全文获取类型
收费全文 | 1582篇 |
免费 | 144篇 |
国内免费 | 1篇 |
专业分类
1727篇 |
出版年
2023年 | 12篇 |
2022年 | 20篇 |
2021年 | 37篇 |
2020年 | 29篇 |
2019年 | 26篇 |
2018年 | 42篇 |
2017年 | 27篇 |
2016年 | 49篇 |
2015年 | 70篇 |
2014年 | 75篇 |
2013年 | 82篇 |
2012年 | 121篇 |
2011年 | 115篇 |
2010年 | 57篇 |
2009年 | 59篇 |
2008年 | 73篇 |
2007年 | 86篇 |
2006年 | 79篇 |
2005年 | 60篇 |
2004年 | 68篇 |
2003年 | 37篇 |
2002年 | 39篇 |
2001年 | 34篇 |
2000年 | 33篇 |
1999年 | 20篇 |
1998年 | 12篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 28篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 12篇 |
1986年 | 15篇 |
1985年 | 8篇 |
1984年 | 22篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 14篇 |
1975年 | 11篇 |
1972年 | 9篇 |
1970年 | 13篇 |
1966年 | 9篇 |
1965年 | 7篇 |
排序方式: 共有1727条查询结果,搜索用时 15 毫秒
91.
The structure of an active mutant of (S)-mandelate dehydrogenase (MDH-GOX2) from Pseudomonas putida has been determined at 2.15 A resolution. The membrane-associated flavoenzyme (S)-mandelate dehydrogenase (MDH) catalyzes the oxidation of (S)-mandelate to give a flavin hydroquinone intermediate which is subsequently reoxidized by an organic oxidant residing in the membrane. The enzyme was rendered soluble by replacing its 39-residue membrane-binding peptide segment with a corresponding 20-residue segment from its soluble homologue, glycolate oxidase (GOX). Because of their amphipathic nature and peculiar solubilization properties, membrane proteins are notoriously difficult to crystallize, yet represent a large fraction of the proteins encoded by genomes currently being deciphered. Here we present the first report of such a structure in which an internal membrane-binding segment has been replaced, leading to successful crystallization of the fully active enzyme in the absence of detergents. This approach may have general application to other membrane-bound proteins. The overall fold of the molecule is that of a TIM barrel, and it forms a tight tetramer within the crystal lattice that has circular 4-fold symmetry. The structure of MDH-GOX2 reveals how this molecule can interact with a membrane, although it is limited by the absence of a membrane-binding segment. MDH-GOX2 and GOX adopt similar conformations, yet they retain features characteristic of membrane and globular proteins, respectively. MDH-GOX2 has a distinctly electropositive surface capable of interacting with the membrane, while the opposite surface is largely electronegative. GOX shows no such pattern. MDH appears to form a new class of monotopic integral membrane protein that interacts with the membrane through coplanar electrostatic binding surfaces and hydrophobic interactions, thus combining features of both the prostaglandin synthase/squaline-hopine cyclase and the C-2 coagulation factor domain classes of membrane proteins. 相似文献
92.
Chronic parasitic infections are a major risk factor for cancer development in many underdeveloped countries. Oxidative damage of DNA may provide a mechanism linking these processes. Eosinophil recruitment is a hallmark of parasitic infections and many forms of cancer, and eosinophil peroxidase (EPO), a secreted hemoprotein, plays a central role in oxidant production by these cells. However, mechanisms through which EPO may facilitate DNA oxidation have not been fully characterized. Here, we show that EPO effectively uses plasma levels of bromide as a cosubstrate to brominate bases in nucleotides and double-stranded DNA, forming several stable novel brominated adducts. Products were characterized by HPLC with on-line UV spectroscopy and electrospray ionization tandem mass spectrometry (LC/ESI/MS/MS). Ring assignments for brominated purine bases as their 8-bromo adducts were identified by NMR spectroscopy. Using stable isotope dilution LC/ESI/MS/MS, we show that while guanine is the preferred purine targeted for bromination as a free nucleobase, 8-bromoadenine is the major purine oxidation product generated following exposure of double-stranded DNA to either HOBr or the EPO/H(2)O(2)/Br(-) system. Bromination of nucleobases was inhibited by scavengers of hypohalous acids such as the thioether methionine, but not by a large molar excess of primary amines. Subsequently, N-monobromoamines were demonstrated to be effective brominating agents for both free nucleobases and adenine within intact DNA. A rationale for selective modification of adenine, but not guanine, in double-stranded DNA based upon stereochemical criteria is presented. Collectively, these results suggest that specific brominated DNA bases may serve as novel markers for monitoring oxidative damage of DNA and the nucleotide pool by brominating oxidants. 相似文献
93.
Immobilization of cyanobacterium Spirulina platensis in sodium alginate (1.5 %) gave the best quality of bead and 15-16 beads were formed per mL of aqueous solution of alginate. The immobilized cells were used in a batch process for treatment of diluted sewage. After 8 days, 95 % of BOD5, 77 % of COD, 90 % of ammonia, and 94 % of TSS were removed. 相似文献
94.
Mitra SK Venkataranganna MV Gopumadhavan S Sundaram R 《Indian journal of experimental biology》1999,37(4):409-410
In the present study HD-03, a herbal formulation was investigated for its anti-cholestatic activity in TAA-induced cholestasis in anaesthetized guinea pigs. Administration of TAA at a dose of 100 mg/kg body wt significantly reduced the bile flow, bile acid and bile salt excretion. Pretreatment with HD-03 at a dose of 750 mg/kg body wt per orally for 15 days in guinea pigs significantly prevented thioacetamide-induced changes in bile flow, bile acids and bile salts excretion. Thus, HD-03 can serve as a potent choleretic and anti-cholestatic agent. 相似文献
95.
Effect of reactive oxygen species on the metabolism of tryptophan in rat brain: Influence of age 总被引:1,自引:0,他引:1
In an earlier study, oxidation of tryptophan hydroxylase was implicated as its affinity was decreased with aging in rat brain. To establish any potential link between its oxidative damage and aging, we have determined the activities of antioxidant enzymes in midbrain, pons and medulla of 2, 12 and 24 month old Fisher 344 BNF1 rats. The results obtained suggest that the activities of antioxidant enzymes varied considerably with age and brain regions studied. Activities of Cu/Zn superoxide dismutase and glutathione peroxidase were found to increase from 2 to 12 months and then decrease in 24 month old rats. However catalase activity decreased consistently with the age. A parallel increase in the carbonyl content was observed in these brain regions indicating the oxidation of proteins. Reactive oxygen species when included in the incubation mixture decreased the activity of tryptophan hydroxylase in a concentration dependent manner. The loss of tryptophan hydroxylase activity induced by hydrogen peroxide and superoxide anion was prevented by catalase. However superoxide dismutase did not provide such protection. Sulfhydryl agents, cysteine, glutathione and dithiothreitol partially prevented the loss of activity. These studies suggest an involvement of reactive oxygen species for sulfhydryl oxidation of tryptophan hydroxylase in aging. 相似文献
96.
HIV-1 Tat protein reprograms cellular gene expression of infected as well as uninfected cells apart from its primary function of transactivating HIV-1 long terminal repeat (LTR) promoter by binding to a nascent RNA stem–loop structure known as the transactivator response region (TAR). Tat also induces chromatin remodeling of proviral LTR-mediated gene expression by recruiting histone acetyl transferases to the chromatin, which results in histone acetylation. Furthermore several studies have shown convincing evidence that Tat can transactivate HIV-1 gene expression in the absence of TAR, the molecular mechanism of which remains to be elucidated. Here we show a direct interaction of Tat with nuclear factor kappa B (NFκB) enhancer, a global regulatory sequence for many cellular genes both in vitro and in vivo. This interaction not only provides a novel molecular basis to explain TAR-independent transactivation in HIV-1, but also points toward the potential mechanism of Tat- mediated modulation of cellular genes. 相似文献
97.
Method for enhancing solubility of the expressed recombinant proteins in Escherichia coli 总被引:1,自引:0,他引:1
Ghosh S Rasheedi S Rahim SS Banerjee S Choudhary RK Chakhaiyar P Ehtesham NZ Mukhopadhyay S Hasnain SE 《BioTechniques》2004,37(3):418, 420, 422-418, 420, 423
The production of correctly folded protein in Escherichia coli is often challenging because of aggregation of the overexpressed protein into inclusion bodies. Although a number of general and protein-specific techniques are available, their effectiveness varies widely. We report a novel method for enhancing the solubility of overexpressed proteins. Presence of a dipeptide, glycylglycine, in the range of 100 mM to 1 M in the medium was found to significantly enhance the solubility (up to 170-fold) of the expressed proteins. The method has been validated using mycobacterial proteins, resulting in improved solubilization, which were otherwise difficult to express as soluble proteins in E. coli. This method can also be used to enhance the solubility of other heterologous recombinant proteins expressed in a bacterial system. 相似文献
98.
Multiple connections link FAK to cell motility and invasion 总被引:28,自引:0,他引:28
The ability of intracellular signaling networks to orchestrate a complex biological response such as cell motility requires that individual signaling proteins must act as integrators, responding to multiple extracellular inputs and regulating multiple signaling pathway outputs. In this review, we highlight recent findings that place focal adhesion kinase (FAK) in an important receptor-proximal position in the regulation of growth factor and integrin-stimulated cell motility. Emphasis is placed on the molecular mechanisms of FAK activation, connections of FAK to focal contact formation as well as turnover, and the potential that FAK function in promoting cell invasion may be distinct from its role in cell motility. 相似文献
99.
Wiederhold L Leppard JB Kedar P Karimi-Busheri F Rasouli-Nia A Weinfeld M Tomkinson AE Izumi T Prasad R Wilson SH Mitra S Hazra TK 《Molecular cell》2004,15(2):209-220
The paradigm for repair of oxidized base lesions in genomes via the base excision repair (BER) pathway is based on studies in Escherichia coli, in which AP endonuclease (APE) removes all 3' blocking groups (including 3' phosphate) generated by DNA glycosylase/AP lyases after base excision. The recently discovered mammalian DNA glycosylase/AP lyases, NEIL1 and NEIL2, unlike the previously characterized OGG1 and NTH1, generate DNA strand breaks with 3' phosphate termini. Here we show that in mammalian cells, removal of the 3' phosphate is dependent on polynucleotide kinase (PNK), and not APE. NEIL1 stably interacts with other BER proteins, DNA polymerase beta (pol beta) and DNA ligase IIIalpha. The complex of NEIL1, pol beta, and DNA ligase IIIalpha together with PNK suggests coordination of NEIL1-initiated repair. That NEIL1/PNK could also repair the products of other DNA glycosylases suggests a broad role for this APE-independent BER pathway in mammals. 相似文献
100.