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High resolution studies on the crystal structure of glycogen phosphorylase b have identified the catalytic site to which the substrate glucose-1-phosphate binds strongly with some local conformational changes. The site is situated 8 A (phosphate-to-phosphate distance) from pyridoxal phosphate, an essential cofactor of all glycogen phosphorylases. The catalytic site is 33 A from the site in the N-terminal portion of the molecule to which adenine nucleotides bind. In contrast to phosphorylase a (the active form of the enzyme which is phosphorylated at Ser 14), the positions of the first 19 residues of phosphorylase b are not well defined. 相似文献
84.
Olfactory signals are transduced by a large family of odorant receptor proteins, each of which corresponds to a unique glomerulus in the first olfactory relay of the brain. Crosstalk between glomeruli has been proposed to be important in olfactory processing, but it is not clear how these interactions shape the odour responses of second-order neurons. In the Drosophila antennal lobe (a region analogous to the vertebrate olfactory bulb), we selectively removed most interglomerular input to genetically identified second-order olfactory neurons. Here we show that this broadens the odour tuning of these neurons, implying that interglomerular inhibition dominates over interglomerular excitation. The strength of this inhibitory signal scales with total feedforward input to the entire antennal lobe, and has similar tuning in different glomeruli. A substantial portion of this interglomerular inhibition acts at a presynaptic locus, and our results imply that this is mediated by both ionotropic and metabotropic receptors on the same nerve terminal. 相似文献
85.
One of the most dramatic perturbations to the Earth system during the past 100 million years was the rapid onset of Antarctic glaciation near the Eocene/Oligocene epoch boundary (approximately 34 million years ago). This climate transition was accompanied by a deepening of the calcite compensation depth--the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution. Changes in the global carbon cycle, rather than changes in continental configuration, have recently been proposed as the most likely root cause of Antarctic glaciation, but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear. Here we use a global biogeochemical box model to test competing hypotheses put forward to explain the Eocene/Oligocene transition. We find that, of the candidate hypotheses, only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor. In our simulations, glacioeustatic sea-level fall associated with the growth of Antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves, leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth. At the same time, fresh limestones are exposed to erosion, thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater delta13C. Our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the Cenozoic era. 相似文献
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Organization and sequences of the variable, joining and constant region genes of the human T-cell receptor alpha-chain 总被引:1,自引:0,他引:1
An essential property of the immune system is its ability to generate great diversity in antibody and T-cell immune responses. The genetic and molecular mechanisms responsible for the generation of antibody diversity have been investigated during the past several years. The gene for the variable (V) region, which determines antigen specificity, is assembled when one member of each of the dispersed clusters of V gene segments, diversity (D) elements (for heavy chains only) and joining (J) segments are fused by DNA rearrangement. The cloning of the beta-chain of the T-cell antigen receptor revealed that the organization of the beta-chain locus, which is similar to that of immunoglobulin genes, is also composed of noncontiguous segments of V, D, J and constant (C) region genes. The structure of the alpha-chain seems to consist of a V and a C domain connected by a J segment. We report here that the human T-cell receptor alpha-chain gene consists of a number of noncontiguous V and J gene segments and a C region gene. The V region gene segment is interrupted by a single intron, whereas the C region contains four exons. The J segments, situated 5' of the C region gene, are dispersed over a distance of at least 35 kilobases (kb). Signal sequences, which are presumably involved in DNA recombination, are found next to the V and J gene segments. 相似文献
88.
D. J. Wilson 《Cellular and molecular life sciences : CMLS》1985,41(2):269-271
Summary The extraembryonic vascular membranes of 3-day-18-day chick embryos were examined for the presence of mast cells. As early as 3.5 ddays mast cells were found on the area vasculosa. It is suggested that these cells have a role in angiogenesis of the chick extraembryonic vascular system.The work is supported by The Wellcome Trust. 相似文献
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Mast cells are present during angiogenesis in the chick extraembryonic vascular system 总被引:2,自引:0,他引:2
D J Wilson 《Experientia》1985,41(2):269-271
The extraembryonic vascular membranes of 3-day-18-day chick embryos were examined for the presence of mast cells. As early as 3.5 days mast cells were found on the area vasculosa. It is suggested that these cells have a role in angiogenesis of the chick extraembryonic vascular system. 相似文献