共查询到20条相似文献,搜索用时 31 毫秒
1.
The effects of binding of various effector ligands on the dynamics of aspartate transcarbamylase (ATCase, c 6r 6) and on its regulatory (r 2) and catalytic (c 3) subunits were characterized by examining succinimide quenching of the intrinsic fluorescence, and by measurement of the lifetime-resolved anisotropies. The lifetimes of the tryptophan residues in c 3 and c 6r 6 are about 1.7 ns while those of tyrosine residues in r 2 are 2.7 ns. These lifetimes are not significantly altered by the binding of various substrates, substrate analogs and nucleotides. The effects of ligand binding on the accessibility of both tyrosine and tryptophan residues to the quencher are modest in all cases, though the changes are in the same direction as seen using other physicochemical techniques such as hydrogen exchange (M. Lennick and N.M. Allewell, Proc. Natl. Acad. Sci. U.S.A. 78 (1981) 6759). The tryptophan residues in both c 3 and c 6r 6 are immobilized whereas the tyrosine residues of r 2 have some motional freedom. Ligands have no effect on the immobilized tryplophan residues in c 3 and c 6r 6, while binding of nucleotides to r 2 results in a small decrease in the motional freedom of the tyrosine residues. These results suggest that the protein matrix around the aromatic arnino acids in r 2, c 3 and c 6r 6 is rather rigid and that local effects of ligands on the dynamics of these residues, and that of the surrounding protein matrix, are minor. They are in general agreement with the results of the crystal structure determination (R.B. Honzatko et al., J. Mol. Biol. 160 (1982) 219). 相似文献
2.
When human spermatozoa are extracted in the presence of 0.05 M benzamidine, the resulting solutions show a time dependent, sigmoidal increase of trypsin-like activity upon incubation at pH 8. Gel permeation chromatography of these extracts separates two species, P 1 and P 2, with apparent molecular weights of 75,000 and 42,000 respectively. P 1 and P 2 are both autoactivatable at pH 7–8 and the kinetic parameters of activated P 1 and P 2 are indistinguishable from those of human acrosin. That P 1 and P 2 are inactive precursors of human acrosin is shown by the fact that, in the presence of benzamidine, they are obtained instead of and in greater yield than acrosin. That P 1 and P 2 are zymogens is shown by the features of the activation process. 相似文献
4.
Possibilities of the linear-polarized infrared (IR-LD) spectroscopy of oriented colloid suspensions in nematic liquid crystals, for structural and local structural elucidation for first time are demonstrated of inorganic compounds and glasses. The advantages of the method for tellurite and borate glasses are shown. The IR-band assignment of the typical local structural units in the glasses are proposed by a comparison with the IR-characteristics of appropriate crystalline analogues as α-TeO 2, V 2O 5, MoO 3 · H 2O and its high temperature form. The IR-spectroscopic characteristics of BO 3, BO 4 and boroxol ring are elucidated, using crystalline β-BaB 2O 4, SrB 4O 7, H 3BO 3 and B 2O 3 as model systems, where the structural moieties have been refined by single crystal X-ray diffraction. 相似文献
5.
Testicular levels of prostaglandin E 2 and F 2α were measured in decapsulated adult rat testis following hCG stimulation. Basal levels were, respectively, 342 ± 74 and 502 ± 89 pg/testis. Following hCG administration these basal values are not significantly modified up to 2 hours. From 2 to 24 hours the concentrations are clearly increased above the basal level: at 12 hrs they are 1925 ± 165 for E 2 and 3200 ± 190 for F 2α. Levels are back to normal at 48 hrs and remain so until 144 hrs. An identical pattern of prostaglandin release is observed in vitro in Leydig cell preparations isolated at different times following in vivo hCG injection. This suggests that prostaglandins are secreted by Leydig cells. In hypophysectomized animals the release of both prostaglandins E 2 and F 2α is similar to controls indicating that prostaglandin secretion is not directly linked to testosterone production. alternatively testosterone injections (10 mg) does not modify prostaglandin levels. Binding sites for prostaglandins E 1, E 2 and F 2α are present on the Leydig cells and consequently Leydig cell function may be modulated by endogenous or exogenous prostaglandins. Their level is slightly increased at 24 hrs following hCG stimulation. Since the acute changes in prostaglandin E 2 and F 2α secretion occur during the period of “desensitization” and of acute “down regulation” of the LH-hCG receptor in the Leydig cells it is suggested that prostaglandins are involved in both phenomena. 相似文献
6.
The activities of populations in complex anaerobic microbial communities that perform complete bioconversion of organic matter to CH 4 and CO 2 are reviewed. Species of eubacteria produce acetate, H 2, and CO 2 from organic substrates, and methanogenic species of archaebacteria transform the acetate, H 2, and CO 2 to CH 4. The characteristics and activities of the methanogenic bacteria are described. The impact of the use of H 2 by methanogens on the fermentations that produce acetate, H 2, and CO 2 and the importance of syntrophy in complete bioconversion are discussed. 相似文献
7.
Phenolic components and peroxidases are localized in vacuoles. Vacuolar peroxidase can oxidize phenolics when H2O2 is formed in vacuoles or tonoplasts, or when H2O2 formed outside of vacuoles is diffused into the organelles. In a mixture of phenolics containing a good and a poor substrate for peroxidase, a radical transfer reaction is possible from the radicals of the good substrate to the poor substrate, resulting in the enhancement of oxidation of the poor substrate. Phenoxyl radicals formed by peroxidase-dependent reactions are reduced by ascorbate in vacuoles. So, as long as ascorbate is present in vacuoles, the accumulation of oxidation products of phenolics is not significant. This suggests that ascorbate/phenolics/peroxidase systems in the vacuoles can scavenge H2O2. During aging, some phenolics are accumulated in vacuoles and the apoplast, and the accumulated phenolics are oxidized to brown components by peroxidase-dependent reactions. The brown components can produced O2
? and H2O2 by autooxidation. The significance and the mechanisms of browning are discussed in tobacco leaves and onion scales. 相似文献
8.
The effects of Cd‐doping on the thermoelectric properties of Sn 1–xPb xTe are investigated and compared to the properties of the corresponding Sn 1–xPb xTe solid solutions. The addition of Cd results in a reduction in the carrier concentration and changes in the physical properties, as well as in the conduction type of Sn 1–xPb xTe. A significant increase in the power factor accompanied by a reduction in the thermal conductivity result in a higher figure of merit (ZT) for (Sn 1–xPb x) 0.97Cd 0.03Te than that of undoped Sn 1–xPb xTe. The maximum ZT (~0.7) values are observed for p‐type material with x = 0.36 at 560 K. Much higher values (ZT ~ 1.2 at 560 K for x = 0.73) are obtained on n‐type samples. 相似文献
10.
Both sphincter and dilator muscle preparations of the cat iris contract to prostaglandins; F 2α and E 2 are the most potent and A 1 and B 1 the least. Ciliary muscle strips relax to PG's provided that the strips are precontracted. E 1, E 2 and often F 2α are more potent relaxants than the remaining PG's. The effects of PG's are not altered by α or β blockade nor by atropine; however, propranolol blocks the PG induced relaxation of the ciliary muscle. The effects of PG's on the sphincter are antagonized by catecholamines; but the latter act synergistically in contracting the dilator and in relaxing the ciliary muscle. Indomethacin markedly potentiates the effects of PG's on all three muscle preparations. 相似文献
11.
In the central nervous system (CNS), the inhibitory transmitter GABA interacts with three subtypes of GABA receptors, type
A, type B, and type C. Historically, GABA receptors have been classified as either the inotropic GABA A receptors or the metabotropic GABA B receptors. Over the past 10 yr, studies have shown that a third class, called the GABA C receptor, also exists. GABA C receptors are found primarily in the vertebrate retina and to some extent in other parts of the CNS. Although GABA A and GABA C receptors both gate chloride channels, they are pharmacologically, molecularly, and functionally distinct. The ρ subunit
of the GABA C receptor, which has about 35% amino acid homology to GABA A receptor subunits, was cloned from the retina and, when expressed in Xenopus oocytes, has properties similar to retinal GABA C receptors. There are probably distinct roles for GABA C receptors in the retina, because they are found on only a subset of neurons, whereas GABA A receptors are ubiquitous. This article reviews recent electrophysiological and molecular studies that have characterized
the unique properties of GABA C receptors and describes the roles that these receptors may play in visual information processing in the retina. 相似文献
12.
[Cu(NCCH 3) 6][B(C 6F 5) 4] 2 and [Cu(NCCH 3) 6][B{C 6H 3(CF 3) 2} 4] 2 are immobilized on poly(4-vinylpyridine). Both resulting materials (Cu(II) complexes immobilized on polymer) are applicable as catalysts for the cyclopropanation of olefins at room temperature. The immobilized Cu(II) compounds are quite stable and recyclable for several catalytic runs, however with some decrease in the catalytic activity. 相似文献
13.
The quadrupole split asymmetric 125Te Mössbauer spectrum recorded from the compound (Te 2) 2(I 2), in which monomolecular planar layers of iodine molecules are intercalated between layers of tellurium, is a reflection of the distorted environment of tellurium atoms in a two-dimensional layered compound in which the elongated flat crystals are preferentially orientated. The differences between the Mössbauer parameters recorded from (Te 2) 2(I 2) and those recorded from elemental tellurium and the tellurium(0) species in the compound Te 3Cl 2 are associated with small differences between the environments of tellurium in the three compounds. The Mössbauer spectra recorded from (Te 2) 2(I 2) are consistent with a recently proposed model on which the electronic band structure of (Te 2) 2(I 2) has been derived. 相似文献
14.
Transport of inorganic phosphate (P i) through plant membranes is mediated by a number of families of transporter proteins. Studies on the topology, function, regulation and sites of expression of the genes that encode the members of these transporter families are enabling roles to be ascribed to each of them. The Pht1 family, of which there are nine members in the Arabidopsis genome, includes proteins involved in the uptake of P i from the soil solution and the redistribution of P i within the plant. Members of this family are H 2PO 4
–/H + symporters. Most of the genes of the Pht1 family that are expressed in roots are up-regulated in P-stressed plants. Two members of the Pht1 family have been isolated from the cluster roots of white lupin. These same genes are expressed in non-cluster roots. The evidence available to date suggests that there are no major differences between the types of transport systems that cluster roots and non-cluster roots use to acquire P i. Differences in uptake rates between cluster and non-cluster roots can be ascribed to more high-affinity P i transporters in the plasma membranes of cluster roots, rather than any difference in the characteristics of the transporters. The efficient acquisition of P i by cluster roots arises primarily from their capacity to increase the availability of soil P i immediately adjacent to the rootlets by excretion of carboxylates, protons and phosphatases within the cluster. This paper reviews P i transport processes, concentrating on those mediated by the Pht1 family of transporters, and attempts to relate those processes involved in P i acquisition to likely P i transport processes in cluster roots. 相似文献
15.
When rat platelets are incubated with phospholipase A 2, thromboxane A 2-like activity and prostaglandins are formed. The amounts are approximately similar, whether aggregation is induced after the incubation or not. No aggregation is observed when the platelets are incubated with phospholinase A 2. In the platelets of essential fatty acid deficient rats, only small amounts of thromboxane A 2-like activity and prostaglandins are formed. No formation of these substances occurs when human and rabbit platelets are incubated with phospholipase A 2.The results indicate that formation of thromboxane A 2-like activity enhances aggregation in rat platelets, but that aggregation is not induced. 相似文献
16.
Intraspecific hybrids are always fertile. In interspecific crosses total or nearly total seed sterility regularly occurs in F 1 or F 2 individuals or even in the involved pods of P generations: Sterility barriers range from F 1 failure to moderate reduction of average hybrid fertility, and are reflected by the taxonomy of the group. Diploid F 1 hybrids are able to produce triploid descendants, triploid F 1 plants however pentaploid to hexaploid ones. Intermediary and dominant inheritance of indument characters and new character combinations in F 2 generations are observed. 相似文献
17.
多不饱和脂肪酸具有包括离子通道在内的众多作用靶点,通过作用于这些靶点,可以有效保护免疫系统、神经系统和心血管系统的功能,在一定程度上保护人体健康。电压门控钾离子通道家族K V7通道和大电导钙离子激活的钾离子通道(BK Ca)广泛表达于机体的各类组织中,具有重要的生理或病理功能。本综述围绕K V7和BK Ca通道,根据对已有报道的汇总,多不饱和脂肪酸可以增大K V7和BK Ca通道的电流幅值,其中对K V7通道电流的影响主要是改变其电压依赖特性和最大电导值,而对BK Ca通道电流的影响主要是改变其孔道区域关闭态的构象。此外,多不饱和脂肪酸对K V7和BK Ca通道功能的调节也会受到共表达的辅助亚基影响,但相关机制有待进一步阐明。深入理解多不饱和脂肪酸对K V7和BK Ca通道调节作用效果和分子机制,有助于全面理解K V7和BK 相似文献
18.
Monoclonal antibodies are a remarkably successful class of therapeutics used to treat a wide range of indications. There has been growing interest in smaller antibody fragments such as Fabs, scFvs and domain antibodies in recent years. In particular, the development of human V H and V L single-domain antibody therapeutics, as stand-alone affinity reagents or as “warheads” for larger molecules, are favored over other sources of antibodies due to their perceived lack of immunogenicity in humans. However, unlike camelid heavy-chain antibody variable domains (V HHs) which almost unanimously resist aggregation and are highly stable, human V Hs and V Ls are prone to aggregation and exhibit poor solubility. Approaches to reduce V H and V L aggregation and increase solubility are therefore very active areas of research within the antibody engineering community. Here we extensively chronicle the various mutational approaches that have been applied to human V Hs and V Ls to improve their biophysical properties such as expression yield, thermal stability, reversible unfolding and aggregation resistance. In addition, we describe stages of the V H and V L development process where these mutations could best be implemented. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody. 相似文献
19.
The analysis of hydrocarbons and fatty acids in ten fern species indicate unique differences from plants in a higher phylogenetic order. Significant concentrations of fatty acids above C 20 are present. Distributions of hydrocarbons range from C 15–C 33 with a trend towards two maxima at C 17 and C 29. Homologous series of n-alkenes are present in all species. Pristane and phytane are large components representing up to 27% of the alkanes. Distinct alkane and fatty acid differences between fern families are observed while species variations within families are slight. 相似文献
20.
Abstract Atomistic lattice calculations are reported of the energetics associated with the formation of oxygen interstitial defects in La 2CuO 4, Nd 2CuO 4, LaNdCuO 4, and YBa 2Cu 3O 6. The location and charge state of the additional oxygen are considered and hole-interstitial association energies estimated. The last of these varies considerably from system to system. The implications for the high- Tc behaviour of these materials, including the effects of fluorination on La 2CuO 4 and Nd 2CuO 4, are discussed in detail. 相似文献
|