铁钼辅因子

某些固氮生物(如棕色固氮菌)的变种,它们的固氮酶中铁蛋白有催化活性,但由于钼铁蛋白处于不活化状态,所以整个固氮酶不能显示出固氮活性,这种不能固氮的固氮酶可为从野生型固氮菌固氮酶钼铁蛋白中分离得到的物质(或因子)所活化或恢复固氮活性,此种活化因子叫钼铁辅因子。这种钼铁辅因子已从多种固氮生物中分离到,其分子量为1000～3000左右。按钼铁蛋白分子量为270,000计算,其中铝、铁、硫三种原     

固氮蓝藻与棕色固氮菌固氮酶组分的交叉互补功能

本文报告了藻菌之间固氮酶组分的交叉互补试验。初步结果证明：固氮蓝藻(Anabaena azotica水生686)的钼铁蛋白与棕色固氮菌(Azotobacter vinelandii)的铁蛋白之间存在着明显的互补功能。但这种蓝藻的铁蛋白在非细胞形态下很不稳定,易于失活。本实验为不同生理类型和不同进化程度的固氮生物之间固氮酶组分的交叉互补研究提供了新的资料。     

固氮蓝藻与棕色固氮菌固氮酶组分的交叉互补功能

本文报告了藻菌之间固氮酶组分的交叉互补试验。初步结果证明:固氮蓝藻(Anabacnaazotica水生686)的钼铁蛋白与棕色固氮菌(Azotobacter vinelandii)的铁蛋白之间存在着明显的互补功能。但这种蓝藻的铁蛋白在非细胞形态下很不稳定,易于失活。本实验为不同生理类型和不同进化程度的固氮生物之间固氮酶组分的交叉互补研究提供了新的资料。       

有催化活性的固氮酶二聚态钼铁蛋白

利用离子交换层析和凝胶过滤,第一次成功地从棕色固氮菌提取液中分离出接近电泳纯的能重组固氮酶活性的二聚态钼铁蛋白。它的分子量约为150,000,每分子含钼约0.5原子。当与铁蛋白重组固氮酶时,表现出相当高的活性。这个结果对固氮酶底物络合中心的结构提供了重要的启示,指出与目前较为流行的双钼多核中心相比,含单钼的多核中心至少同样是不可忽视的可能结构。     

光在钼影响蓝藻固氮中的作用

我们过去的工作曾指出,蓝藻及其离体异形胞的固氮是一个依赖于光的过程,光对蓝藻固氮酶的合成和活性有调节作用。此外,多年的生化研究也已确定,钼是包括蓝藻在内的一切固氮生物生长和参与固氮酶催化活性中心结构的必需与关键性的金属元素,它对钼铁蛋白有调控功能。这样,     

固氮酶

指固氮生物中对固定空气中分子氮并将其转变成氨起特殊催化功能的酶蛋白质。已从16种不同类型固氮生物中分离到固氮酶,它可分为两种独立的酶蛋白:(1)含钼和铁,称钼铁蛋白;(2)含铁,称铁蛋白。一般认为前者是络合和还原氮的中心,后者是电子活化中心。两者单独存在时不能固氮,只有合起来才能重组固氮活性。固氮酶(尤其是铁蛋白)对氧和摄氏零度左右的低温很敏感,易于失活。由于两种蛋白重组固氮活性时的确切比例尚不清楚,加上它们的均一制剂又未得到,所以对整个固氮酶的分子量还无法判断。目     

植物的共生固氮体系

氮是植物生命活动中不可缺少的重要元素之一。大气中的氮尽管为79％,但这种游离氮只有少数固氮细菌和蓝藻才能吸收利用,绿色植物却不能直接利用。这些细菌、蓝藻把大气中的游离氮固定转化为含氮化合物,成为植物所能吸收的氮,就称为生物固氮作用。固氮生物之所以能催化还原N2成NH3,是由于它含有固氮酶。固氮酶是一种结构复杂、功能特异的酶,由铁蛋白和钼铁蛋白组成。     

棕色固氮菌钼铁蛋白结晶及一些物理化学性质

获得了棕色固氮菌固氮酶钼铁蛋白的针状结晶,大小为2-2.5×25-65微米,成晶所要求的条件不是很严格的。SDS 电泳和氨基酸分析结果表明,钼铁蛋白分子量为250,000。只观察到一种类型亚单位。其中酸性氨基酸占优势。分子氧使针状晶体演变为发状晶丝,晶丝又密集成束。晶束可保存较长时间。氧使钼铁蛋白在电泳时形成高聚合态。       

不同底物与固氮酶及其钼铁蛋白相互作用的荧光光谱分析

 本文研究了不同底物(N_2,H_2,N_2O,NaN_3,C_2H_2)对棕色固氮菌固氮酶及其钼铁蛋白荧光光谱的影响。结果表明,上述底物均能络合在钼铁蛋白及固氮酶上,但络合程度不同,从而为固氮酶系统有多个不同的底物络合中心,底物络合中心在钼铁蛋白分子上,铁蛋白对钼铁蛋白有变构作用,提供了光谱学证据。     

纯化柱胞鱼腥藻(Anabaena cylindrca)固氮酶组份(铁蛋白)的厌氧制备电泳法

各种固氮生物的固氮酶对氧都很敏感,无论是制备固氮酶的组份Ⅰ(钼铁蛋白)或组份Ⅱ(铁蛋白),也无论采取什么方法(如DEAE-纤维素层析法、硫酸鱼精朊沉淀法、胶滤、制备凝胶电泳等等)都必需在严格厌氧条件下进行,铁蛋白对氧更加敏感,因此要获得较纯而又具活力的铁蛋白,其分离、纯化过程既要严格厌氧又要迅速。到目前为止,仅红螺菌(Rhodospirillum rubrum)和棕色     

A Unique Protease Cleavage Site Predicted in the Spike Protein of the Novel Pneumonia Coronavirus (2019-nCoV) Potentially Related to Viral Transmissibility

正Dear Editor,In December 2019, a novel human coronavirus caused an epidemic of severe pneumonia(Coronavirus Disease 2019,COVID-19) in Wuhan, Hubei, China(Wu et al. 2020; Zhu et al. 2020). So far, this virus has spread to all areas of China and even to other countries. The epidemic has caused 67,102 confirmed infections with 1526 fatal cases     

Curcuminoids as inhibitors of thioredoxin reductase: A receptor based pharmacophore study with distance mapping of the active site

Curcumin is the yellow pigment of turmeric that interacts irreversibly forming an adduct with thioredoxin reductase (TrxR), an enzyme responsible for redox control of cell and defence against oxidative stress. Docking at both the active sites of TrxR was performed to compare the potency of three naturally occurring curcuminoids, namely curcumin, demethoxy curcumin and bis-demethoxy curcumin. Results show that active sites of TrxR occur at the junction of E and F chains. Volume and area of both cavities is predicted. It has been concluded by distance mapping of the most active conformations that Se atom of catalytic residue SeCYS498, is at a distance of 3.56 from C13 of demethoxy curcumin at the E chain active site, whereas C13 carbon atom forms adduct with Se atom of SeCys 498. We report that at least one methoxy group in curcuminoids is necessary for interation with catalytic residues of thioredoxin. Pharmacophore of both active sites of the TrxR receptor for curcumin and demethoxy curcumin molecules has been drawn and proposed for design and synthesis of most probable potent antiproliferative synthetic drugs.     

Comparative morphology of the carpel in the Liliaceae: Hewardieae, Petrosavieae, and Tricyrteae

The young pistils in the melanthioid tribes, Hewardieae, Petrosavieae and Tricyrteae, are uniformly tricarpellate and syncarpous. They lack raphide idioblasts. All are multiovulate, with bitegmic ovules. The Petrosavieae are marked by the presence of septal glands and incomplete syncarpy. Tepals and stamens adhere to the ovary in the Hewardieae and the Petrosavieae but not in the Tricyrteae. Two vascular bundles occur in the stamens of the Hewartlieae and Tricyrtis latifolia. Ventral bundles in the upper part of the ovary of the Hewardieae are continuous with compound septal bundles and placental bundles in the lower part. Putative ventral bundles occur in the alternate position in the Tricyrteae and putative placental bundles in the opposite. position in the Petrosavieae. The dichtomously branched stigma in each carpel of the Tricyrteae is supplied by a bifurcated dorsal bundle.     

Selection with two alleles and complete dominance

For a plant selection model with frequency-independent viabilities, fertilities and selfing rates, it is shown that apart from global fixation, for certain parameter combinations a protected polymorphism and facultative fixation (either allele may become fixed according to initial frequencies) may both occur. Facultative fixation requires different selling rates for the dominant and recessive type. Protection of the polymorphism requires resource allocation for male and female function. In this connection the problem of purely genetically caused population extinction is discussed.
For general frequency dependence and regular segregation, the chances for establishment of a completely recessive gene are compared to those of a completely dominant gene. It is proven that the process of establishment of the recessive gene, despite a fitness advantage, may be considerably endangered by drift effects if random mating prevails. The recessive gene may reach the same effectivity in establishment as a dominant gene, only if the recessive homozygote mates exclusively with its own type during the period of establishment.     

Composition and diversity of bird communities in Swedish farmland–forest mosaic landscapes

CapsuleThe amount of forest (at local and landscape scales) and occurrence of residual habitats at the local scale are shown to be the major factors influencing bird community composition in farmland–forest landscapes in central Sweden.

Aims To investigate the importance of local habitat and landscape structure for breeding birds in farmland–forest landscapes in central Sweden.

Methods Breeding birds were censused at 292 points. A detailed habitat mapping was made within 300 m of the points. Within a 300–600 m radius only two major habitats (forests and arable fields) were identified.

Results Cluster analyses of bird communities identified three site types that also differed in habitat composition: (i) partially forested sites in forested landscapes; (ii) heterogeneous sites with residual habitats in mosaic landscapes; and (iii) field-dominated farmland sites in open landscapes. A total of 19 of 25 farmland bird species (restricted to farmland or using both farmland and forest) had the highest abundance in farmland sites with mosaics of forest and farmland, while only six farmland species had the highest abundance in field-dominated sites. The bird community changed from being dominated by farmland species to being dominated by forest species (common in forest landscapes without farmland) at small proportions (10–20%) of forest at the local scale. A major difference in habitat composition between heterogeneous and field-dominated sites was the occurrence of different residual habitats (e.g. shrubby areas and seminatural grasslands). These habitats seemed to influence bird community composition more than land-use, despite covering <10% of the area. Seminatural grasslands were important for bird community composition and species-richness, but grazing seemed to be less important. Among different land-use types, cereal crops were the least preferred fields. Set-asides with tall vegetation and short rotation coppices were positively associated with species-richness of farmland birds.

Conclusion In general, the composition of the landscape was important for bird community composition, although amount and distribution of forests, occurrence of residual habitats and land-use of fields at the local scale had the strongest influence on bird community composition. The possible implications of these patterns for managing farmland–forest landscapes are discussed.     

Chemical constituents from the fruits of Rhus typhina L. and their chemotaxonomic significance

This work describes the isolation and characterization of twenty-nine compounds from the fruits of Rhus typhina L., including eleven flavonoids (1–11), eleven phenols (12–22), two pentacyclic triterpenes (23–24), two organic acids (25–26), one lumichrome (27), one courmarin (28) and one pyrimidine (29) on the basis of their spectroscopic data. Compounds apigenin (1), daidzein (4), orobol (5), 3′, 5, 5′, 7-tetrahydroxyflavanone (6), naringenin (7), butein (8), (-)-catechin (9), quercetin-3-O-α-L-(3″-O-galloyl)-rhamnoside (11), 2-hydroxybenzoic acid (13), 4-hydroxybenzaldehyde (14), vanillin (15), methyl 3,4-dihydroxybenzoate (16), 3,5-dihydroxybenzamide (18), tyrosol (19), caffeic acid (20), 3-(2,4,6-trihydroxyphenyl)-1-(4-hydroxyphenyl)-propan-1-one (21), phlorizin (22), friedelin (23), oleanolic acid (24), 4,4-dimethyl-heptanedioic acid (25), anthranilic acid (26), lumichrome (27), scoparone (28) and uracil (29) have not been recorded before in this plant. This is the first report on the occurrence of compounds 4–7, 9, 11, 13–14, 16, 18–21, 25–29 from the genus Rhus. Moreover, the chemotaxonomic significance of these isolated compounds was also summarized.