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关于"pH对酶活性的影响"3种实验方案的比较分析 总被引:1,自引:1,他引:0
关于"pH对酶活性的影响"的实验教学中,学生设计了3种不同的实验方案,在课堂交流研讨的过程中引起学生激烈的争论.现将这3种实验方案陈述如下,并将学生的争论进行了整理. 相似文献
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人类关注公正, 非人灵长类也表现出公正行为。本文先以现有研究资料为基础, 以理毛为例分析后认为, 非人灵长类关注投入—收益的对称性, 说明它们可能具备不公正规避这一心理特质; 关于非人灵长类公正行为的实验也表明, 它们不仅比较自身的投入—收益对称性, 而且能在社会比较过程中与其它个体相比。有实验得出期望假设和挫折效应能更好地解释被试的行为, 本文认为, 这些实验结论不一致的主要原因, 是研究者未充分考虑"投入"对被试行为的影响。文章在最后进行了总结并提出了三点研究展望。 相似文献
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Mammalian carbonic anhydrase III has previously been shown to catalyze the hydrolysis of p-nitrophenyl phosphate in addition to possessing the conventional CO2 hydratase and p-nitrophenylacetate esterase activities. Modification of pig muscle carbonic anhydrase III with the arginine reagent phenylglyoxal yielded two clearly distinctive results. Reaction of the enzyme with phenylglyoxal at concentrations equivalent to those of the enzyme yielded stoichiometric inactivation titration of the enzyme's phosphatase activity, approaching 100% loss of activity with the simultaneous modification of one arginine residue, the latter based on a 1:1 reaction of phenylglyoxal with arginine. At this low ratio of phenylglyoxal to enzyme, neither the CO2 hydratase activity nor the acetate esterase activity was affected. When the modification was performed with a significant excess of phenylglyoxal, CO2 hydratase and acetate esterase activities were diminished as well. That loss of activity was accompanied by the incorporation of an additional half dozen phenylglyoxals and, presumably, the modification of an equal number of arginine residues. The data in their entirety are interpreted to show that the p-nitrophenylphosphatase activity is a unique property of carbonic anhydrase III and that excessive amounts of the arginine-modifying reagent lead to unspecific structural changes of the enzyme as a result of which all of its enzymatic activities are inactivated. 相似文献
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The relationship between aspartokinase activity and fruiting body formation in Myxococcus xanthus was investigated. Two required amino acids, methionine and isoleucine, which stimulated the enzyme in vitro also inhibited fruiting body formation when added to 0.1% Casitone agar. Threonine, a potent feedback inhibitor of the aspartokinase, completely reversed the effects of methionine and isoleucine both on enzyme activity and fruiting body formation. A mutant, M. xanthus FB-S, which had the unusual property of forming fruiting bodies on 1.0% Casitone agar, also exhibited an altered regulation of aspartokinase activity. Spermidine, which is a strong stimulator of the enzyme in vitro, interfered with the developmental cycle of both M. xanthus FB and FS-S. During glycerol induction of myxospores the level of aspartokinase dropped more than 75% during the first hour. These data indicate a strong correlation between aspartokinase activity and the induction of the developmental cycle in M. xanthus. It is suggested that the decrease in aspartokinase activity results in diaminopimelic acid starvation, blockage of cell wall growth, and subsequent induction of the developmental cycle. 相似文献
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Merlino A Ercole C Picone D Pizzo E Mazzarella L Sica F 《Journal of molecular biology》2008,376(2):427-437
Bovine seminal ribonuclease exists in the native state as an equilibrium mixture of a swapped and an unswapped dimer. The molecular envelope and the exposed surface of the two isomers are practically indistinguishable and their diversity is almost completely buried in the interior of the protein. Surprisingly, the cytotoxic and antitumor activity of the enzyme is a peculiar property of the swapped dimer. This buried diversity comes into light in the reducing environment of the cytosol, where the unswapped dimer dissociates into monomers, whereas the swapped one generates a metastable dimeric form (NCD-BS) with a quaternary assembly that allows the molecule to escape the protein inhibitor of ribonucleases. The stability of this quaternary shape was mainly attributed to the combined presence of Pro19 and Leu28. We have prepared and fully characterized by X-ray diffraction the double mutant P19A/L28Q (PALQ) of the seminal enzyme. While the swapped and unswapped forms of the mutant have structures very similar to that of the corresponding wild-type forms, the non-covalent form (NCD-PALQ) adopts an opened quaternary structure, different from that of NCD-BS. Moreover, model building clearly indicates that NCD-PALQ can be easily sequestered by the protein inhibitor. In agreement with these results, cytotoxic assays have revealed that PALQ has limited activity, whereas the single mutants P19A and L28Q display cytotoxic activity against malignant cells almost as large as the wild-type enzyme. The significant increase in the antitumor activity, brought about by the substitution of just two residues in going from the double mutant to the wild-type enzyme, suggests a new strategy to improve this important biological property by strengthening the interface that stabilizes the quaternary structure of NCD-BS. 相似文献
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一株产纤溶酶菌株的分离鉴定及其纤溶组分分析 总被引:1,自引:0,他引:1
【目的】筛选性能良好的产纤溶酶菌株,对菌株进行多项分类鉴定,分析其纤溶酶系的组成特征及纤溶能力。【方法】通过酪蛋白培养基初筛,琼脂-纤维蛋白双层平板复筛,从海泥、土壤等环境中筛选纤维蛋白降解菌,以尿激酶为标准测定纤溶酶活性。通过形态学、生理生化特征研究,结合16S rDNA基因序列分析菌株种类及系统分类地位。通过SDS-PAGE和纤维蛋白酶谱法分析胞外纤溶酶系的组成特征。【结果】筛选到一株能降解纤维蛋白的细菌CNY16,鉴定其为沙福芽孢杆菌(Bacillus safensis)。该酶为胞外酶,SDS-PAGE和纤维蛋白酶谱结果表明该纤溶酶系有至少两种分子量大小不同的纤溶酶,分别约33 kD和23 kD。能有效溶解血块中纤维蛋白,并且对红细胞无降解作用。【结论】细菌CNY16是一株新的纤溶酶产生菌,纤溶酶活性及稳定性较好,具有潜在开发价值。为获取新型纤溶酶提供了一种新的菌源。 相似文献
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A simple kinetic model of hysteretic enzymes with the influx of substrate or ion (transported by the enzyme) is considered. Two alternative steady activity levels are shown to arise in the system with a hysteretic enzyme. The transition between these levels can proceed in an oscillatory manner. The duration of the initial steady activity level is shown to be determined by the initial substrate (or ion) level, and the oscillatory transition between the activity levels is the property of hysteretic enzymes. It was shown for plasma membrane Ca2+-ATPase as an example that the level of the signal can be encoded into the time interval in which the enzyme retains the memory about this signal. 相似文献
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Allosteric enzymes are part of a unique class of enzymes which regulate metabolic pathways. On the molecular level, allosteric regulation is the result of interactions between discrete binding sites on the enzyme. In order to accommodate these multiple binding sites, allosteric enzymes have evolved with oligomeric quaternary structures. However, only a few oligomeric enzymes are known to have regulatory interactions between binding sites. Is regulatory activity an inherent property of oligomeric enzymes? The trimeric Bacillus subtilis aspartate transcarbamoylase catalyzes the first committed step of the pyrimidine biosynthetic pathway and is not known to be a regulatory enzyme. When an alanine residue is substituted for the active-site residue Arg-99 by site-specific mutagenesis, the regulatory activity of homotropic substrate cooperativity (Hill coefficient of 1.5) is observed in the resulting mutant enzyme. These results suggest that homotropic regulation may have evolved by a relatively small number of mutations to an oligomeric enzyme. 相似文献
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The effects of heparin on trypsin have recently been demonstrated to involve inhibition of catalytic activity and degradation of the enzyme by means of an oxidative mechanism. The possibility that alpha-phenyl N-tert-butylnitrone protects heparin-induced radical formation on trypsin was investigated by measuring amidolytic activity and changes in the structure of trypsin in the presence of heparin with and without alpha-phenyl N-tert-butylnitrone. The results show that alpha-phenyl N-tert-butylnitrone does not only prevent, but it even significantly enhances effects of heparin on the enzyme. This is due to the unique property of alpha-phenyl N-tert-butylnitrone, independently of spin-trapping capacity, to modify the trypsin structure by binding irreversibly to the catalytic triad, at sites distinct from those to which heparin binds. 相似文献