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151.
Shainoff JR Smejkal GB DiBello PM Sung SS Bush LA Di Cera E 《The Journal of biological chemistry》2002,277(22):19367-19373
Fibrin formation depends on the release of the two N-terminal fibrinopeptides A (FPA) from fibrinogen, and its formation is accompanied by an intermediate, alpha-profibrin, which lacks only one of the FPA. In this study, we confirm that the maximal levels of alpha-profibrin found over the course of thrombin reactions with human fibrinogen are only half of what would be expected if the first and second FPA were being released independently with equal rate constants. The rapidity of release of the fibrinopeptides by thrombin had been shown to depend on an allosteric transformation that is induced when Na(+) binds to a site defined by the 215-227 residues of thrombin, a transformation that results in the exposure of its fibrinogen-binding exosites transforming the thrombin from a slow to a fast acting form toward fibrinogen. When choline was substituted for sodium to transform thrombin to its slow form, the maximal levels of alpha-profibrin rose to those expected for independent release of the two FPA. Thus, it is only the fast thrombin that releases the second FPA fast, and that fast release only occurs when both FPA are present because of a partial coupling of its release with that of the first FPA. The release of the FPA from purified alpha-profibrin with the first FPA already missing is no faster than the release of any FPA. Surprisingly, we also found that slow thrombin became increasingly transformed to a fast form in the absence of sodium when the fibrinogen was elevated to high concentrations. This potentiation by concentrated fibrinogen also occurs with the recombinant mutant thrombin (Y225P), which is otherwise slow in both the presence and absence of Na(+). The potentiation of thrombin by fibrinogen must be short-lived so that the thrombin reverts to its slow acting form in the interim among encounters with other fibrinogen molecules in dilute fibrinogen solutions lacking Na(+), whereas at high fibrinogen concentrations the thrombin encounters other molecules before it reverts back to the slow form. 相似文献
152.
A method is introduced to identify amino acid residues that dictate the functional diversity acquired during evolution in a protein family. Using over 80 enzymes of the chymotrypsin family, we demonstrate that the general organization of the phylogenetic tree and its functional branch points are fully accounted for by a limited number of residues that cluster around the active site of the protein and define the contact region with the P1-P4 residues of substrate. 相似文献
153.
Dorsoventral asymmetry in flowers of Antirrhinum depends on expression of the cycloidea gene in dorsal regions of floral meristems. To determine how cycloidea might be regulated we analysed its expression in several contexts. We show that cycloidea is activated shortly after floral induction, and that in addition to flowers, cycloidea can be asymmetrically expressed in shoots, even though these shoots show no marked dorsoventral asymmetry. Shoots expressing cycloidea include secondary branches lying just below the inflorescence, and shoots of floricaula mutants. Asymmetric cycloidea expression may also be observed within organ primordia, such as the sepals of terminal flowers produced by centroradialis mutants. Later expression of cycloidea within flowers can be modified by mutations in organ identity genes. Taken together, the results suggest that cycloidea can respond to a common dorsoventral pre-pattern in the apex and that the specific effects of cycloidea on the flower depend on interactions with floral-specific genes. 相似文献
154.
Rescigno A Sollai F Pisu B Rinaldi A Sanjust E 《Journal of enzyme inhibition and medicinal chemistry》2002,17(4):207-218
The active site of tyrosinase is described with a view to depicting its interactions with substrates and inhibitors. Occurrence and mechanism(s) of tyrosinase-mediated browning of agrofood products are reviewed, with regard to both enzymic and chemical reactions, and their control, modulation, and inhibition. Technical and applicational implications are discussed. 相似文献
155.
156.
Santucci D Francia N Aloe L Alleva E 《Journal of gravitational physiology : a journal of the International Society for Gravitational Physiology》2002,9(1):P39-P40
Behavioural responses of CD-1 mice exposed to 2 g hypergravity (HG; 60 or 120 min) were investigated during adolescence and at adulthood. To characterise motion sickness (MS), pica behaviour, a reliable MS index measured through kaolin consumption, and spontaneous activity were evaluated before, during and after HG exposure. Short- and/or long-lasting effects on emotional responses, exploratory behaviour and spatial learning performances were also investigated and brain levels of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) assessed. An increased pica behaviour during post rotational days and a reduction in spontaneous activity during rotation indicated a mild sickness associated with HG, and susceptibility to MS was sex and age related. Short- and long-lasting effects of exposure were also observed, rotated mice showing altered emotional/anxiety behavioural profiles and impaired spatial learning performances. Moreover, central NGF levels were markedly increased after rotation, while minor changes were observed in BDNF levels. 相似文献
157.
Determinants of the recognition of enteroviral cloverleaf RNA by coxsackievirus B3 proteinase 3C 下载免费PDF全文
The initiation of enteroviral positive-strand RNA synthesis requires the presence of a functional ribonucleoprotein complex containing a cloverleaf-like RNA secondary structure at the 5' end of the viral genome. Other components of the ribonucleoprotein complex are the viral 3CD proteinase (the precursor protein of the 3C proteinase and the 3D polymerase), the viral 3AB protein and the cellular poly(rC)-binding protein 2. For a molecular characterization of the RNA-binding properties of the enteroviral proteinase, the 3C proteinase of coxsackievirus B3 (CVB3) was bacterially expressed and purified. The recombinant protein is proteolytically active and forms a stable complex with in vitro-transcribed cloverleaf RNA of CVB3. The formation of stable complexes is also demonstrated with cloverleaf RNA of poliovirus (PV) 1, the first cloverleaf of bovine enterovirus (BEV) 1, and human rhinovirus (HRV) 2 but not with cloverleaf RNA of HRV14 and the second cloverleaf of BEV1. The apparent dissociation constants of the protein:RNA complexes range from approx. 1.7 to 4.6 microM. An electrophoretic mobility shift assay with subdomain D of the CVB3 cloverleaf demonstrates that this RNA is sufficient to bind the CVB3 3C proteinase. Binding assays using mutated versions of CVB3 and HRV14 cloverleaf RNAs suggest that the presence of structural features rather than a defined sequence motif of loop D are important for 3C proteinase-RNA interaction. 相似文献
158.
Recent delineation of the serine protease cascade controlling dorsal-ventral patterning during Drosophila embryogenesis allows this cascade to be compared with those controlling clotting and complement in vertebrates and invertebrates. The identification of discrete markers of serine protease evolution has made it possible to reconstruct the probable chronology of enzyme evolution and to gain new insights into functional linkages among the cascades. Here, it is proposed that a single ancestral developmental/immunity cascade gave rise to the protostome and deuterostome developmental, clotting and complement cascades. Extensive similarities suggest that these cascades were built by adding enzymes from the bottom of the cascade up and from similar macromolecular building blocks. 相似文献
159.
160.
NAD glycohydrolases are enzymes that catalyze the hydrolisis of NAD to produce ADP-ribose and nicotinamide. Regulation of these enzymes has not been fully elucidated. We have identified an NAD-glycohydrolase activity associated with the outer surface of the plasma membrane in human lung epithelial cell line A549. This activity is negatively regulated by its substrate -NAD but not by -NAD. Partial restoration of NADase activity after incubation of the cells with arginine or histidine, known ADP-ribose acceptors, suggests that inhibition be regulated by ADP-ribosylation. A549 do not undergo to apoptosis upon NAD treatment indicating that this effect be likely mediated by a cellular component(s) lacking in epithelial cells. 相似文献