共查询到20条相似文献,搜索用时 31 毫秒
1.
M. V. Tarasova V. V. Kuznetsov N. A. Netesova D. A. Gonchar S. Kh. Degtyarev 《Moscow University Biological Sciences Bulletin》2011,66(2):76-78
DNA methyltransferases genes of the BspACI restriction-modification system from Bacillus psychrodurans AC have been cloned in E. coli cells. Analysis of amino acid sequences of the proteins showed that both of these genes belong to C5 DNA methyltransferases.
Gene M1.BspACI has been subcloned in pJW2 vector. A high-purity recombinant enzyme has been obtained using chromatography on different carriers.
It has been shown that M1.BspACI modifies the first cytosine residue in the sequence 5′-CCGC-3′. Kinetic parameters of DNA
methylation by the enzyme have been determined. Catalytic constant appears to be 0.095 ± 0.002 min−1. K
mphage is λ DNA—0.053 ± 0.007 μM, and K
mSAM is 5.1 ± 0.3 μM. 相似文献
2.
Tarasova MV Kuznetsov VV Netesova NA Gonchar DA Degtyarev SKh 《Biochemistry. Biokhimii?a》2010,75(12):1484-1490
A restriction-modification system from Bacillus psychrodurans AC (recognition sequence 5′-CCGC-3′) comprises two DNA methyltransferases: M1.BspACI and M2.BspACI. The bspACIM1 gene was cloned in the pJW2 vector and expressed in Escherichia coli cells. High-purity M1.BspACI preparation has been obtained by chromatography on different carriers. M1.BspACI has a temperature
optimum of 30°C and demonstrates maximum activity at pH 8.0. M1.BspACI modifies the first cytosine in the recognition sequence
5′-CCGC-3′. The kinetic parameters of M1.BspACI DNA methylation are as follows: K
m for phage λ DNA is 0.053 μM and K
m for S-adenosyl-L-methionine is 5.1 μM. The catalytic constant (k
cat) is 0.095 min−1. 相似文献
3.
V. A. Chernukhin V. V. Kuznetsov D. A. Gonchar Yu. G. Kashirina N. A. Netesova S. Kh. Degtyarev 《Biochemistry. Biokhimii?a》2010,75(1):63-71
Optimal conditions for DNA methylation by the M3.BstF5I enzyme from Bacillus stearothermophilus and kinetic parameters of λ phage DNA modification and that of a number of oligonucleotide substrates are established. Comparison
of M1.BstF5I and M3.BstF5I kinetic parameters revealed that with similar temperature optima and affinity for DNA, M3.BstF5I
has nearly fourfold lower turnover number (0.24 min−1) and modifies the hemimethylated recognition site with lower efficiency under optimal conditions than the unmethylated one.
In contrast to another three methylases of the BstF5I restriction-modification system, the M3.BstF5I enzyme is able to optionally
modify the noncanonical 5′-GGATC-3′ DNA sequence with a rate more than one order of magnitude lower than the methylation rate
of the canonical 5′-GGATG-3′ recognition site. 相似文献
4.
5.
Bacteroids formed by Mesorhizobium ciceri CC 1192 in symbiosis with chickpea plants (Cicer arietinum L.) contained a single form of citrate synthase [citrate oxaloacetate-lyase (CoA-acetylating) enzyme; EC 4.1.3.7], which
had the same electrophoretic mobility as the enzyme from the free-living cells. The citrate synthase from CC 1192 bacteroids
had a native molecular mass of 228 ± 32 kDa and was activated by KCl, which also enhanced stability. Double reciprocal plots
of initial velocity against acetyl-CoA concentration were linear, whereas the corresponding plots with oxaloacetate were nonlinear.
The K
m value for acetyl-CoA was 174 μM in the absence of added KCl, and 88 μM when the concentration of KCl in reaction mixtures
was 100 mM. The concentrations of oxaloacetate for 50% of maximal activity were 27 μM without added KCl and 14 μM in the presence
of 100 mM KCl. Activity of citrate synthase was inhibited 50% by 80 μM NADH and more than 90% by 200 μM NADH. Inhibition by
NADH was linear competitive with respect to acetyl-CoA (K
is = 23.1 ± 3 μM) and linear noncompetitive with respect to oxaloacetate (K
is = 56 ± 3.8 μM and K
ii = 115 ± 15.4 μM). NADH inhibition was relieved by NAD+ and by micromolar concentrations of 5′-AMP. In the presence of 50 or 100 mM KCl, inhibition by NADH was apparent only when
the proportion of NADH in the nicotinamide adenine dinucleotide pool was greater than 0.6. In the microaerobic environment
of bacteroids, NADH may be at concentrations that are inhibitory for citrate synthase. However, this inhibition is likely
to be relieved by NAD+ and 5′-AMP, allowing carbon to enter the tricarboxylic acid cycle.
Received: 14 July 1999 / Accepted: 20 September 1999 相似文献
6.
Parashar V Capalash N Xu SY Sako Y Sharma P 《Applied microbiology and biotechnology》2006,72(5):917-923
TspMI, a thermostable isoschizomer of XmaI from a Thermus sp., has been characterized. The enzyme was purified to homogeneity using Cibacron-Blue 3GA agarose, Heparin agarose, SP sephadex C50, and Mono-Q fast protein liquid chromatography and was found to be a homodimer of 40 kDa. Restriction mapping and run-off sequencing of TspMI-cleaved DNA ends depicted that it cleaved at 5′C/CCGGG3′ to generate a four-base, 5′-CCGG overhang. The enzyme was sensitive to methylation of second and third cytosines in its recognition sequence. TspMI worked optimally at 60°C with 6 mM Mg2+, no Na+/K+, and showed no star activity in the presence of 25% glycerol. The enzyme could efficiently digest the DNA labeled with a higher concentration of YOYO-I (one dye molecule to one nucleotide), making it a useful candidate for real-time imaging experiments. Single molecule interaction between TspMI and λ DNA was studied using total internal reflection fluorescence microscopy. The enzyme survived 30 polymerase chain reaction (PCR) cycles in the presence of 10% glycerol and 0.5 M trehalose without any activity loss and, hence, is suitable for incorporation in restriction-endonuclease-mediated selective-PCR for various applications.Electronic Supplementary Material Supplementary material for this article is available at 相似文献
7.
Masłyk M Kochanowicz E Zieliński R Kubiński K Hellman U Szyszka R 《Molecular and cellular biochemistry》2008,312(1-2):61-69
Since Svf1 is phosphoprotein, we investigated whether it was a substrate for protein kinase CK2. According to the amino acid
sequence Svf1 harbours 20 putative CK2 phosphorylation sites. Here, we have reported cloning, overexpression, purification
and characterization of yeast Svf1 as a substrate for three forms of yeast CK2. Svf1 serves as a substrate for both the recombinant
CK2α (K
m 0.35 μM) and CK2α′ (K
m 0.18 μM) as well as CK2 holoenzyme (K
m 1.1 μM). Different K
m values argue that CK2β(β′) subunit has an inhibitory effect on the activity of both CK2α and CK2α′ towards surviving factor
Svf1. Reconstitution of α′2ββ′ isoform of CK2 holoenzyme shows that β/β′ subunits have regulatory effect depending on the kind of CK2 catalytic subunit.
This effect was not observed in the case of α2ββ′ isoform, which may be due to interaction between Svf1 and regulatory CK2β subunit (shown by co-immunoprecipitation experiments).
Interactions between CK2 subunits and Svf1 protein may have influence on ATP as well as ATP-competitive inhibitors (TBBt and
TBBz) binding. CK2 phosphorylates up to six serine residues in highly acidic peptide K199EVIPESDEEESSADEDDNEDEDEESGDSEEESGSEEESDSEEVEITYED248 of the Svf1 protein in vitro. Presented data may help to elucidate the role of protein kinase CK2 and Svf1 in the regulation
of cell survival pathways. 相似文献
8.
《Molekuliarnaia biologiia》2004,38(6):997-1004
Genes encoding DNA-methyltransferases which recognize the same sequence 5'-GCATC-3' from SfaNI and Bst19I restriction-modification systems have been cloned and primary structures of these have been determined. It has been revealed that restriction-modification system Bst19I contains two DNA-methyltransferases M1.Bst19I and M2.Bst19I, whereas RM system SfaNI include only one DNA-methyltransferase M.SfaNI, N- and C-domain of which are homologous of M2.Bst19I and M1.Bst19I, respectively. M1.Bst19I and M2.Bst19I as well as both domains of M.SfaNI contain conservative elements in an order that is typical for N6-adenine DNA-methyltransferases alpha class. SfaNI and Bst19I DNA-methyltransferases share high homology level with methylases of FokI and BstF5I RM systems. Probably this reflects presence of the common DNA sequence 5'-GATG-3' in the recognition sites of all these RM systems. Basing on primary structures homology of methylases, highly conserved amino acid residues on known spatial model of DNA-methyltransferase M.DpnIIA have been determined. 相似文献
9.
Rücker B Almeida ME Libermann TA Zerbini LF Wink MR Sarkis JJ 《Molecular and cellular biochemistry》2007,306(1-2):247-254
In the present study we investigate the biochemical properties of the members of NPP family in synaptosomes prepared from
rat heart left ventricles. Using p-nitrophenyl-5′-thymidine monophosphate (p-Nph-5′-TMP) as substrate for E-NPPs in rat cardiac synaptosomes, we observed an alkaline pH dependence, divalent cation dependence
and the K
M
value corresponded to 91.42 ± 13.97 μM and the maximal velocity (V
max
) value calculated was 63.79 ± 3.59 nmol p-nitrophenol released/min/mg of protein (mean ± SD, n = 4). Levamisole (1 mM), was ineffective as inhibitor of p-Nph-5′-TMP hydrolysis in pH 8.9 (optimum pH for the enzyme characterized). Suramin (0.25 mM) strongly reduced the hydrolysis
of p-Nph-5′-TMP by about 46%. Sodium azide (10 and 20 mM) and gadolinium chloride (0.3 and 0.5 mM), E-NTPases inhibitors, had
no effects on p-Nph-5′-TMP hydrolysis. RT-PCR analysis of left ventricle demonstrated the expression of NPP2 and NPP3 enzymes, but excluded
the presence of NPP1 member. By quantitative real-time PCR we identified the NPP3 as the enzyme with the highest expression
in rat left ventricle. The demonstration of the presence of the E-NPP family in cardiac system, suggest that these enzymes
could contribute with the fine-tuning control of the nucleotide levels at the nerve terminal endings of left ventricles that
are involved in several cardiac pathologies. 相似文献
10.
Siu Wah Wong-Deyrup Youngbae Kim Sonya J. Franklin 《Journal of biological inorganic chemistry》2006,11(1):17-25
The DNA-binding behavior and target sequences of two designed metallopeptides have been investigated with an iterative electrophoresis
mobility shift assay followed by PCR amplification, and by circular dichroism spectroscopy. Peptides P3W and P5b were designed
based on the structural similarity of the helix–turn–helix motif of homeodomains and the EF-hand motifs of calmodulin, as
previously described for P3W. Like P3W, P5b binds both Eu(III) (K
d=12.6±1.9 μM) and Ca(II) (K
d=70±8 μM) with reasonable affinity. Binding selection from a library of randomized 8-mer DNA oligonucleotide sequences identified
one target family for CaP5b [5′-pur-T-pur-G-(G/C)-3′], and two target sites for CaP3W [5′-(A/T)-G-G-G-(T/C)-3′ and 5′-A-T-(G/T)-T-G-3′].
Circular dichroism studies indicate that unlike EuP3W, EuP5b is poorly folded in the absence of DNA. In the presence of DNA
containing target-binding sites for both peptides, both EuP3W and EuP5b increase in helical content, in the latter case significantly.
These results suggest that EuP5b binding to target DNA involves an induced-fit mechanism. These small chimeric metallopeptides
have been found to bind selectively to DNA targets, analogous to natural protein–DNA interactions. This corroborates our earlier
conclusions (J. Am. Chem. Soc. 125:6656, 2003) that sequence-preferential DNA cleavage by Ce(IV)P3W was due to sequence recognition.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
11.
Cloning and characterization of a new laccase from Bacillus licheniformis catalyzing dimerization of phenolic acids 总被引:1,自引:0,他引:1
Koschorreck K Richter SM Ene AB Roduner E Schmid RD Urlacher VB 《Applied microbiology and biotechnology》2008,79(2):217-224
A new laccase gene (cotA) was cloned from Bacillus licheniformis and expressed in Escherichia coli. The recombinant protein CotA was purified and showed spectroscopic properties, typical for blue multi-copper oxidases. The
enzyme has a molecular weight of ~65 kDa and demonstrates activity towards canonical laccase substrates 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic
acid) (ABTS), syringaldazine (SGZ) and 2,6-dimethoxyphenol (2,6-DMP). Kinetic constants K
M and k
cat for ABTS were of 6.5 ± 0.2 μM and 83 s−1, for SGZ of 4.3 ± 0.2 μM and 100 s−1, and for 2,6-DMP of 56.7 ± 1.0 μM and 28 s−1. Highest oxidizing activity towards ABTS was obtained at 85°C. However, after 1 h incubation of CotA at 70°C and 80°C, a
residual activity of 43% and 8%, respectively, was measured. Furthermore, oxidation of several phenolic acids and one non-phenolic
acid by CotA was investigated. CotA failed to oxidize coumaric acid, cinnamic acid, and vanillic acid, while syringic acid
was oxidized to 2,6-dimethoxy-1,4-benzoquinone. Additionally, dimerization of sinapic acid, caffeic acid, and ferulic acid
by CotA was observed, and highest activity of CotA was found towards sinapic acid. 相似文献
12.
Aftab Alam 《Journal of biosciences》1986,10(1):37-47
Dihydrofolate reductase has been purified from a methotrexate-resistant human lymphoid cell line (CCRF/CEM-R3) and up to 1
mg of enzyme has been obtained from 5 litres of culture. The enzyme has a molecular weight of 22000 ±500 as determined by
gel filtration. The pH activity profile shows a single optimum at pH 7.7, where marked activation is observed by addition
of 0.2 M NaCl. TheK
m for NADPH is 3μM and dihydrofolate 0.7μM. The binding constant for the inhibitor, methotrexate, is 29 pM 相似文献
13.
The gene encoding carboxylesterase from the hyperthermophilic bacterium Thermotoga maritima (tm0053) was cloned. The recombinant protein (EST53) was overexpressed in Escherichia coli without its NH2-terminal hydrophobic region, and with a C-terminal hexahistidine sequence. The enzyme was purified to homogeneity by heat
treatment, followed by Ni2+ affinity chromatography, and then characterized. Among the p-nitrophenyl esters tested, the best substrate was p-nitrophenyl decanoate with K
m and k
cat values of 3.1 μM and 10.8 s−1, respectively, at 60°C and pH 7.5. The addition of O,O′-bis(2-aminoethyl)ethyleneglycol-N,N,N′,N′-tetraacetic acid decreased the esterase activity, indicating that EST53 is dependent on the presence of Ca2+ ion. In addition, its activity was inhibited by the addition of phenylmethylsulfonyl fluoride and diethyl pyrocarbonate,
indicating that it contains serine and histidine residues, which play key roles in the catalytic mechanism. EST53 shows a
relatively high degree of similarity to Burkholderia lipases that belong to family I.2 of the lipolytic enzymes, whereas the local sequence around the pentapeptide of EST53 is
most similar to those of Bacillus lipases belonging to family I.4. 相似文献
14.
Betul Can Gulnihal Kulaksiz Erkmen Ozlem Dalmizrak I. Hamdi Ogus Nazmi Ozer 《The protein journal》2010,29(4):250-256
Glutathione reductase [GR, E.C.1.8.1.7] catalyses NADPH dependent reduction of glutathione disulfide (GSSG) to reduced glutathione
(GSH). Thus, it is the crucial enzyme to maintain high [GSH]/[GSSG] ratio and physiological redox status in cells. Kidney
and liver tissues were considered as a rich source of GR. In this study, rat kidney GR was purified and some of its properties
were investigated. The enzyme was purified 2,356 fold with a yield of 16% by using heat-denaturation and Sephadex G25 gel
filtration, 2′,5′-ADP Agarose 4B, PBE94 column chromatographies. The purified enzyme had a specific activity (Vm) of 250 U/mg protein and the ratio of absorbances at wavelengths of A
273/A
463,
A
280/A
460, A
365/A
460, and A
379/A
463, were 7.1, 6.8, 1.2 and 1.0, respectively. Each mol of GR subunit bound 0.97 mol of FAD. NADH was used as a coenzyme by rat
kidney GR but with a lower efficiency (32.7%) than NADPH. Its subunit molecular weight was estimated as 53 kDa. An optimum
pH of 6.5 and optimum temperature of 65 °C were found for rat kidney GR. Its activation energy (Ea) and temperature coefficient
(Q10) were calculated as 7.02 kcal/mol and 1.42, respectively. The Km(NADPH) and kcat/Km (NADPH) values were found to be 15.3 ± 1.4 μM and 1.68 × 107 M−1 s−1 for the concentration range of 10-200 μM NADPH and when GSSG is the variable substrate, the Km(GSSG) and the kcat/Km(GSSG) values of 53.1 ± 3.4 μM and 4.85 × 106 M−1 s−1 were calculated for the concentration range of 20–1,200 μM GSSG. 相似文献
15.
Christopher A. Dieni Kenneth B. Storey 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2010,180(8):1133-1142
Glucose-6-phosphate dehydrogenase (G6PDH) and the pentose phosphate pathway play a key role in reductive biosynthesis and
antioxidant defense, while diverting glucose from other cellular functions. G6PDH was isolated from liver of the wood frog,
Rana sylvatica, a freeze tolerant species that uses glucose as a cryoprotectant. Analysis of kinetic parameters (K
m and V
max) of G6PDH showed a significant increase in K
m G6P (from 98.2 ± 3.8 to 121 ± 5.3 μM) and K
m NADP+ (from 65.5 ± 2.3 to 89.1 ± 4.8 μM) in frogs following freezing exposure, indicating lower affinity for G6PDH substrates in
this state. Subsequent analyses indicated that differential phosphorylation of G6PDH between the two states was responsible
for the altered kinetic properties. Thus, two differentially charged forms of G6PDH were resolved by DEAE ion-exchange chromatography
and, compared with controls, the proportion of G6PDH activity in peak I decreased and in peak II increased in liver from frozen
frogs. G6PDH in peak I had a K
m G6P of 94.1 ± 1.1 μM and K
m NADP+ of 61.2 ± 3.5 μM, whereas Peak II G6PDH showed higher values (K
m G6P was 172 ± 4.3 μM, K
m NADP+ was 98.2 ± 3.3 μM). G6PDH from each peak was incubated with ions and second messengers to stimulate the actions of protein
kinases with results indicating that G6PDH can be phosphorylated by protein kinase G, protein kinase C, AMP-activated protein
kinase, or calmodulin-dependent protein kinase. The data indicate that in control frogs, G6PDH is in a high phosphate form
and displays a high substrate affinity, whereas in frozen frogs G6PDH is less phosphorylated, with lower substrate affinity. 相似文献
16.
The xylose reductase (XR) gene (xyl1) from Candida shehatae was cloned and expressed in Escherichia coli, and purified as a His6-tagged fusion protein. The recombinant XR had Km values for NADH than NADPH of 150 μM and 20 μM, respectively. The optimal reaction was at pH 6.5 and 35°C. The enzyme was
specific for d-xylese. 相似文献
17.
K+-conductive pathways were evaluated in isolated surface and crypt colonic cells, by measuring 86Rb efflux. In crypt cells, basal K+ efflux (rate constant: 0.24 ± 0.044 min−1, span: 24 ± 1.3%) was inhibited by 30 mM TEA and 5 mM Ba2+ in an additive way, suggesting the existence of two different conductive pathways. Basal efflux was insensitive to apamin,
iberiotoxin, charybdotoxin and clotrimazole. Ionomycin (5 μM) stimulated K+ efflux, increasing the rate constant to 0.65 ± 0.007 min−1 and the span to 83 ± 3.2%. Ionomycin-induced K+ efflux was inhibited by clotrimazole (IC50 of 25 ± 0.4 μM) and charybdotoxin (IC50 of 65 ± 5.0 nM) and was insensitive to TEA, Ba2+, apamin and iberiotoxin, suggesting that this conductive pathway is related to the Ca2+-activated intermediate-conductance K+ channels (IKca). Absence of extracellular Ca2+ did neither affect basal nor ionomycin-induced K+ efflux. However, intracellular Ca2+ depletion totally inhibited the ionomycin-induced K+ efflux, indicating that the activation of these K+ channels mainly depends on intracellular calcium liberation. K+ efflux was stimulated by intracellular Ca2+ with an EC50 of 1.1 ± 0.04 μM. In surface cells, K+ efflux (rate constant: 0.17 ± 0.027 min−1; span: 25 ± 3.4%) was insensitive to TEA and Ba2+. However, ionomycin induced K+ efflux with characteristics identical to that observed in crypt cells. In conclusion, both surface and crypt cells present
IKCa channels but only crypt cells have TEA- and Ba2+-sensitive conductive pathways, which would determine their participation in colonic K+ secretion. 相似文献
18.
The malic enzyme-encoding cDNA (GQ372891) from the oleaginous yeast Lipomyces starkeyi AS 2.1560 was isolated, which has an 1719-bp open reading frame flanked by a 290-bp 5′ untranslated sequence and a 92-bp
3′ untranslated sequence. The proposed gene, LsME1, encoded a protein with 572 amino acid residues. The protein presented 58% sequence identity with the malic enzymes from
Yarrowia lipolytica CLIB122 and Aspergillus fumigatus Af293. The LsME1 gene was cloned into the vector pMAL-p4x to express a fusion protein (MBP-LsME1) in Escherichia coli TB1. The fusion protein was purified and then cleaved by Factor Xa to give the recombinant LsME1. This purified enzyme took
either NAD+ or NADP+ as the coenzyme but preferred NAD+. The K
m values for malic acid, NAD+ and NADP+ were 0.85 ± 0.05 mM, 0.34 ± 0.08 mM, and 7.4 ± 0.32 mM, respectively, at pH 7.3. 相似文献
19.
Kim SB Lee DW Cheigh CI Choe EA Lee SJ Hong YH Choi HJ Pyun YR 《Journal of industrial microbiology & biotechnology》2006,33(6):436-444
We have isolated a bacterium (TP-6) from the Indonesian fermented soybean, Tempeh, which produces a strong fibrinolytic protease and was identified as Bacillus subtilis. The protease (TPase) was purified to homogeneity by ammonium sulfate fractionation and octyl sepharose and SP sepharose chromatography. The N-terminal amino acid sequence of the 27.5 kDa enzyme was determined, and the encoding gene was cloned and sequenced. The result demonstrates that TPase is a serine protease of the subtilisin family consisting of 275 amino acid residues in its mature form. Its apparent K
m and V
max for the synthetic substrate N-succinyl-Ala-Ala-Pro-Phe-pNA were 259 μM and 145 μmol mg−1 min−1, respectively. The fibrinogen degradation pattern generated by TPase as a function of time was similar to that obtained with plasmin. In addition, N-terminal amino acid sequence analysis of the fibrinogen degradation products demonstrated that TPase cleaves Glu (or Asp) near hydrophobic acids as a P1 site in the α- and β-chains of fibrinogen to generate fragments D′, E′, and D′ similar to those generated by plasmin. On plasminogen-rich fibrin plates, TPase did not seem to activate fibrin clot lysis. Moreover, the enzyme converted the active plasminogen activator inhibitor-1 to the latent form.Seong-Bo Kim and Dong-Woo Lee contributed equally to the work. 相似文献
20.
Three fenamates—niflumic, flufenamic and mefenamic acids—were tested for effects on substrate-induced currents of glutamate
and glycine transporters (EAAT1, EAAT2, GLYT1b and GLYT2a) expressed in Xenopus laevis oocytes. All fenamates inhibited EAAT1 currents; 100 μM flufenamic acid produced the most inhibition, decreasing the I
max by 53 ± 4% (P < 0.001). EAAT2 currents were less sensitive, but 100 μM flufenamic acid inhibited the I
max by 34 ± 5% (P = 0.006). All fenamates inhibited GLYT1b currents; 100 μM flufenamic acid produced the most inhibition, decreasing the I
max by 61 ± 1% (P < 0.001). At 100 μM, effects on the GLYT2a I
max were mixed: 13 ± 2% inhibition by flufenamic acid (P = 0.002), 30 ± 6% enhancement by niflumic acid (P = 0.002), and no effect by mefenamic acid. Minor effects on substrate affinity suggested non-competitive mechanisms. These
data could contribute to the development of selective transport modulators. 相似文献