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991.
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993.
氨基酸作为生物体内组成生命物质的小分子化合物, 在天然产物生物合成中扮演了非常重要的作用。色氨酸含有一个独特的吲哚环, 相对复杂的吲哚环平面结构使得色氨酸相比其他氨基酸具有更多的修饰空间。在微生物天然产物生物合成研究中, 色氨酸及其衍生物经常作为组成模块参与到天然产物的生物合成中, 本文概述了色氨酸几种不同的生物修饰方式, 包括烷基化修饰、卤化修饰、羟基化修饰、以及吲哚环的开环重排反应等。分析并总结色氨酸在天然产物生物合成中的作用可以增加我们对天然产物结构多样性的认识和推动天然产物生物合成机制的研究。 相似文献
994.
Structural analysis and antiviral activity of a sulfated galactan from the red seaweed Schizymenia binderi (Gigartinales, Rhodophyta) 总被引:4,自引:0,他引:4
Matsuhiro B Conte AF Damonte EB Kolender AA Matulewicz MC Mejías EG Pujol CA Zúñiga EA 《Carbohydrate research》2005,340(15):2392-2402
Aqueous extraction of gametophytic Schizymenia binderi afforded a polysaccharide composed of galactose and sulfate groups in a molar ratio of 1.0:0.89 together with uronic acids (6.8 wt%) and minor amounts of other neutral sugars. Alkali-treatment of the polysaccharide afforded a polysaccharide devoid of 3,6-anhydrogalactose. 13C NMR spectroscopy of the desulfated alkali-treated polysaccharide showed a backbone structure of alternating 3-linked beta-D-galactopyranosyl and 4-linked alpha-galactopyranosyl units that are predominantly of the D-configuration and partly of the L-configuration. Methylation, ethylation and NMR spectroscopic studies of the alkali-treated polysaccharide indicated that the sulfate groups are located mainly at positions O-2 of 3-linked beta-D-galactopyranosyl residue and at position O-3 of 4-linked-alpha-galactopyranosyl residues, the latter is partially glycosylated at position O-2. The sulfated galactan from S. binderi exhibited highly selective antiviral activity against Herpes simplex virus types 1 and 2, with selectivity indices (ratio cytotoxicity/antiviral activity) >1000 for all assayed virus strains. This compound was shown to interfere with the initial adsorption of viruses to cells. 相似文献
995.
In order to investigate the mechanisms creating and maintaining variability at the major histocompatibility (MH) class II alpha (DAA) locus we examined patterns of polymorphism in two isolated Atlantic salmon populations which share a common post-glacial origin. As expected from their common origin, but contrary to the observation at the MH class I locus, these populations shared the majority of DAA alleles: out of 17 sequences observed, 11 were common to both populations. Recombination seems to play a more important role in the origin of new alleles at the class II alpha locus than at the class I locus. A greater than expected proportion of sites inferred to be positively selected (potentially peptide binding residues, PBRs) were found to be involved in recombination events, suggesting a mechanism for increasing MH variability through an interaction between recombination and natural selection. Thus it appears that although selection and recombination are important mechanisms for the evolution of both class II alpha and class I loci in the Atlantic salmon, the pattern of variability differs markedly between these classes of MH loci. 相似文献
996.
The HLA-A*11 subtype includes 17 naturally occurring variants (-A*1101 to -A*1117) distributed among different ethnic groups worldwide. At present, only HLA-A*1101 has been characterized at the molecular, structural, and immunological level. Developing similar knowledge on other HLA-A*11 alleles is highly important for bone marrow and graft transplantation. This is also important to better understand disease linkages within the HLA-A*11 subtype given that HLA-A*11 molecules are associated with resistance to acquisition of HIV-1 infection and various autoimmune diseases. To broaden our understanding of HLA-A*11 molecules, we have determined the impact of natural polymorphism on the peptide-binding properties of several HLA-A*11 molecules: -A*1103, -A*1106, -A*1108, -A*1110, -A*1111, and -A*1114. We used an approach that combines data from thermal stability studies of recombinant, soluble forms of these molecules in complex with HIV-1 peptides, together with a detailed structural analysis of the resulting HLA-A*11 molecule/peptide complexes based on crystal and molecular model structures. Our analysis shows that natural polymorphism within the HLA-A*11 subtype is distributed along the 1 and 2 helices of the peptide-binding groove, in marked contrast to the pattern of polymorphism in HLA-A*2 and HLA-B*27 subtypes. Natural polymorphism greatly altered the abilities of individual -A*11 molecules to form stable complexes with HIV-1 peptides. In comparison to -A*1101, natural polymorphism altered the peptide-presenting properties of -A*1103, -A*1108, and -A*1114 and has the potential to affect the peptide-selecting properties of -A*1106, -A*1110, and -A*1111 as well. Overall, our findings suggest that HLA-A*11 molecules may stimulate alloreactive CD8+ cytotoxic T-cell responses. 相似文献
997.
Haug T Stensvåg K Olsen M ØM Sandsdalen E Styrvold OB 《Journal of invertebrate pathology》2004,85(2):112-119
A search for antibacterial activity in different organs/tissues of the horse mussel, Modiolus modiolus, was conducted. Dried samples were extracted with 60% (v/v) acetonitrile, containing 0.1% (v/v) trifluoroacetic acid. Due to high salt content, two liquid phases were obtained; an acetonitrile-rich phase (ACN extract) and an aqueous phase. The aqueous phase was further subjected to solid phase extraction (SPE). Eluates from SPE and ACN extracts were tested for antibacterial, lysozyme, and toxic activity. Antibacterial activity was demonstrated in extracts from several tissues, including plasma, haemocytes, labial palps, byssus, mantle, and gills. Some of the extracts were sensitive to proteinase K treatment, indicating antibacterial peptides and/or proteins. Lysozyme-like activity and toxic activity against Artemia salina nauplii was detected in fractions from the gills, mantle, muscle, and haemocytes. Results from this study indicate that M. modiolus is a promising source for identifying novel drug lead compounds. 相似文献
998.
An analysis of the histone H1 subtype, H1-1, in eight legumes belonging to four genera of the tribe Vicieae (Pisum, Lathyrus, Lens, and Vicia), revealed an extended region consisting of the tandemly repeated AKPAAK motifs. We named this region the Regular zone (RZ). The AKPAAK motifs are organized into two blocks separated by a short (two or six amino acids) intervening sequence (IS). The distal block contains six AKPAAK motifs, while the number of repeats in the proximal block varies from six in V. faba to seven in the other species. In V. hirsuta, the first two repeated units of the proximal block are octapeptides AKAKPAAK. The apparent rate of synonymous substitutions in the blocks of RZ is much higher than in the rest of the gene. This can be explained by repeat shuffling within each block. In the C-domain of the orthologous H1 subtype from Medicago truncatula (tribe Trifolieae), a region corresponding to the RZ of Vicieae species was found. It also consists of two blocks of AKPAAK motifs (four and three repeats in the proximal and distal blocks, respectively). These blocks are separated by a 20-amino acid IS. The first 20 amino acids of the Medicago RZ are not part of AKPAAK repeats. We hypothesise that the RZ has most probably evolved as a result of an expansion of AKPAAK repeats from two separate sites in the C-domain. This process started tens of millions of years ago and was most likely directed by positive selection.[Reviewing Editor: Dr. Martin Kreitman] 相似文献
999.
1000.
Carbon dioxide effects on stomatal responses to the environment and water use by crops under field conditions 总被引:2,自引:0,他引:2
Bunce JA 《Oecologia》2004,140(1):1-10
The structural complexity of habitats has been espoused as an important factor influencing natural-enemy abundance and food-web dynamics in invertebrate-based communities, but a rigorous synthesis of published studies has not heretofore been conducted. We performed a meta-analytical synthesis of the density response of natural enemies (invertebrate predators and parasitoids) to experimental increases and decreases in the structural complexity of their habitats using data from 43 published studies, reporting 62 independent taxa. Studies varied in structural complexity at two spatial scales (habitat and within-plant architecture) and comprised a diverse array of natural-enemy taxa (natural-enemy assemblage at large, the entire spider assemblage, hunting spiders, web-building spiders, mites, hemipterans, coccinellid beetles, carabid beetles, ants, and parasitoids). For all taxa combined, increasing habitat structure resulted in a large and significant increase in natural enemy abundance. Similarly, decreasing habitat structure significantly diminished natural enemy abundance. Separate meta-analyses at two spatial scales (habitat and within-plant architecture) found that increasing habitat complexity resulted in significant increases in abundance. In particular, manipulating levels of detritus at the habitat spatial scale had the strongest effect on natural enemy abundance. In general, most guilds of natural enemies were significantly affected when the structural complexity of the habitat was altered. Seven of nine natural enemy guilds were more abundant under conditions of increased habitat complexity, with hunting spiders and web-building spiders showing the strongest response followed by hemipterans, mites, and parasitoids. Spiders in particular were negatively affected when habitat structure was simplified. The mechanisms underlying the accumulation of natural enemies in complex-structured habitats are poorly known. However, refuge from intraguild predation, more effective prey capture, and access to alternative resources (alternative prey, pollen, or nectar), are possible candidates. Our analysis was unable to confirm that predators aggregate in complex-structured habitats because prey (mostly herbivores) are more abundant there. The results of this meta-analysis support the view that basal resources mediate top-down impacts on herbivores, and provide encouragement that manipulations of habitat complexity can be made in agroecosystems that will enhance the effectiveness of the natural enemy complex for more effective pest suppression.Electronic Supplementary Material Supplementary material is available in the online version of this article at 相似文献