全文获取类型
收费全文 | 7410篇 |
免费 | 691篇 |
国内免费 | 6篇 |
专业分类
8107篇 |
出版年
2023年 | 52篇 |
2022年 | 116篇 |
2021年 | 232篇 |
2020年 | 154篇 |
2019年 | 184篇 |
2018年 | 184篇 |
2017年 | 158篇 |
2016年 | 210篇 |
2015年 | 359篇 |
2014年 | 384篇 |
2013年 | 471篇 |
2012年 | 552篇 |
2011年 | 548篇 |
2010年 | 323篇 |
2009年 | 289篇 |
2008年 | 381篇 |
2007年 | 394篇 |
2006年 | 329篇 |
2005年 | 339篇 |
2004年 | 317篇 |
2003年 | 281篇 |
2002年 | 273篇 |
2001年 | 68篇 |
2000年 | 45篇 |
1999年 | 66篇 |
1998年 | 83篇 |
1997年 | 41篇 |
1996年 | 33篇 |
1995年 | 36篇 |
1994年 | 31篇 |
1993年 | 42篇 |
1992年 | 44篇 |
1991年 | 31篇 |
1990年 | 31篇 |
1989年 | 40篇 |
1988年 | 29篇 |
1987年 | 37篇 |
1986年 | 33篇 |
1985年 | 28篇 |
1984年 | 36篇 |
1982年 | 43篇 |
1981年 | 44篇 |
1980年 | 42篇 |
1979年 | 23篇 |
1978年 | 39篇 |
1977年 | 29篇 |
1976年 | 23篇 |
1975年 | 34篇 |
1974年 | 30篇 |
1973年 | 29篇 |
排序方式: 共有8107条查询结果,搜索用时 15 毫秒
971.
We have shown by theoretical studies of alanine peptides that the CαDα stretch frequency could be particularly useful for determining peptide structure because of its sensitivity to the φ,ψ torsion angles at the Cα atom. To demonstrate that this is a robust methodology worthy of experimental exploration, we have also shown that this mode is even more determinative of conformation in aqueous solution, mainly as a result of the development of differential Cα? Dα···O(water) interactions. As further assurance, we now determine the influence of the side chain on this band, showing for aliphatic, a polar, and an aromatic side chains that the dependence is minor and explaining why this is also expected for other side chains. These results should stimulate new experimental methodologies in the field of peptide structure determination. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 1065–1071, 2010. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com 相似文献
972.
Beum Jun Kim David J. Schneider Samuel W. Cartinhour Michael L. Shuler 《Biotechnology and bioengineering》2010,105(5):955-964
The growth of a model plant pathogen, Pseudomonas syringae pv. tomato DC3000, was investigated using a chemostat culture system to examine environmentally regulated responses. Using minimal medium with iron as the limiting nutrient, four different types of responses were obtained in a customized continuous culture system: (1) stable steady state, (2) damped oscillation, (3) normal washout due to high dilution rates exceeding the maximum growth rate, and (4) washout at low dilution rates due to negative growth rates. The type of response was determined by a combination of initial cell mass and dilution rate. Stable steady states were obtained with dilution rates ranging from 0.059 to 0.086 h?1 with an initial cell mass of less than 0.6 OD600. Damped oscillations and negative growth rates are unusual observations for bacterial systems. We have observed these responses at values of initial cell mass of 0.9 OD600 or higher, or at low dilution rates (<0.05 h?1) irrespectively of initial cell mass. This response suggests complex dynamics including the possibility of multiple steady states. Iron, which was reported earlier as a growth limiting nutrient in a widely used minimal medium, enhances both growth and virulence factor induction in iron‐supplemented cultures compared to unsupplemented controls. Intracellular iron concentration is correlated to the early induction (6 h) of virulence factors in both batch and chemostat cultures. A reduction in aconitase activity (a TCA cycle enzyme) and ATP levels in iron‐limited chemostat cultures was observed compared to iron‐supplemented chemostat cultures, indicating that iron affects central metabolic pathways. We conclude that DC3000 cultures are particularly dependent on the environment and iron is likely a key nutrient in determining physiology. Biotechnol. Bioeng. 2010;105: 955–964. © 2009 Wiley Periodicals, Inc. 相似文献
973.
VPS4 proteins are AAA+ ATPases required to form multivesicular bodies, release viral particles, and complete cytokinesis. They act by disassembling ESCRT-III heteropolymers during or after their proposed function in membrane scission. Here we show that purified human VPS4A is essentially inactive but can be stimulated to hydrolyze ATP by ESCRT-III proteins in a reaction that requires both their previously defined MIT interacting motifs and ∼50 amino acids of the adjacent sequence. Importantly, C-terminal fragments of all ESCRT-III proteins tested, including CHMP2A, CHMP1B, CHMP3, CHMP4A, CHMP6, and CHMP5, activated VPS4A suggesting that it disassembles ESCRT-III heteropolymers by affecting each component protein. VPS4A is thought to act as a ring-shaped cylindrical oligomer like other AAA+ ATPases, but this has been difficult to directly demonstrate. We found that concentrating His6-VPS4A on liposomes containing Ni2+-nitrilotriacetic acid-tagged lipid increased ATP hydrolysis, confirming the importance of inter-subunit interactions for activity. We also found that mutating pore loops expected to line the center of a cylindrical oligomer changed the response of VPS4A to ESCRT-III proteins. Based on these data, we propose that ESCRT-III proteins facilitate assembly of functional but transient VPS4A oligomers and interact with sequences inside the pore of the assembled enzyme. Deleting the N-terminal MIT domain and adjacent linker from VPS4A increased both basal and liposome-enhanced ATPase activity, indicating that these elements play a role in autoinhibiting VPS4A until it encounters ESCRT-III proteins. These findings reveal new ways in which VPS4 activity is regulated and specifically directed to ESCRT-III polymers. 相似文献
974.
Claudia R. Oliva Susan E. Nozell Anne Diers Samuel G. McClugage III Jann N. Sarkaria James M. Markert Victor M. Darley-Usmar Shannon M. Bailey G. Yancey Gillespie Aimee Landar Corinne E. Griguer 《The Journal of biological chemistry》2010,285(51):39759-39767
Temozolomide (TMZ) is an oral alkylating agent used for the treatment of high-grade gliomas. Acquired chemoresistance is a severe limitation to this therapy with more than 90% of recurrent gliomas showing no response to a second cycle of chemotherapy. Efforts to better understand the underlying mechanisms of acquired chemoresistance to TMZ and potential strategies to overcome chemoresistance are, therefore, critically needed. TMZ methylates nuclear DNA and induces cell death; however, the impact on mitochondria DNA (mtDNA) and mitochondrial bioenergetics is not known. Herein, we tested the hypothesis that TMZ-mediated alterations in mtDNA and respiratory function contribute to TMZ-dependent acquired chemoresistance. Using an in vitro model of TMZ-mediated acquired chemoresistance, we report 1) a decrease in mtDNA copy number and the presence of large heteroplasmic mtDNA deletions in TMZ-resistant glioma cells, 2) remodeling of the entire electron transport chain with significant decreases of complexes I and V and increases of complexes II/III and IV, and 3) pharmacologic and genetic manipulation of cytochrome c oxidase, which restores sensitivity to TMZ-dependent apoptosis in resistant glioma cells. Importantly, human primary and recurrent pairs of glioblastoma multiforme (GBM) biopsies as well as primary and TMZ-resistant GBM xenograft lines exhibit similar remodeling of the ETC. Overall these results suggest that TMZ-dependent acquired chemoresistance may be due to a mitochondrial adaptive response to TMZ genotoxic stress with a major contribution from cytochrome c oxidase. Thus, abrogation of this adaptive response may reverse chemoresistance and restore sensitivity to TMZ, providing a strategy for improved therapeutic outcomes in GBM patients. 相似文献
975.
Samuel G. Gattis Marcy Hernick Carol A. Fierke 《The Journal of biological chemistry》2010,285(44):33788-33796
UDP-3-O-((R)-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) catalyzes the deacetylation of UDP-3-O-((R)-3-hydroxymyristoyl)-N-acetylglucosamine to form UDP-3-O-myristoylglucosamine and acetate in Gram-negative bacteria. This second, and committed, step in lipid A biosynthesis is a target for antibiotic development. LpxC was previously identified as a mononuclear Zn(II) metalloenzyme; however, LpxC is 6–8-fold more active with the oxygen-sensitive Fe(II) cofactor (Hernick, M., Gattis, S. G., Penner-Hahn, J. E., and Fierke, C. A. (2010) Biochemistry 49, 2246–2255). To analyze the native metal cofactor bound to LpxC, we developed a pulldown method to rapidly purify tagged LpxC under anaerobic conditions. The metal bound to LpxC purified from Escherichia coli grown in minimal medium is mainly Fe(II). However, the ratio of iron/zinc bound to LpxC varies with the metal content of the medium. Furthermore, the iron/zinc ratio bound to native LpxC, determined by activity assays, has a similar dependence on the growth conditions. LpxC has significantly higher affinity for Zn(II) compared with Fe(II) with KD values of 60 ± 20 pm and 110 ± 40 nm, respectively. However, in vivo concentrations of readily exchangeable iron are significantly higher than zinc, suggesting that Fe(II) is the thermodynamically favored metal cofactor for LpxC under cellular conditions. These data indicate that LpxC expressed in E. coli grown in standard medium predominantly exists as the Fe(II)-enzyme. However, the metal cofactor in LpxC can switch between iron and zinc in response to perturbations in available metal ions. This alteration may be important for regulating the LpxC activity upon changes in environmental conditions and may be a general mechanism of regulating the activity of metalloenzymes. 相似文献
976.
977.
Sorensen GL Madsen J Kejling K Tornoe I Nielsen O Townsend P Poulain F Nielsen CH Reid KB Hawgood S Falk E Holmskov U 《American journal of physiology. Heart and circulatory physiology》2006,290(6):H2286-H2294
Surfactant protein D (SP-D) is an important innate immune defense molecule that mediates clearance of pathogens and modulates the inflammatory response. Moreover, SP-D is involved in lipid homeostasis, and pulmonary accumulation of phospholipids has previously been observed in SP-D-deficient (Spd-/-) mice. Atherogenesis involves both inflammation and lipid deposition, and we investigated the role of SP-D in the development of atherosclerosis. SP-D synthesis was localized to vascular endothelial cells. Atherosclerotic lesion areas were 5.6-fold smaller in the aortic roots in Spd-/- mice compared with wild-type C57BL/6N mice on an atherogenic diet. HDL cholesterol (HDL-C) was significantly elevated in Spd-/- mice. Treatment of Spd-/- mice with a recombinant fragment of human SP-D resulted in decreases of HDL-C (21%) as well as total cholesterol (26%), and LDL cholesterol (28%). Plasma TNF-alpha was reduced in Spd-/- mice (45% difference). SP-D was proatherogenic in the mouse model used. The effect is likely to be due to the observed disturbances of plasma lipid metabolism and alteration of the inflammatory process, which underlie the reduced susceptibility to atherosclerosis in Spd-/- mice. 相似文献
978.
Mora-Montes HM López-Romero E Zinker S Ponce-Noyola P Flores-Carreón A 《FEMS microbiology letters》2006,257(1):50-56
A soluble alpha-mannosidase from Candida albicans CAI-4 was purified by conventional methods of protein isolation. Analytical electrophoresis of the purified preparation revealed two polypeptides of 52 and 27 kDa, the former being responsible for enzyme activity. The purified, 52 kDa enzyme trimmed Man9GlcNAc2, producing Man8GlcNAc2 isomer B and mannose, and was inhibited preferentially by 1-deoxymannojirimycin. These properties are consistent with an endoplasmic reticulum-resident alpha1,2-mannosidase of the glycosyl hydrolase family 47. Moreover, a proteolytic activity responsible for converting the 52 kDa alpha-mannosidase into a polypeptide of 43 kDa retaining full enzyme activity, was demonstrated in membranes of ATCC 26555, but not in CAI-4 strain. 相似文献
979.
Pal S Wu J Murray JK Gellman SH Wozniak MA Keely PJ Boyer ME Gomez TM Hasso SM Fallon JF Bresnick EH 《The Journal of cell biology》2006,174(7):1047-1058
Establishment of angiogenic circuits that orchestrate blood vessel development and remodeling requires an exquisite balance between the activities of pro- and antiangiogenic factors. However, the logic that permits complex signal integration by vascular endothelium is poorly understood. We demonstrate that a "neuropeptide," neurokinin-B (NK-B), reversibly inhibits endothelial cell vascular network assembly and opposes angiogenesis in the chicken chorioallantoic membrane. Disruption of endogenous NK-B signaling promoted angiogenesis. Mechanistic analyses defined a multicomponent pathway in which NK-B signaling converges upon cellular processes essential for angiogenesis. NK-B-mediated ablation of Ca2+ oscillations and elevation of 3'-5' [corrected] cyclic adenosine monophosphate (cAMP) reduced cellular proliferation, migration, and vascular endothelial growth factor receptor expression and induced the antiangiogenic protein calreticulin. Whereas NK-B initiated certain responses, other activities required additional stimuli that increase cAMP. Although NK-B is a neurotransmitter/ neuromodulator and NK-B overexpression characterizes the pregnancy-associated disorder preeclampsia, NK-B had not been linked to vascular remodeling. These results establish a conserved mechanism in which NK-B instigates multiple activities that collectively oppose vascular remodeling. 相似文献
980.
Nikrodhanond AA Ortiga-Carvalho TM Shibusawa N Hashimoto K Liao XH Refetoff S Yamada M Mori M Wondisford FE 《The Journal of biological chemistry》2006,281(8):5000-5007
Hypothalamic thyrotropin-releasing hormone (TRH) stimulates thyroid-stimulating hormone (TSH) secretion from the anterior pituitary. TSH then initiates thyroid hormone (TH) synthesis and release from the thyroid gland. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback is thought to be the primary regulator. This hypothesis, however, has yet to be proven in vivo. To elucidate the relative importance of TRH and TH in regulating the hypothalamic-pituitary-thyroid axis, we have generated mice that lack either TRH, the beta isoforms of TH receptors (TRbeta KO), or both (double KO). TRbeta knock-out (KO) mice have significantly higher TH and TSH levels compared with wild-type mice, in contrast to double KO mice, which have reduced TH and TSH levels. Unexpectedly, hypothyroid double KO mice also failed to mount a significant rise in serum TSH levels, and pituitary TSH immunostaining was markedly reduced compared with all other hypothyroid mouse genotypes. This impaired TSH response, however, was not due to a reduced number of pituitary thyrotrophs because thyrotroph cell number, as assessed by counting TSH immunopositive cells, was restored after chronic TRH treatment. Thus, TRH is absolutely required for both TSH and TH synthesis but is not necessary for thyrotroph cell development. 相似文献