全文获取类型
收费全文 | 2035篇 |
免费 | 140篇 |
国内免费 | 1篇 |
专业分类
2176篇 |
出版年
2022年 | 13篇 |
2021年 | 31篇 |
2020年 | 8篇 |
2019年 | 11篇 |
2018年 | 17篇 |
2017年 | 17篇 |
2016年 | 39篇 |
2015年 | 53篇 |
2014年 | 85篇 |
2013年 | 101篇 |
2012年 | 119篇 |
2011年 | 111篇 |
2010年 | 79篇 |
2009年 | 65篇 |
2008年 | 119篇 |
2007年 | 102篇 |
2006年 | 95篇 |
2005年 | 105篇 |
2004年 | 76篇 |
2003年 | 121篇 |
2002年 | 89篇 |
2001年 | 28篇 |
2000年 | 26篇 |
1999年 | 33篇 |
1998年 | 44篇 |
1997年 | 24篇 |
1996年 | 33篇 |
1995年 | 31篇 |
1994年 | 23篇 |
1993年 | 34篇 |
1992年 | 36篇 |
1991年 | 26篇 |
1990年 | 27篇 |
1989年 | 26篇 |
1988年 | 14篇 |
1987年 | 22篇 |
1986年 | 12篇 |
1985年 | 29篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 22篇 |
1979年 | 25篇 |
1978年 | 11篇 |
1977年 | 14篇 |
1976年 | 12篇 |
1975年 | 9篇 |
1974年 | 12篇 |
1973年 | 11篇 |
排序方式: 共有2176条查询结果,搜索用时 15 毫秒
101.
Guillerm G Guillerm D Vandenplas-Vitkowski C Glapski C De Clercq E 《Bioorganic & medicinal chemistry letters》2003,13(10):1649-1652
Synthesis of 5'-S-vinyl-5'-thioadenosine 5, 5'-S-ethynyl-5'-thioadenosine 7 and 5'-S-cyano-5'-thioadenosine 9 is described. Incubation of AdoHcy hydrolase with 5, 7 and 9 resulted in time- and concentration-dependent inactivation of the enzyme and partial depletion of its NAD(+) content. From these results and characterisation of metabolites released during the inactivation process, hypothetical mechanisms are suggested. The antiviral activity of 5, 7 and 9 was examined. Significant activities were noted with 5 against Vaccinia, Junin and Taccaribe viruses. 相似文献
102.
HLM1, an essential signaling component in the hypersensitive response,is a member of the cyclic nucleotide-gated channel ion channel family 总被引:3,自引:0,他引:3 下载免费PDF全文
Balagué C Lin B Alcon C Flottes G Malmström S Köhler C Neuhaus G Pelletier G Gaymard F Roby D 《The Plant cell》2003,15(2):365-379
The hypersensitive response (HR) in plants is a programmed cell death that is commonly associated with disease resistance. A novel mutation in Arabidopsis, hlm1, which causes aberrant regulation of cell death, manifested by a lesion-mimic phenotype and an altered HR, segregated as a single recessive allele. Broad-spectrum defense mechanisms remained functional or were constitutive in the mutant plants, which also exhibited increased resistance to a virulent strain of Pseudomonas syringae pv tomato. In response to avirulent strains of the same pathogen, the hlm1 mutant showed differential abilities to restrict bacterial growth, depending on the avirulence gene expressed by the pathogen. The HLM1 gene encodes a cyclic nucleotide-gated channel, CNGC4. Preliminary study of the HLM1/CNGC4 gene pro-duct in Xenopus oocytes (inside-out patch-clamp technique) showed that CNGC4 is permeable to both K(+) and Na(+) and is activated by both cGMP and cAMP. HLM1 gene expression is induced in response to pathogen infection and some pathogen-related signals. Thus, HLM1 might constitute a common downstream component of the signaling pathways leading to HR/resistance. 相似文献
103.
Two multiple mutants of a psychrophilic alpha-amylase were produced, bearing five mutations (each introducing additional weak interactions found in pig pancreatic alpha-amylase) with or without an extra disulfide bond specific to warm-blooded animals. Both multiple mutants display large modifications of stability and activity arising from synergic effects in comparison with single mutations. Newly introduced weak interactions and the disulfide bond confer mesophilic-like stability parameters, as shown by increases in the melting point t(m), in the calorimetric enthalpy DeltaH(cal) and in protection against heat inactivation, as well as by decreases in cooperativity and reversibility of unfolding. In addition, both kinetic and thermodynamic activation parameters of the catalyzed reaction are shifted close to the values of the porcine enzyme. This study confirms the central role of weak interactions in regulating the balance between stability and activity of an enzyme in order to adapt to the environmental temperature. 相似文献
104.
Van Petegem F Collins T Meuwis MA Gerday C Feller G Van Beeumen J 《The Journal of biological chemistry》2003,278(9):7531-7539
Enzymes from psychrophilic organisms differ from their mesophilic counterparts in having a lower thermostability and a higher specific activity at low and moderate temperatures. The current consensus is that they have an increased flexibility, enhancing accommodation and transformation of the substrates at low energy costs. Here we describe the structure of the xylanase from the Antarctic bacterium Pseudoalteromonas haloplanktis at 1.3 A resolution. Xylanases are usually grouped into glycosyl hydrolase families 10 and 11, but this enzyme belongs to family 8. The fold differs from that of other known xylanases and can be described as an (alpha/alpha)(6) barrel. Various parameters that may explain the cold-adapted properties were examined and indicated that the protein has a reduced number of salt bridges and an increased exposure of hydrophobic residues. The crystal structures of a complex with xylobiose and of mutant D144N were obtained at 1.2 and 1.5 A resolution, respectively. Analysis of the various substrate binding sites shows that the +3 and -3 subsites are rearranged as compared to those of a family 8 homolog, while the xylobiose complex suggests the existence of a +4 subsite. A decreased acidity of the substrate binding cleft and an increased flexibility of aromatic residues lining the subsites may enhance the rate at which substrate is bound. 相似文献
105.
Weber M Hagège H Lutfalla G Dandolo L Brunel C Cathala G Forné T 《Analytical biochemistry》2003,320(2):252-258
Allele-specific epigenetic modifications are crucial for several important biological functions, including genomic imprinting and X-inactivation in mammals. Consequently, an ever increasing number of investigations requires accurate quantification of the relative abundance of parental alleles of a specific sequence in a DNA sample. Here, combining the use of polymorphic restriction sites with real-time polymerase chain reaction (PCR) amplification, we describe a simple and quantitative assay to measure allele ratios. The efficiency of the assay was assessed on genomic DNA for several polymorphic restriction sites located in the mouse Igf2/H19 imprinted locus. The assay was also successfully applied to quantify allele ratio in cDNA samples. In addition, we provide an experimental procedure for detection and correction of potential PCR amplification bias which significantly extends the range of application of the assay. 相似文献
106.
A mechanism for P-glycoprotein-mediated apoptosis as revealed by verapamil hypersensitivity 总被引:6,自引:0,他引:6
Selection of tumor cell lines with anticancer drugs has led to the appearance of multidrug-resistant (MDR) subclones with P-glycoprotein 1 (P-gp1) expression. These cells are cross-resistant to several structurally and functionally dissimilar drugs. Interestingly, in the process of gaining resistance, MDR cells become hypersensitive or collaterally sensitive to membrane-active agents, such as calcium channel blockers, steroids, and local anaesthetics. In this report, hypersensitivity to the calcium channel blocker, verapamil, was analyzed in sensitive and resistant CHO cell lines. Our results show that treatment with verapamil preferentially induced apoptosis in MDR cells compared to drug-sensitive cells. This effect was independent of p53 activity and could be inhibited by overexpression of the Bcl-2 gene. The induction of apoptosis by verapamil had a biphasic trend in which maximum cell death occurred at 10 microM, followed by improved cell survival at higher concentrations (50 microM). We correlated this effect to a similar biphasic trend in P-gp1 ATPase activation by verapamil in which low concentrations of verapamil (10 microM) activated ATPase, followed by inhibition at higher concentrations. To confirm the relationship between apoptosis and ATPase activity, we used two inhibitors of P-gp1 ATPase, PSC 833 and ivermectin. These ATPase inhibitors reduced hypersensitivity to verapamil in MDR cells. In addition, low concentrations of verapamil resulted in the production of reactive oxygen species (ROS) in MDR cells. Taken together, these results show that apoptosis was preferentially induced by P-gp1 expressing cells exposed to verapamil, an effect that was mediated by ROS, produced in response the high ATP demand by P-gp1. 相似文献
107.
The unc-52 gene of Claenorhabditis elegans encodes a homologue of the basement membrane heparan sulfate proteoglycan perlecan. Viable alleles reduce the abundance of UNC-52 in late larval stages and increase the frequency of distal tip cell (DTC) migration defects caused by mutations disrupting the UNC-6/netrin guidance system. These unc-52 alleles do not cause circumferential DTC migration defects in an otherwise wild-type genetic background. The effects of unc-52 mutations on DTC migrations are distinct from effects on myofilament organization and can be partially suppressed by mutations in several genes encoding growth factor-like molecules, including EGL-17/FGF, UNC-129/TGF-beta, DBL-1/TGF-beta, and EGL-20/WNT. We propose that UNC-52 serves dual roles in C. elegans larval development in the maintenance of muscle structure and the regulation of growth factor-like signaling pathways. 相似文献
108.
Nezer C Collette C Moreau L Brouwers B Kim JJ Giuffra E Buys N Andersson L Georges M 《Genetics》2003,165(1):277-285
We herein describe the fine mapping of an imprinted QTL with major effect on muscle mass that was previously assigned to distal SSC2p in the pig. The proposed approach exploits linkage disequilibrium in combination with QTL genotyping by marker-assisted segregation analysis. By identifying a haplotype shared by all "Q" chromosomes, we map the QTL to an approximately 250-kb chromosome segment containing INS and IGF2 as the only known paternally expressed genes. This considerably reinforces the candidacy of these genes, justifying their detailed analysis. 相似文献
109.
Moritella cold-active dihydrofolate reductase: are there natural limits to optimization of catalytic efficiency at low temperature? 下载免费PDF全文
Adapting metabolic enzymes of microorganisms to low temperature environments may require a difficult compromise between velocity and affinity. We have investigated catalytic efficiency in a key metabolic enzyme (dihydrofolate reductase) of Moritella profunda sp. nov., a strictly psychrophilic bacterium with a maximal growth rate at 2 degrees C or less. The enzyme is monomeric (Mr=18,291), 55% identical to its Escherichia coli counterpart, and displays Tm and denaturation enthalpy changes much lower than E. coli and Thermotoga maritima homologues. Its stability curve indicates a maximum stability above the temperature range of the organism, and predicts cold denaturation below 0 degrees C. At mesophilic temperatures the apparent Km value for dihydrofolate is 50- to 80-fold higher than for E. coli, Lactobacillus casei, and T. maritima dihydrofolate reductases, whereas the apparent Km value for NADPH, though higher, remains in the same order of magnitude. At 5 degrees C these values are not significantly modified. The enzyme is also much less sensitive than its E. coli counterpart to the inhibitors methotrexate and trimethoprim. The catalytic efficiency (kcat/Km) with respect to dihydrofolate is thus much lower than in the other three bacteria. The higher affinity for NADPH could have been maintained by selection since NADPH assists the release of the product tetrahydrofolate. Dihydrofolate reductase adaptation to low temperature thus appears to have entailed a pronounced trade-off between affinity and catalytic velocity. The kinetic features of this psychrophilic protein suggest that enzyme adaptation to low temperature may be constrained by natural limits to optimization of catalytic efficiency. 相似文献
110.
Athmer L Kindrachuk J Georges F Napper S 《The Journal of biological chemistry》2002,277(34):30502-30507
Proteins are vulnerable to spontaneous, covalent modifications that may result in alterations to structure and function. Asparagines are particularly labile, able to undergo deamidation through the formation of a succinimide intermediate to produce either aspartate or isoaspartate residues. Although aspartates cannot undergo deamidation they can form a succinimide and result in the same products. Isoaspartyls are the principal product of succinimide hydrolysis, accounting for 65-85% of the emerging residues. The variability in the ratio of products emerging from succinimide hydrolysis suggests the ability of protein structure to influence succinimide outcome. In the H15D histidine-containing protein (HPr), phosphorylation of the active site aspartate catalyzes the formation of a cyclic intermediate. Resolution of this species is exclusively to aspartate residues, suggestive of either a succinimide with restrained hydrolysis, or an isoimide, from which aspartyl residues are the only possible product. Deletion of the C-terminal residue of this protein does not influence the ability for phosphorylation or ring formation, but it does allow for isoaspartyl formation, verifying a succinimide as the cyclic intermediate in H15D HPr. Isoaspartyl formation in H15D Delta85 is rationalized to occur as a consequence of elimination of steric restrictions imposed by the C terminus on the main-chain carbonyl of the succinimide, the required point of nucleophilic attack of a water molecule for isoaspartyl formation. This is the first reported demonstration of the influence of protein structure on the products emerging from succinimide hydrolysis. 相似文献