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1.
Voltage‐gated sodium channels are essential for electrical signalling across cell membranes. They exhibit strong selectivities for sodium ions over other cations, enabling the finely tuned cascade of events associated with action potentials. This paper describes the ion permeability characteristics and the crystal structure of a prokaryotic sodium channel, showing for the first time the detailed locations of sodium ions in the selectivity filter of a sodium channel. Electrostatic calculations based on the structure are consistent with the relative cation permeability ratios (Na+ ≈ Li+ ≫ K+, Ca2+, Mg2+) measured for these channels. In an E178D selectivity filter mutant constructed to have altered ion selectivities, the sodium ion binding site nearest the extracellular side is missing. Unlike potassium ions in potassium channels, the sodium ions in these channels appear to be hydrated and are associated with side chains of the selectivity filter residues, rather than polypeptide backbones.  相似文献   
2.
黑皮质素系统来自阿片-促黑素细胞皮质素原,在中枢摄食行为和能量平衡代谢中起到重要作用,此系统生理功能的发挥主要通过与下丘脑神经元细胞上特定膜受体(黑皮质素受体)结合完成。黑皮质素受体(MCR)有五种亚型(MC1R-MC5R),其中参与体重调节的受体主要是黑皮质素受体3(MC3R)和黑皮质素受体4(MC4R)。MC4R属于G蛋白耦联受体,具有七次跨膜结构。作为一种膜受体,MC4R发挥体重调节作用,一方面受外界激动剂或拮抗剂的调节;另一方面,此受体活化后会影响到细胞内的信号调节通路。研究MC4R的功能首先要了解受体的结构,本文对G蛋白耦联受体的结构进行了较详细的叙述,MC4R经信号调节通路,激活腺苷酸环化酶,增加cAMP的浓度,最终通过影响细胞内基因的转录和翻译,来调节体重和能量的消耗。  相似文献   
3.

Background  

The present study aimed to evaluate the efficacy of the hyaluronic acid (HA) binding assay in the selection of motile spermatozoa with normal morphology at high magnification (8400x).  相似文献   
4.
  总被引:2,自引:0,他引:2  
The conversion of substrate into product catalyzed by an enzyme with a prosthetic group is known to involve a number of stages during which the substrate is bound covalently with the coenzyme. For pyridoxal enzymes, and particularly for transaminases, Schiff bases formed by amino acids and pyridoxal-5-phosphate (PLP) are considered the major type of these intermediate complexes. From a purely chemical viewpoint it is evident, that besides these major forms there must also exist others, e.g., containing no double bond between coenzyme and substrate.We have recently proposed, that compounds which are structural analogues of the substrate-coenzyme complexes may be inhibitors of prosthetic group-containing enzymes, particularly, of the PLP-enzymes /1/. In the present paper we report the synthesis of previously unknown N-(pyridoxyl-5′-phosphate)-amino acids (P-Pyl-amino acids) and the results of the studies on their interaction with some transaminases whose coenzyme is PLP and whose substrates are L-amino acids.  相似文献   
5.
Re-examination, using molecular tools, of the diversity of haemosporidian parasites (among which the agents of human malaria are the best known) has generally led to rearrangements of traditional classifications. In this study, we explored the diversity of haemosporidian parasites infecting vertebrate species (particularly mammals, birds and reptiles) living in the forests of Gabon (Central Africa), by analyzing a collection of 492 bushmeat samples. We found that samples from five mammalian species (four duiker and one pangolin species), one bird and one turtle species were infected by haemosporidian parasites. In duikers (from which most of the infected specimens were obtained), we demonstrated the existence of at least two distinct parasite lineages related to Polychromophilus species (i.e., bat haemosporidian parasites) and to sauropsid Plasmodium (from birds and lizards). Molecular screening of sylvatic mosquitoes captured during a longitudinal survey revealed the presence of these haemosporidian parasite lineages also in several Anopheles species, suggesting a potential role in their transmission. Our results show that, differently from what was previously thought, several independent clades of haemosporidian parasites (family Plasmodiidae) infect mammals and are transmitted by anopheline mosquitoes.  相似文献   
6.
    
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7.
The type series of Progynotaenia evaginata Fuhrmann, 1909 from Burhinus senegalensis in Sudan, P. foetida Meggitt, 1928 from B. oedicnemus in Egypt and Angularia australis Maplestone, 1921 from B. grallarius in Australia are redescribed. As a comparative material, specimens of P. evaginata from B. oedicnemus in Kazakhstan were studied. The type-series of P. evaginata and P. foetida were found to be heterogeneous due to the presence of scoleces and fragments of cestodes of the genus Stenovaria Spasskii & Borgarenko, 1973 (Dilepididae). For P. foetida, a lectotype is designated. P. foetida is recognised as a synonym of P. evaginata (new synonymy). Angularia australis, previously considered a member of the Dilepididae, is transferred to the Progynotaeniidae as a synonym of P. evaginata (new synonymy). The synonymy of P. indica Johri, 1963 with P. evaginata, proposed by Ryzhikov & Tolkacheva (1981), is supported. The host range and the geographical distribution of P. evaginata are restricted to birds of the genus Burhinus from the Eastern Hemisphere.  相似文献   
8.
Two populations of Anadara senilis were sampled periodicallyin a lagoon of Senegal. Spatfall occurs in summer during therainy season. Population density decreases from about 50, 000.m–2,two months after spatfall, down to 6, 750 one year later and750 after 28 months. The average growth, measured on the surfaceof the shell, following the umbo rim axis is 0.8 mm per month.The main reduction in growth rate occurs in winter and endsby the deposition of a growth ring. A second growth reductiontakes place during the rainy season and sometimes ends by aweak growth ring. The microgrowth increments are conspicuousand widest during spring equinox. They are not directly relatedto the tidal cycle, but it was impossible to determine if theyhave a tidal or a daily periodicity. (Received 11 February 1990; accepted 16 July 1993)  相似文献   
9.
记述中国西北地区东鳖甲属2新种:巴丹东鳖甲A.badainica Ba,Ren&Liu sp.nov.和古尔班东鳖甲A.gurbantunggutica Ba&Ren sp.nov.,提供了主要鉴别特征和形态图,并简要讨论了其昼夜活动规律.  相似文献   
10.
柳脊虎天牛Xylotrechus namanganensis Heyden是危害胡杨Populus euphratica Oliv、榆树Ulmus pumila L.等多种阔叶树的蛀干性害虫,2013年柳脊虎天牛被国家林业局列入“林业危险性有害生物名单”。因其隐蔽性极强,龄数划分十分困难。本研究通过在额济纳旗天然胡杨林区定期收集不同发育阶段的柳脊虎天牛幼虫,并选取体长、体宽、头壳宽度、触角孔间距、前胸背板宽度5项形态指标进行测定。利用频次分布图初步判定龄数,同时运用Crosby生长法则和回归分析等方法进行验证分析。结果表明柳脊虎天牛幼虫龄数可以划分为6龄,体宽、头壳宽度、触角孔间距、前胸背板均符合Dyar法则,测量值与龄数呈现显著的拟合关系。其中前胸背板宽(Y)与虫龄(X)的拟合度最高,回归方程为Y= 0.5192e0.3782x(r2 =0.9667),是幼虫龄数划分的最佳指标。本文对研究柳脊虎天牛发生规律及生物生态学特性,制定防治策略和预测预报害虫发生规律都有一定的指导价值,同时也可为其他地区珍稀胡杨林保护提供可借鉴的经验。  相似文献   
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