全文获取类型
收费全文 | 2143篇 |
免费 | 225篇 |
国内免费 | 9篇 |
出版年
2022年 | 28篇 |
2021年 | 33篇 |
2020年 | 25篇 |
2019年 | 48篇 |
2018年 | 43篇 |
2017年 | 55篇 |
2016年 | 59篇 |
2015年 | 87篇 |
2014年 | 92篇 |
2013年 | 125篇 |
2012年 | 118篇 |
2011年 | 139篇 |
2010年 | 103篇 |
2009年 | 85篇 |
2008年 | 98篇 |
2007年 | 108篇 |
2006年 | 106篇 |
2005年 | 97篇 |
2004年 | 86篇 |
2003年 | 66篇 |
2002年 | 75篇 |
2001年 | 63篇 |
2000年 | 65篇 |
1999年 | 51篇 |
1998年 | 24篇 |
1997年 | 18篇 |
1996年 | 15篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1992年 | 46篇 |
1991年 | 30篇 |
1990年 | 27篇 |
1989年 | 34篇 |
1988年 | 25篇 |
1987年 | 20篇 |
1986年 | 17篇 |
1985年 | 26篇 |
1984年 | 16篇 |
1983年 | 18篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1979年 | 8篇 |
1978年 | 15篇 |
1975年 | 10篇 |
1974年 | 12篇 |
1973年 | 17篇 |
1972年 | 11篇 |
1971年 | 7篇 |
1968年 | 8篇 |
排序方式: 共有2377条查询结果,搜索用时 15 毫秒
171.
172.
173.
174.
The genetic construction of cancer-prone mice, combined with the capacity to control transgene expression in vivo, provides new opportunities to study the role of oncogenes in the maintenance of fully formed tumors. These inducible cancer models provide a means to dissect how specific oncogenic signals influence host-tumor symbiosis, to validate the importance of a given oncogenic lesion in established advanced tumors, and to predict the biological response and adaptations to therapies targeted to that cancer-causing genetic alteration. 相似文献
175.
Kuo CC Tsai LC Chin TY Chang GG Chou WY 《Biochemical and biophysical research communications》2000,270(3):821-825
Alanine-scanning site-directed mutagenesis was carried out on all conserved lysine residues of pigeon cytosolic NADP(+)-dependent malic enzyme. Only two mutant enzymes, K162A and K340A, showed significant effect on their kinetic parameters. Both mutant enzymes have K(m) values for Mn(2+) and l-malate similar to those of wild-type. The K(m) value for NADP(+) of K162A is identical to that of wild-type. However, K162A demonstrated a 235-fold decrease in the k(cat) value (0.17 +/- 0.01 vs 40.0 +/- 1.3 s(-1)). These data suggested that the side chain of K162 is important for the enzyme catalytic reaction. We propose that the epsilon-amino group of K162 may serve as a general acid to protonate the 3-carbon of enolpyruvate after decarboxylation. The K340A mutant demonstrated no effect on the k(cat) value. However, its K(m) value for NADP(+) was increased by a factor of 65 (225.7 +/- 5.07 vs 3.49 +/- 0.05 microM). We propose that the NADP(+) specificity is determined by the electrostatic interaction between the epsilon-amino group of K340 and 2'-phosphate of NADP(+). 相似文献
176.
Sunna A Gibbs MD Chin CW Nelson PJ Bergquist PL 《Applied and environmental microbiology》2000,66(2):664-670
Genomic walking PCR was used to obtained a 4,567-bp nucleotide sequence from Caldibacillus cellulovorans. Analysis of this sequence revealed that there were three open reading frames, designated ORF1, ORF2, and ORF3. Incomplete ORF1 encoded a putative C-terminal cellulose-binding domain (CBD) homologous to members of CBD family IIIb, while putative ORF3 encoded a protein of unknown function. The putative ManA protein encoded by complete manA ORF2 was an enzyme with a novel multidomain structure and was composed of four domains in the following order: a putative N-terminal domain (D1) of unknown function, an internal CBD (D2), a beta-mannanase catalytic domain (D3), and a C-terminal CBD (D4). All four domains were linked via proline-threonine-rich peptides. Both of the CBDs exhibited sequence similarity to family IIIb CBDs, while the mannanase catalytic domain exhibited homology to the family 5 glycosyl hydrolases. The purified recombinant enzyme ManAd3 expressed from the cloned catalytic domain (D3) exhibited optimum activity at 85 degrees C and pH 6.0 and was extremely thermostable at 70 degrees C. This enzyme exhibited high specificity with the substituted galactomannan locust bean gum, while more substituted galacto- and glucomannans were poorly hydrolyzed. Preliminary studies to determine the effect of the recombinant ManAd3 and a recombinant thermostable beta-xylanase on oxygen-delignified Pinus radiata kraft pulp revealed that there was an increase in the brightness of the bleached pulp. 相似文献
177.
Neocarzinostatin is a potent enediyne antitumor antibiotic complex in which a chromophore is noncovalently bound to a carrier protein. The protein regulates availability of the drug by proper release of the biologically active chromophore. To understand the physiological mechanism of the drug delivery system, we have examined the trifluoroethanol (TFE)-induced conformational changes of the protein with special emphasis on their relation to the release of the chromophore from holoneocarzinostatin. The effect of the alpha helix-inducing agent, TFE, on all the beta-sheet neocarzinostatin proteins was studied by circular dichroism, fluorescence, and (1)H NMR studies. By using binding of anilinonaphthalene sulfonic acid as a probe, we observed that the protein exists in a stable, partially structured intermediate state around 45-50% TFE, which is consistent with the results from tryptophan fluorescence and circular dichroism studies. The native state is stable until 20% TFE and is half-converted into the intermediate state at 30% TFE, which starts to collapse beyond 50%. High pressure liquid chromatographic analysis of the release of the chromophore caused by TFE treatment at 0 degrees C suggests that the release process, which occurs below 20% TFE, does not result from an observable conformational change in the protein. Kinetic measurements of the release of chromophore at 25 degrees C reveal that TFE does stimulate the rate of release, which increases sharply at 15% and reaches a maximum at 20% TFE, although no major secondary or tertiary structural change of the carrier protein is observed under these same conditions. Our data suggest that chromophore release results from a fluctuation of the protein structure that is stimulated by TFE. Complete release of the chromophore occurs at TFE concentrations where no overall observable unfolding of the apoprotein is seen. Thus, the results suggest that denaturation of the protein by TFE is not a necessary step for release of the tightly bound chromophore. 相似文献
178.
Chin LS Nugent RD Raynor MC Vavalle JP Li L 《The Journal of biological chemistry》2000,275(2):1191-1200
Synaptosome-associated protein of 25 kDa (SNAP-25) is a presynaptic membrane protein that has been clearly implicated in membrane fusion in both developing and mature neurons, although its mechanisms of action are unclear. We have now identified a novel SNAP-25-interacting protein named SNIP. SNIP is a hydrophilic, 145-kDa protein that comprises two predicted coiled-coil domains, two highly charged regions, and two proline-rich domains with multiple PPXY and PXXP motifs. SNIP is selectively expressed in brain where it co-distributes with SNAP-25 in most brain regions. Biochemical studies have revealed that SNIP is tightly associated with the brain cytoskeleton. Subcellular fractionation and immunofluorescence localization studies have demonstrated that SNIP co-localizes with SNAP-25 as well as the cortical actin cytoskeleton, suggesting that SNIP serves as a linker protein connecting SNAP-25 to the submembranous cytoskeleton. By using deletion analysis, we have mapped the binding domains of SNIP and SNAP-25, and we have demonstrated that the SNIP-SNAP-25 association is mediated via coiled-coil interactions. Moreover, we have shown that overexpression of SNIP or its SNAP-25-interacting domain inhibits Ca(2+)-dependent exocytosis from PC12 cells. These results indicate that SNIP is involved in regulation of neurosecretion, perhaps via its interaction with SNAP-25 and the cytoskeleton. 相似文献
179.
Transforming growth factor-beta 1 inhibition of macrophage activation is mediated via Smad3 总被引:5,自引:0,他引:5
Werner F Jain MK Feinberg MW Sibinga NE Pellacani A Wiesel P Chin MT Topper JN Perrella MA Lee ME 《The Journal of biological chemistry》2000,275(47):36653-36658
Activated macrophages are critical cellular participants in inflammatory disease states. Transforming growth factor (TGF)-beta1 is a growth factor with pleiotropic effects including inhibition of immune cell activation. Although the pathway of gene activation by TGF-beta1 via Smad proteins has recently been elucidated, suppression of gene expression by TGF-beta1 remains poorly understood. We found that of Smad1-Smad7, Smad3 alone was able to inhibit expression of markers of macrophage activation (inducible nitric-oxide synthase and matrix metalloproteinase-12) following lipopolysaccharide treatment in gene reporter assays. Transient and constitutive overexpression of a dominant negative Smad3 opposed the inhibitory effect of TGF-beta1. Domain swapping experiments suggest that both the Smad MH-1 and MH-2 domains are required for inhibition. Mutation of a critical amino acid residue required for DNA binding in the MH-1 of Smad3 (R74A) resulted in the loss of inhibition. Transient overexpression of p300, an interactor of the Smad MH-2 domain, partially alleviated the inhibition by TGF-beta1/Smad3, suggesting that inhibition of gene expression may be due to increased competition for limiting amounts of this coactivator. Our results have implications for the understanding of gene suppression by TGF-beta1 and for the regulation of activated macrophages by TGF-beta1. 相似文献
180.