA tertiary structure) decreases up to approximately 45% in aminoacylation and up to 70% in IF-2 X GTP-dependent binding to the ribosomes. Irradiation of complexes 70S ribosome-MS2-RNA-fMet-azido-tRNAfMet results in covalent linking of the tRNA derivative to the ribosomes. Both subunits are labeled, the 30S to a larger extent than 50S. It is shown that fMet-azido-tRNAfMet-II labels proteins S1, S7, S9, L27 whereas fMet-azido-tRNAfMet-1--proteins S1, S3, S5, S9, S14, L1, L2, L7/L12.
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PMID:[Photoaffinity modification of Escherichia coli ribosomes by fMet-tRNAf Met derivatives in the 70S initiation complex]. 243 42

A lysine-reactive cross-linker has been coupled to the minor base 3-(3-amino-3-carboxypropyl)uridine in the variable loop of the Escherichia coli elongator methionine tRNA (tRNA(mMet]. Incubation of the derivatized tRNA with E. coli methionyl-tRNA synthetase (MetRS) resulted in covalent coupling of the protein and nucleic acid and loss of amino acid acceptor activity of the enzyme. One mole of tRNA was cross-linked per mole of enzyme inactivated. Enzyme activity was largely restored by release of the bound tRNA following cleavage of the disulfide bond in the cross-linker with a sulfhydryl reagent. The cross-linking reaction was effectively inhibited by unmodified tRNA(mMet) but not by noncognate tRNA(Phe). The covalent complex was digested with trypsin, and the resulting tRNA-bound peptides were isolated by anion-exchange chromatography. The cross-linked peptides were released from the tRNA by cleavage in the disulfide bond of the cross-linker and purified by reverse-phase high-pressure liquid chromatography, yielding one major peptide plus several minor peptides. Amino acid analysis indicated that the major product was an octadecapeptide cross-linked to tRNA(mMet) through lysine residue 596 in the primary sequence of MetRS. The N-terminal sequence of the peptide was determined to be Val-Ala-Leu-Ile-Glu-Asn-Ala-Glu-Phe-Val, corresponding to residues 582-591 in MetRS. The procedures described here should be applicable to the determination of peptide sequences near the variable loop of other tRNAs containing the 3-(3-amino-3-carboxypropyl)uracil base when such tRNAs are bound to specific proteins.
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PMID:Covalent coupling of the variable loop of the elongator methionine tRNA to a specific lysine residue in Escherichia coli methionyl-tRNA synthetase. 310 75

The thermal denaturation of E. coli unfractionated tRNA in ethanol/water mixtures has been studied as a function of alcohol concentration in the water-rich region (mole fraction of co-solvent chi 2 less than 0.2). The results show that with increasing alcohol concentration the melting temperature of tRNA first reaches a minimum at an intermediate composition chi *2 approximately equal to 0.055 and then increases with increasing chi 2. The value of chi *2 is close to that at which structural changes in the mixture occur as inferred from compressibility and optical absorption measurements. The present experimental data support the assumptions that the dominant mechanism by which ethanol affects the thermal stability of tRNA molecules is through its effect on the structure of water.
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PMID:Effect of ethanol on the thermal stability of tRNA molecules. 354 10

Pressure inhibition of cell-free polypeptide synthesis is manifested in the same manner as that observed in the intact cell: (i) starting at approximately 200 atm, there is a progressive inhibition with increasing pressures; (ii) there is complete inhibition at 680 atm; (iii) incorporation into polypeptide is instantaneously reversible after pressure release and proceeds at a rate parallel to an atmospheric control; and (iv) the volume change of activation (DeltaV*) is 100 cm(3)/mole. Peptide bond formation per se can occur at a pressure level which is totally inhibitory to polypeptide synthesis. The one investigated step in translation that is inhibited in an identical manner is the binding of aminoacyl-transfer ribonucleic acid (AA-tRNA) to the ribosome-messenger RNA (mRNA) complex. The volume change of activation (DeltaV*) calculated for the binding reaction is also 100 cm(3)/mole. Thus, the inability of AA-tRNA to bind to ribosomes and mRNA under pressure, possibly in conjunction with translocation, appears to be responsible for the observed inhibition of the translational mechanism.
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PMID:Inhibition of cell-free protein synthesis by hydrostatic pressure. 456 35

A sensitive method of polyamine estimation has been adapted to the study of the organic cations of small amounts of nucleic acid. A procedure utilizing phenol extraction, alcohol precipitation, and separation on Sephadex G100 has been devised for the isolation of tRNA at low ionic strength. The procedure is applicable to the isolation of tRNA from liter batches of bacterial culture. With these methods we have examined the polyamines of tRNA isolated from polyauxotrophic strains of E. coli incubated under various physiological conditions and have found the following: (1) The tRNA from relaxed bacteria (TAU rel) harvested during exponential growth is heterogeneous with respect to polyamine content. Some portions of the population contain about one mole of spermidine per mole of tRNA. Some putrescine and an unknown amine are also present in low concentration. (2) After exponential TAU rel is incubated with thymine and uracil in the absence of arginine, the tRNA population is far more homogeneous with respect to polyamine content. The various fractions contain two moles of spermidine per mole of tRNA and a small amount of putrescine. (3) After exponential TAU rel is incubated in the absence of both arginine and uracil, the polyamine pattern of the tRNA resembles that isolated from exponential cells. (4) The tRNA from stringent bacterias harvested during exponential growth is heterogeneous with respect to polyamine distribution and some fractions contain relatively high concentrations of the unknown amine.
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PMID:The polyamine content of the tRNA of E. coli. 490 5

Glutamyl-tRNA synthetase has been isolated from an extreme thermophile, Thermus thermophilus HB8. The enzyme has been purified to homogeneity by successive chromatography on columns of DEAE-cellulose, DEAE-Sephacel, phosphocellulose and hydroxyapatite. 11.7 mg of purified enzyme has been obtained from 2 kg of T. thermophilus cells, with a purification factor of 600 with an 11% yield. From gel permeation chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis, the enzyme is found to be a monomer protein with a molecular weight of 50,000. The optimum temperature for the aminoacylation of T. thermophilus tRNAGlu is 65 degrees C, and the optimum pH range is 8.0-9.0, in the presence of 5 mM Mg2+. The Km values for ATP, L-glutamate, and T. thermophilus tRNAGlu are 230 microM, 70 microM, and 0.65 microM, respectively, in the presence of 50 mM KCl and 10 mM MgCl2 at pH 8.0 at 65 degrees C. Escherichia coli tRNA2Glu is also a good substrate with a Km value of 0.60 microM at 65 degrees C. The mole fractions of Arg and Leu residues are higher and that of Asx residues is lower than those of E. coli glutamyl-tRNA synthetase. Glutamyl-tRNA synthetase from T. thermophilus is remarkably thermostable; even after incubation for 9 h at 65 degrees C, 70% of the enzyme activity is retained in the absence of any protecting factors. Such an extremely thermostable enzyme with a low molecular weight will be useful for detailed physiochemical analyses on the molecular mechanism of strict recognition by aminoacyl-tRNA synthetases.
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PMID:Purification and characterization of glutamyl-tRNA synthetase from an extreme thermophile, Thermus thermophilus HB8. 652 23

The effect of modification of carboxylic groups of phenylalanyl-tRNA synthetase by p-toluene sulfonate N-cyclohexyl-N'-beta-(4-methylmorpholine) ethylcarbodiimide (CMEC) on the activity of the enzyme was investigated. It was shown that modification of two moles of carboxylic groups per mole of the enzymes leads to the diminution of negative charge of the enzyme and to inactivation in ATP-[32P]PPi-exchange and aminoacylation reactions. The inactivation is completely reversed by mild alkaline hydrolysis. ATP in concentration 2 X 10(-4) M partially protects the enzyme against inactivation, protective effect being stimulated by Mg2+ and 0.4-0.7 moles of carboxylic groups per mole of the enzyme are protected against inactivation is observed although the depth of modification is increased. Other substrates do not have protective effect. Modification of the enzyme by CMEC increases Kdiss value of [14C]-Phe-tRNA enzyme complex and Km value for tRNAPhe in aminoacylation by factor of three. Vmax for all substrates in both aminoacylation and leads to 40% increase of Hill's coefficient for ATP in ATP-[32P]PPi-exchange reaction but not in aminoacylation. The carboxylic groups modified by CMEC are assumed to take part in ATP recognition and in catalysis of the ATP conversion and in catalysis of transfer of activated amino acid residues on tRNA.
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PMID:[Role of the carboxylic groups in interaction of phenylalanyl-tRNA synthetase with substrates]. 703 45

The imino proton resonances of 15N labeled tRNA appear as asymmetric doublet signals, the asymmetry being dependent on the applied magnetic field strength. Assuming a tautomerism of the type N-H...N not equal to N...H-N in the base pairs the line shapes can be simulated. The most important parameters fitted in the simulation are the rate constants of the proton transfer and the mole fractions of either tautomeric state. The rate constants are of the order of 100s-1 and the mole fractions of the non dominant tautomer about 0.1 depending on the temperature and on the nature of the base pairing. The observations are attributed to a double proton transfer in the base pairs. The unexpectedly slow rates of the double proton transfer process may be connected with a concomitant conformational change of the duplex structure.
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PMID:Evidence for tautomerism in nucleic acid base pairs. 1H NMR study of 15N labeled tRNA. 717 56


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