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71.
Two groups control light-induced schiff base deprotonation and the proton affinity of asp(85) in the Arg(82)His mutant of bacteriorhodopsin 总被引:1,自引:0,他引:1 下载免费PDF全文
ES Imasheva SP Balashov TG Ebrey N Chen RK Crouch DR Menick 《Biophysical journal》1999,77(5):2750-2763
Arg(82) is one of the four buried charged residues in the retinal binding pocket of bacteriorhodopsin (bR). Previous studies show that Arg(82) controls the pK(a)s of Asp(85) and the proton release group and is essential for fast light-induced proton release. To further investigate the role of Arg(82) in light-induced proton pumping, we replaced Arg(82) with histidine and studied the resulting pigment and its photochemical properties. The main pK(a) of the purple-to-blue transition (pK(a) of Asp(85)) is unusually low in R82H: 1.0 versus 2.6 in wild type (WT). At pH 3, the pigment is purple and shows light and dark adaptation, but almost no light-induced Schiff base deprotonation (formation of the M intermediate) is observed. As the pH is increased from 3 to 7 the M yield increases with pK(a) 4.5 to a value approximately 40% of that in the WT. A transition with a similar pK(a) is observed in the pH dependence of the rate constant of dark adaptation, k(da). These data can be explained, assuming that some group deprotonates with pK(a) 4.5, causing an increase in the pK(a) of Asp(85) and thus affecting k(da) and the yield of M. As the pH is increased from 7 to 10.5 there is a further 2.5-fold increase in the yield of M and a decrease in its rise time from 200 &mgr;s to 75 &mgr;s with pK(a) 9. 4. The chromophore absorption band undergoes a 4-nm red shift with a similar pK(a). We assume that at high pH, the proton release group deprotonates in the unphotolyzed pigment, causing a transformation of the pigment into a red-shifted "alkaline" form which has a faster rate of light-induced Schiff base deprotonation. The pH dependence of proton release shows that coupling between Asp(85) and the proton release group is weakened in R82H. The pK(a) of the proton release group in M is 7.2 (versus 5.8 in the WT). At pH < 7, most of the proton release occurs during O --> bR transition with tau approximately 45 ms. This transition is slowed in R82H, indicating that Arg(82) is important for the proton transfer from Asp(85) to the proton release group. A model describing the interaction of Asp(85) with two ionizable residues is proposed to describe the pH dependence of light-induced Schiff base deprotonation and proton release. 相似文献
72.
Structure and function of the mammalian egg zona pellucida. 总被引:3,自引:0,他引:3
P Wassarman J Chen N Cohen E Litscher C Liu H Qi Z Williams 《The Journal of experimental zoology》1999,285(3):251-258
The zona pellucida is a thick extracellular coat that surrounds all mammalian eggs and preimplantation embryos. The zona pellucida supports communication between oocytes and follicle cells during oogenesis; protects oocytes, eggs, and embryos during development, and regulates interactions between ovulated eggs and free-swimming sperm during and following fertilization. Mutant females that produce eggs that lack a zona pellucida are infertile. The functions of the zona pellucida during fertilization now can be ascribed to certain of its glycoproteins. Here we describe some aspects of zona pellucida structure and function as they relate to mammalian fertilization. J. Exp. Zool. (Mol. Dev. Evol.) 285:251-258, 1999. 相似文献
73.
A comment on G. Litscher: Infrared thermography fails to visualize stimulation-induced meridian-like structures. Biomed. Eng. OnLine 2005, 4:38 (15 June 2005), with a response by the author. 相似文献
74.
Simultaneous continuous recording and processing of evoked potentials, various cardiovascular, and respiratory signals was realized with the aid of a PC-controlled monitoring system. The system is at present used for monitoring patients in intensive care units, during neurosurgical interventions and for sleep investigations in infants. 相似文献
75.
Background
The omics fields promise to revolutionize our understanding of biology and biomedicine. However, their potential is compromised by the challenge to analyze the huge datasets produced. Analysis of omics data is plagued by the curse of dimensionality, resulting in imprecise estimates of model parameters and performance. Moreover, the integration of omics data with other data sources is difficult to shoehorn into classical statistical models. This has resulted in ad hoc approaches to address specific problems. 相似文献76.
Background
Due to controversially discussed results in scientific literature concerning changes of electrical skin impedance before and during acupuncture a new measurement system has been developed. 相似文献77.
78.
ISABEL FERNANDES CLÁUDIA PASCOAL HELENA GUIMARÃES RUTE PINTO INÊS SOUSA FERNANDA CÁSSIO 《Freshwater Biology》2012,57(11):2306-2317
1. We investigated the effects of riparian plant diversity (species number and identity) and temperature on microbially mediated leaf decomposition by assessing fungal biodiversity, fungal reproduction and leaf mass loss. 2. Leaves of five riparian plant species were first immersed in a stream to allow microbial colonisation and were then exposed, alone or in all possible combinations, at 16 or 24 °C in laboratory microcosms. 3. Fungal biodiversity was reduced by temperature but was not affected by litter diversity. Temperature altered fungal community composition with species of warmer climate, such as Lunulospora curvula, becoming dominant. 4. Fungal reproduction was affected by litter diversity, but not by temperature. Fungal reproduction in leaf mixtures did not differ or was lower than that expected from the weighted sum of fungal sporulation on individual leaf species. At the higher temperature, the negative effect of litter diversity on fungal reproduction decreased with the number of leaf species. 5. Leaf mass loss was affected by the identity of leaf mixtures (i.e. litter quality), but not by leaf species number. This was mainly explained by the negative correlation between leaf decomposition and initial lignin concentration of leaves. 6. At 24 °C, the negative effects of lignin on microbially mediated leaf decomposition diminished, suggesting that higher temperatures may weaken the effects of litter quality on plant litter decomposition in streams. 7. The reduction in the negative effects of lignin at the higher temperature resulted in an increased microbially mediated litter decomposition, which may favour invertebrate‐mediated litter decomposition leading to a depletion of litter stocks in streams. 相似文献
79.
Seed survival and dispersal of an endemic Atlantic forest palm: the combined effects of defaunation and forest fragmentation 总被引:3,自引:0,他引:3
MAURO GALETTI CAMILA I. DONATTI ALEXANDRA S. PIRES PAULO R. GUIMARÃES JR PEDRO JORDANO 《Botanical journal of the Linnean Society. Linnean Society of London》2006,151(1):141-149
About 45 palm species occur in the Atlantic forest of Brazil, and most of them are affected by loss of seed dispersers resulting from forest fragmentation and hunting. Here we report the effects of habitat loss and defaunation on the seed dispersal system of an endemic palm, Astrocaryum aculeatissimum . We evaluated seed removal, insect and rodent seed predation, and scatter-hoarding in nine sites, ranging from 19 ha to 79 000 ha. We report the seedling, juvenile and adult palm densities in this range of sites. Endocarps remaining beneath the parent palm had a higher probability of being preyed upon by insects in small, mostly fragmented and more defaunated sites. The frequency of successful seed removal, scatter-hoarding and consumption by rodents increased in the larger, less defaunated sites. Successful removal and dispersal collapsed in small (< 1000 ha), highly defaunated sites and frequently resulted in low densities of both seedlings and juveniles. Our results indicate that a large fraction of Atlantic forest palms that rely on scatter-hoarding rodents may become regionally extinct due to forest fragmentation and defaunation. Current management practices including palm extraction and hunting pressure have a lasting effect on Atlantic forest palm regeneration by severely limiting successful recruitment of prereproductive individuals. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 141–149. 相似文献
80.
Zea systematics: ribosomal ITS evidence 总被引:10,自引:0,他引:10
Ribosomal internal transcribed spacer (ITS) sequences were used to evaluate
the phylogenetics of Zea and Tripsacum. Maximum likelihood and polymorphism
parsimony were used for phylogenetic reconstructions. Zea ITS nucleotide
diversity was high compared to other plant species, but approximately
equivalent to other maize loci. Coalescence of ITS alleles was rapid
relative to other nuclear loci; however, there was still much diversity
within populations. Zea and Tripsacum form a clade clearly differentiated
from all other Poaceae. Four Zea ITS pseudogenes were identified by
phylogenetic position and nucleotide composition. The phylogenetic position
of Z. mays ssp. huehuetenangensis was clearly established as basal to the
other Z. mays. The ITS phylogeny disfavored a Z. luxurians and Z.
diploperennis clade, which conflicted with some previous studies. The
introgression of Z. mays alleles into Z. perennis and Z. diploperennis was
also established. The ITS data indicated a near contemporary divergence of
domesticated maize and its two closest wild relatives.
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