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A protein of Mr 26 000 has been shown to be the major component of eye-lens junctions, which are similar but not identical to the gap junctions of liver and other tissues. Cyanogen bromide cleavage of the Mr 26 000 polypeptide from bovine lenses yields a major fragment of Mr 15 000 (fragment 1). However, if the junctions are first treated with trypsin or carboxypeptidase Y, cyanogen bromide treatment yields a fragment of reduced molecular weight. Since protease treatment has been shown to cleave residues almost exclusively from the carboxy-terminal end of the Mr 26 000 polypeptide, it follows that fragment 1 represents the carboxy-terminal half of this molecule, part of which is exposed to proteolytic attack outside the membrane. This latter result is corroborated by the fact that antisera which recognize both the Mr 26 000 polypeptide and fragment 1 fail to do so after preadsorption with intact membranes. In addition, comparative amino acid and partial sequence analyses of the Mr 26 000 polypeptide and fragment 1 indicate that fragment 1 is more hydrophilic in character, suggesting that much of the amino-terminal half of the Mr 26 000 polypeptide is buried within the lipid bilayer.  相似文献   

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Molecular weights and sedimentation coefficients have been measured for different oligomeric forms of phaseolin, the major storage protein in seeds of Phaseolus vulgaris L. The results indicate that phaseolin is a trimer (Mr = 150000) at neutral pH which aggregates further to a dodecamer form (Mr = 596000) at pH 4.5. The subunit size is in good agreement with the recently determined sequence molecular weight, if allowance is made for bound oligosaccharide and phytic acid moieties. The trimeric nature at neutral pH has been confirmed by chemical crosslinking studies using dimethylsuberimidate and dithiobis(succinimidylpropionate). Analyses of optical rotatory dispersion and circular dichroism data have been used to examine the corformation of phaseolin. In common with other seed globulins, a low proportion of α-helix (~ 10%) coupled with a high level of β-sheet (~50%) is predicted. These data are compared with a structural analysis based on the amino acid sequence of a phaseolin subunit polypeptide. The predicted level of α-helix is increased (~20%) when phaseolin is heated in sodium dodecyl sulphate, but not when the detergent is added at room temperature.  相似文献   

5.
Edelstein's model
?E=F(M, E)
,
?M=G(M, E)+D?2M?s2
,
M(s,0)=?(s)
,
E(s,0)=ψ(s)
, where τ ? 0 and ?∞<s<∞, F(M, E>) = (K1+Mm)(K2+Mm)?k1E, G(M, E)= k1E ? k2M, m ? 2, describes the behavior of two basic chemical species during the cellular differentiation in a linear ensemble of the same cell type. We prove the existence and uniqueness of a travelling-wavefront solution. We also demonstrate one kind of stability for this solution.  相似文献   

6.
The luminal plasma membrane of calf urinary bladder epithelium (urothelium) has been isolated by a method designed to preserve enzymic activity as well as structural integrity. The yield was about 80 μg per calf bladder. Low levels of 5′ nucleotidase, Mg2+-ATPase and (Na+ + K+)-ATPase activities were found in the luminal membrane fraction. Cerebroside was the major lipid present and dodecyl sulphate gel electrophoresis revealed a complex protein and glycoprotein composition in the whole membrane. A membrane fraction consisting of only the plaque areas was shown to have a simpler protein composition with major polypeptides of apparent Mr 12 000 and 22 000. These may associate to form a 30 000 apparent Mr complex which could represent the individual ‘particles’ of the dodecameric subunits seen by electron microscopy in the plaque regions.  相似文献   

7.
A plasma membrane-enriched vesicle fraction has been prepared from Trypanosoma brucei by sonication and differential centrifugation on sucrose gradients. This fraction is enriched 5-fold in the plasma membrane marker enzymes adenyl cyclase (EC 4.6.1.1) and ouabain-inhibitable, (Na+ + K+)-dependent adenosine triphosphatase (EC 3.6.1.3). It is also enriched up to 14-fold in iodinated surface proteins, and up to 4-fold in [3H]mannose-labeled glycoproteins, of which the major variable surface coat glycoprotein is the main constituent. Proteins of the plasma membrane fraction and other subcellular fractions have been identified by electrophoretic analysis in sodium dodecyl sulfate-polyacrylamide gradient slab gels. Several high molecular weight surface glycopeptides have been selectively investigated and partially characterized by a combination of metabolic labeling with [3H]mannose, lactoperoxidase-catalyzed surface iodination, and affinity chromatography on Con A-Sepharose. In addition to the major variable surface coat glycoprotein (estimated Mr = 58 000), there are several minor surface glycopeptides (Mr = 76 000, 86 000 and 92 000–100 000) which are apparent extrinsic membrane components, and two surface glycopeptides (Mr = 42 000 and 130 000) which are intrinsic membrane components.  相似文献   

8.
M1 cells, which are cell line cells established from myeloid leukemia cells of the SL strain mouse, can differentiate from blast cells (M1?) to mature macrophages (M1+) within 48 hr, when they are cultured with conditioned medium (CM) obtained from murine embryonic fibroblasts. While M1? cells have no phagocytic activity nor Fc receptor (FcR), M1+ cells possess both characteristics. The appearance of FcR is temperature-dependent and inhibited by a metabolic inhibitor, cycloheximide. FcR on M1+ cells is resistant to trypsin and pronase. M1+ cells improve the viability of macrophage-depleted SL splenic lymphocytes and restore the in vitro secondary plaque forming cell response of macrophage-depleted spleen cells to particulate and soluble antigens. M1? cells lack this macrophage-substituting capacity. Mm1 cells, mutant cells from M1 cells, having FcR and higher phagocytic activity than M1+ cells, are also devoid of this capacity.  相似文献   

9.
This paper presents an interpretation of fluorescence polarization measurements in lipid membranes which are labelled with the apolar probe 1,6-diphenyl-1,3,5-hexatriene. The steady-state fluorescence anisotropy, rS, is resolved into a fast decaying or kinetic component, rf, and an infinitely slow decaying or static component, r. The latter contribution, which predominates in biological membranes, is exclusively determined by the degree of molecular packing (order) in the apolar regions of the membrane; r is proportional to the square of the lipid order parameter. An empirical relation between rS and r is presented, which is in agreement with a prediction based on a theory of rotational dynamics in liquid crystals. This relation enabled us to estimate a lipid structural order parameter directly from simple steady-state fluorescence polarization measurements in a variety of isolated biological membranes. It is shown that major factors determining the order parameter in biomembranes are the temperature, the cholesterol and sphingomyelin content and (in a few systems) the membrane intrinsic proteins.  相似文献   

10.
(1) Analysis of the data from steady-state kinetic studies shows that two reactions between cytochrome c and cytochrome c oxidase sufficed to describe the concave Eadie-Hofstee plots (Km ? 1 · 10?8M and Km ? 2 · 10?5M). It is not necessary to postulate a third reaction of Km ? 10?6M. (2) Change of temperature, type of detergent and type of cytochrome c affected both reactions to the same extent. The presence of only a single catalytic cytochrome c interaction site on the oxidase could explain the kinetic data. (3) Our experiments support the notion that, at least under our conditions (pH 7.8, low-ionic strength), the dissociation of ferricytochrome c from cytochrome c oxidase is the rate-limiting step in the steady-state kinetics. (4) A series of models, proposed to describe the observed steady-state kinetics, is discussed.  相似文献   

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A method is described for the synthesis of high-molecular-weight carrier ampholytes for preparative isoelectric focusing of peptides. A giant polyethylene imine (Mr 40 000–60 000) is mixed with a linear gradient of acrylic acid in a flow-through system and let to react at 80°C for 70 h. Giant carrier ampholytes (Mrrange 50 000–90 000) are thus obtained. These compounds interact very strongly among themselves, probably not by hydrogen bonds or hydrophobic interactions but ionic bonds. In fact, the aggregates are split by high salt (NaCl) or by zwitterionic compounds (Gly, taurine) or at acidic or alkaline pHs. They appear to interact only weakly and reversibly with proteins and no interactions are apparent with model dipeptides (His-Ser, His-Met, His-Phe and His-Lys).  相似文献   

14.
Purified rabbit liver fructose 1,6-bisphosphatase is maximally active with 2 μM fructose 1,6-bisphosphate. Above this concentration the substrate becomes inhibotory. Inhibition is reversed by NH4+ or by physiological concentrations of K+. Substrate inhibition and its modification by monovalent cations may play a role in the regulation of gluconeogenesis at the step catalyzed by fructose 1,6-bisphosphatase.  相似文献   

15.
Two new crystal forms of oxidized uteroglobin have been obtained. An orthorhombic one (P21212, Z = 2, a = 44.48 (5) A?, b = 36.93 (5) A?, c = 32·34 (5) A?) and a monoclinic one (P21, Z = 2, a = 44.56 (5) A?, b = 46.06 (5) A?, c = 37.43 (4) A?, β = 120.92 ° (5)). Both were grown at pH ~7.0 and diffract to a resolution of 2·1 to 2·2 Å. Data collections for native crystals have been recorded with an automatic four-circle diffractometer.  相似文献   

16.
A capacitor microphone was used to measure the enthalpy and volume changes that accompany the electron transfer reactions, PQAhv P+Q?A and PQAQBhv P+QAQ?B, following flash excitation of photosynthetic reaction centers isolated from Rhodopseudomonas sphaeroides. P is a bacteriochlorophyll dimer (P-870), and QA and QB are ubiquinones. In reaction centers containing only QA, the enthalpy of P+Q?A is very close to that of the PQA ground state (ΔHr = 0.05 ± 0.03 eV). The free energy of about 0.65 eV that is captured in the photochemical reaction evidently takes the form of a substantial entropy decrease. In contrast, the formation of P+QAQ?B in reaction centers containing both quinones has a ΔHr of 0.32 ± 0.02 eV. The entropy change must be near zero in this case. In the presence of o-phenanthroline, which blocks electron transfer between Q?A and QB, ΔHr for forming P+Q?AQB is 0.13 ± 0.03 eV. The influence of flash-induced proton uptake on the results was investigated, and the ΔHr values given above were measured under conditions that minimized this influence. Although the reductions of QA and QB involve very different changes in enthalpy and entropy, both reactions are accompanied by a similar volume decrease of about 20 ml/mol. The contraction probably reflects electrostriction caused by the charges on P+ and Q?A or Q?B.  相似文献   

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Photosystem I particles prepared from spinach chloroplast using Triton X-100 were frozen in the dark with the bound iron-sulphur Centre A reduced. Illumination at cryogenic temperatures of such samples demonstrated the photoreduction of the second bound iron-sulphur Centre B. Due to electron spin-electron spin interaction between these two bound iron-sulphur centres, it was not possible to quantify amounts of Centre B relative to the other components of the Photosystem I reaction centre by simulating the line-shape of its EPR spectrum. However, by deleting the free radical signal I from the EPR spectra of reduced Centre A alone or both Centres A plus B reduced, it was possible to double integrate these spectra to demonstrate that Centre B is present in the Photosystem I reaction centre in amounts comparable to those of Centre A and thus also signal I (P-700) and X.Oxidation-reduction potential titrations confirmed that Centre A had Em ? ?550 mV, Centre B had Em ? ?585 mV. These results, and those presented for the photoreduction of Centre B, place Centre B before Centre A in the sequence of electron transport in Photosystem I particles at cryogenic temperatures. When both A and B are reduced, P-700 photooxidation is reversible at low temperature and coupled to the reduction of the component X. The change from irreversible to reversible P-700 photooxidation and the photoreduction of X showed the same potential dependence as the reduction of Centre B with Em ? ?585 mV, substantiating the identification of X as the primary electron acceptor of Photosystem I.  相似文献   

19.
A monoclinic crystal form (P21, a = 140.4 A?, b = 85.0 A?, c = 94.5 A?, β= 130.1 °) of Δ5-3-ketosteroid isomerase from Pseudomonas testosteroni (EC 5.3.3.1), grown at pH 7.0, has been characterized. Crystal-density measurements show that the asymmetric unit contains 12 protomers (Mr = 13,394).  相似文献   

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