全文获取类型
收费全文 | 8131篇 |
免费 | 548篇 |
国内免费 | 1304篇 |
专业分类
9983篇 |
出版年
2023年 | 135篇 |
2022年 | 250篇 |
2021年 | 282篇 |
2020年 | 256篇 |
2019年 | 285篇 |
2018年 | 249篇 |
2017年 | 271篇 |
2016年 | 337篇 |
2015年 | 333篇 |
2014年 | 384篇 |
2013年 | 731篇 |
2012年 | 310篇 |
2011年 | 353篇 |
2010年 | 276篇 |
2009年 | 375篇 |
2008年 | 344篇 |
2007年 | 372篇 |
2006年 | 378篇 |
2005年 | 311篇 |
2004年 | 317篇 |
2003年 | 300篇 |
2002年 | 283篇 |
2001年 | 192篇 |
2000年 | 172篇 |
1999年 | 165篇 |
1998年 | 152篇 |
1997年 | 154篇 |
1996年 | 123篇 |
1995年 | 141篇 |
1994年 | 159篇 |
1993年 | 150篇 |
1992年 | 133篇 |
1991年 | 94篇 |
1990年 | 81篇 |
1989年 | 113篇 |
1988年 | 76篇 |
1987年 | 73篇 |
1986年 | 65篇 |
1985年 | 108篇 |
1984年 | 148篇 |
1983年 | 97篇 |
1982年 | 100篇 |
1981年 | 64篇 |
1980年 | 51篇 |
1979年 | 57篇 |
1978年 | 40篇 |
1977年 | 24篇 |
1976年 | 33篇 |
1974年 | 26篇 |
1973年 | 20篇 |
排序方式: 共有9983条查询结果,搜索用时 15 毫秒
61.
62.
63.
An endopolygalacturonase (endo-PG), was purified from the culture medium of a local isolate of Sclerotinia sclerotiorum with ammonium sulphate precipitation, cation exchange chromatography and gel filtration. The purified endo-PG had a molecular
mass of approximately 18 kDa estimated by gel filtration. The isoelectric point was determined by isoelectric focusing to
be approximately 8, suggesting that PG II possesses a net positive charge at physiological pHs. The pH optimum for the enzyme
was at pH 4.5. The endo-PG showed essentially the same affinity for pectin and polygalacturonic acid as substrates.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
64.
The ability to predict protein function from structure is becoming increasingly important as the number of structures resolved is growing more rapidly than our capacity to study function. Current methods for predicting protein function are mostly reliant on identifying a similar protein of known function. For proteins that are highly dissimilar or are only similar to proteins also lacking functional annotations, these methods fail. Here, we show that protein function can be predicted as enzymatic or not without resorting to alignments. We describe 1178 high-resolution proteins in a structurally non-redundant subset of the Protein Data Bank using simple features such as secondary-structure content, amino acid propensities, surface properties and ligands. The subset is split into two functional groupings, enzymes and non-enzymes. We use the support vector machine-learning algorithm to develop models that are capable of assigning the protein class. Validation of the method shows that the function can be predicted to an accuracy of 77% using 52 features to describe each protein. An adaptive search of possible subsets of features produces a simplified model based on 36 features that predicts at an accuracy of 80%. We compare the method to sequence-based methods that also avoid calculating alignments and predict a recently released set of unrelated proteins. The most useful features for distinguishing enzymes from non-enzymes are secondary-structure content, amino acid frequencies, number of disulphide bonds and size of the largest cleft. This method is applicable to any structure as it does not require the identification of sequence or structural similarity to a protein of known function. 相似文献
65.
Sang-Hoon Kim Minjeong Kim Daechan Park Sujeong Byun Sangkee Rhee 《The Journal of biological chemistry》2022,298(5)
Pseudouridine, one major RNA modification, is catabolized into uracil and ribose-5′-phosphate by two sequential enzymatic reactions. In the first step, pseudouridine kinase (PUKI) phosphorylates pseudouridine to pseudouridine 5′-monophosphate. High-fidelity catalysis of pseudouridine by PUKI prevents possible disturbance of in vivo pyrimidine homeostasis. However, the molecular basis of how PUKI selectively phosphorylates pseudouridine over uridine with >100-fold greater efficiency despite minor differences in their Km values has not been elucidated. To investigate this selectivity, in this study we determined the structures of PUKI from Escherichia coli strain B (EcPUKI) in various ligation states. The structure of EcPUKI was determined to be similar to PUKI from Arabidopsis thaliana, including an α/β core domain and β-stranded small domain, with dimerization occurring via the β-stranded small domain. In a binary complex, we show that Ser30 in the substrate-binding loop of the small domain mediates interactions with the hallmark N1 atom of pseudouridine nucleobase, causing conformational changes in its quaternary structure. Kinetic and fluorescence spectroscopic analyses also showed that the Ser30-mediated interaction is a prerequisite for conformational changes and subsequent catalysis by EcPUKI. Furthermore, S30A mutation or EcPUKI complexed with other nucleosides homologous to pseudouridine but lacking the pseudouridine-specific N1 atom did not induce such conformational changes, demonstrating the catalytic significance of the proposed Ser30-mediated interaction. These analyses provide structural and functional evidence for a pseudouridine-dependent conformational change of EcPUKI and its functional linkage to catalysis. 相似文献
66.
Summary Two thymidine kinase isoenzymes, TK 3 and TK 4, from mononuclear leucocytes from a patient with acute monocytic leukemia, were purified and characterized in regard to the molecular weights and kinetic properties.The molecular weights of TK 3 and TK 4 were 60 000 and 45 000, respectively. In the presence of 2 mM ATP, the molecular weight of TK 3 increased to 200 000, whereas the molecular weight of TK 4 was unchanged.Studies of the kinetic properties showed clear differences between TK 3 and TK 4. With thymidine as substrate, TK 3 showed biphasic kinetics with a Km of 22 µM, and TK 4 showed Michaelis-Menten kinetics with a Km of 0.33 µM With ATP as substrate, TK 3 showed Michaelis-Menten kinetics with a Km of 100 µM, and TK 4 showed biphasic kinetics with a Km of 3.5 µM. With dTTP as inhibitor, TK 3 showed cooperative inhibition kinetics, and TK 4 showed non-cooperative competitive inhibition kinetics. The dTTP concentration at 50% inhibition was 75 µM for TK 3 but 380 µM for TK 4.Comparison of the molecular weights and the kinetic properties of TK 3 and TK 4 with the corresponding data previously obtained for TK 1 and TK 2 from normal human lymphocytes indicate the existence of four thymidine kinase isoenzymes in human leucocytes. 相似文献
67.
Principles of functional and structural organization in the bacterial cell: 'Compartments' and their enzymes 总被引:3,自引:0,他引:3
F. Mayer 《FEMS microbiology letters》1993,104(3-4):327-346
68.
Catechol oxidase was distributed in soluble and particulate fractions of Solanum melongena. The purified preparation appears to be homogeneous by polyacrylamide gel electrophoresis. The enzyme shows two pH maxima—with catechol, 6.5 and 7.5; and with dopa, 6.5 and 7.9. The latent form of the enzyme does not occur in S. melongena. The preparation resembles the enzyme from other sources in substrate specificity towards various mono- and diphenols, having a higher affinity for catechol than dopa; this tendency increases on purification. The cresolase activity decreases with purification and a lag period with p-cresol is observed. The oxidation of mono- and diphenols is inhibited by ascorbic acid, sulphydryl compounds and chelating agents. 相似文献
69.
《Microbes and infection / Institut Pasteur》2020,22(4-5):221-225
SARS-CoV-2, the newly identified human coronavirus causing severe pneumonia pandemic, was probably originated from Chinese horseshoe bats. However, direct transmission of the virus from bats to humans is unlikely due to lack of direct contact, implying the existence of unknown intermediate hosts. Angiotensin converting enzyme 2 (ACE2) is the receptor of SARS-CoV-2, but only ACE2s of certain species can be utilized by SARS-CoV-2. Here, we evaluated and ranked the receptor-utilizing capability of ACE2s from various species by phylogenetic clustering and sequence alignment with the currently known ACE2s utilized by SARS-CoV-2. As a result, we predicted that SARS-CoV-2 tends to utilize ACE2s of various mammals, except murines, and some birds, such as pigeon. This prediction may help to screen the intermediate hosts of SARS-CoV-2. 相似文献
70.
In this study, an ion exchange resin-based downstream-processing concept for imine reductase (IRED)-catalyzed reactions was investigated. As a model reaction, 2-methylpyrroline was converted to its corresponding product (S)-2-methylpyrrolidine with >99% of conversion by the (S)-selective IRED from Paenibacillus elgii B69. Under optimized reaction conditions full conversion was achieved using a substrate concentration of 150 and 500 mmol/L of d -glucose. Seven commercially available cation- and anion-exchange resins were studied with respect to their ability to recover the product from the reaction solution. Without any pretreatment, cation-exchange resins Amberlite IR-120(H), IRN-150, Dowex Monosphere 650C, and Dowex Marathon MSC showed high recovery capacities (up to >90%). A 150-ml preparative scale reaction was performed yielding ~1 g hydrochloride salt product with >99% purity. Any further purification steps, for example, by column chromatography or recrystallization, were not required. 相似文献