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991.
In the HM5 mutant of Haemophilus influenzae, which carries a mutation in the rec-1 gene region and in which the replication of donor-recipient DNA complexes formed in transformation is inhibited, the transformation frequency could be greatly enhanced by inhibition of protein synthesis during transformation, indicating that transformation in the HM5 mutant induces the synthesis of a protein that inhibits the replication of the donor-recipient DNA complexes. This induction occurred in an early step of the recombination. Synthesis of the wild-type Rec-1 protein after transformation of the HM5 mutant with wild-type DNA could diminish the inhibiting effect on DNA replication. The HM5 mutant synthesized an altered Rec-1 protein (molecular weight, 38,000) whose pI differed from that of the wild type. As a result of the mutation in the rec-1 gene, two other proteins (molecular weights, 37,500 and 43,000) are lacking in the HM5 mutant.  相似文献   
992.
31P and 13C nuclear magnetic resonance (NMR) experiments were performed on suspensions of the phototrophic bacterium Chromatium vinosum incubated anaerobically in the dark. 31P NMR spectra revealed that during prolonged dark incubation high ATP levels are maintained. This phenomenon was independent of the presence of the energy reserves polyglucose and polyphosphate. 13C NMR experiments revealed that the amino acids glutamate, aspartate, and alanine are the major products of acetate incorporation in the dark. Apart from these amino acids, poly-beta-hydroxybutyrate was also formed. Acetate metabolism was markedly stimulated by the presence of polyglucose. The specific 13C activity of glutamate C-2 was approximately 50% that of glutamate C-4. The idea is discussed that this difference is the consequence of the maintenance of redox balance during entry of acetate into cell metabolism.  相似文献   
993.
Initial-rate measurements were made of the reduction of pyridine-3-aldehyde and p-carboxybenzaldehyde by NADPH catalyzed by pig liver aldehyde reductase I. The initial velocity analysis and product inhibition data suggest that aldehyde reductase I obeys a compulsory-order mechanism with pyridine-3-aldehyde as substrate but follows a partially random-order pathway with p-carboxybenzaldehyde. The partially random-order pathway would be operative only at high concentrations of p-carboxybenzaldehyde. In both cases, aldehydes and the corresponding alcohol substrates inhibit the enzyme at high concentration. Abortive ternary complexes are shown to be formed with pyridine-3-aldehyde and with p-carboxybenzaldehyde. Dissociation of the coenzyme from the abortive ternary complex seems only to be observed with p-carboxybenzaldehyde. This study suggests overall that an enzyme kinetic mechanism may be different, depending on whether specific interactions can occur between certain amino acid residue(s) of the protein active site and substrates. Finally, the mechanism of the inhibition of pyridine-3-aldehyde reduction by diacid derivatives is discussed.  相似文献   
994.
Proton NMR studies at 500 MHz in aqueous solution were carried out on the G-G chelated deoxytrinucleosidediphosphate platinum complex cis-Pt(NH3)2[d(GpCpG], on the uncoordinated trinucleotide d(GpCpG) and on the constituent monomers cis-Pt(NH3)2[d(Gp)]2, cis-Pt(NH3)2[d(pG)]2, d(Gp), d(pCp) and d(pG). Complete NMR spectral assignments are given and chemical shifts and coupling constants are analysed to obtain an impression of the detailed structure of d(GpCpG) and the distortion of the structure due to chelation with [cis-Pt(NH3)2]2+. Platination of the guanosine monophosphates affects the sugar conformational equilibrium to favour the N conformation of the deoxyribose ring. This feature is also apparent in ribose mononucleotides and is possibly caused by an increased anomeric effect. In cis-Pt(NH3)2[d(pG)]2 the phase angle of pseudorotation of the S-type sugar ring is 20 degrees higher than in 'free' d(pG) which might be an indication for an ionic interaction between the positive platinum and the negatively charged phosphate. It appears that d(GpCpG) reverts from a predominantly random coil to a normal right-handed B-DNA-like single-helical structure at lower temperatures, whereas the conformational features of cis-Pt(NH3)2[d(GpCpG)] are largely temperature-independent. In the latter compound much conformational freedom along the backbone angles is seen. The cytosine protons and deoxyribose protons exhibit almost no shielding effect as should normally be exerted by the guanine bases in stacking positions. This is interpreted in terms of a 'turning away' of the cytosine residue from both chelating guanines. Conformational features of cis-Pt(NH3)2[d(GpCpG)[ are compared with the 'bulge-out' of the ribose-trinucleotide m6(2)ApUpm6(2)A.  相似文献   
995.
Pulse-radiolysis experiments were performed on solutions containing methyl or benzyl viologen and flavodoxin. Viologen radicals are formed after the pulse. The kinetics of the reaction of these radicals with flavodoxin were studied. The kinetics observed depend strongly on the concentration of oxidized viologen. Therefore one must conclude that a relatively stable intermediate is formed after the reduction of flavodoxin. The midpoint potential of the intermediate state is -(480 +/- 30) mV, and is hardly dependent on the pH between 7 and 9.2. Due to a conformational change (k2 approximately equal to 10(5)S-1) the intermediate state decays to the stable semiquinone form of flavodoxin. The delta G of the conformational change at pH 8 is about 29 kJ mol -1 (0.3 eV). This means that the upper limit for the pK of N-5 in the semiquinone form will be 13. The activation energy of the conformational change is 43 kJ mol -1 (0.45 eV). The reaction between methyl viologen radicals and the semiquinone of flavodoxin can be described by a normal bimolecular reaction. The reaction is diffusion-controlled with a forward rate constant of (7 +/- 1) X 10(8) M -1S -1 (pH 8, I = 55 mM). The midpoint potential of the semiquinone/hydroquinone was found to be -(408 +/- 5) mV. A consequence of the intermediate state is that flavodoxin (Fld) could be reduced by a two-electron process, the midpoint potential of which should be located between -440 mV less than Em (Fld/FldH-) less than -290 mV. The exact value will depend on the delta G of the conformational change between the fully reduced flavodoxin with its structure in the oxidized form and the fully reduced flavodoxin with its structure in the hydroquinone form. The conditions are discussed under which flavodoxin could behave as a two-electron donor.  相似文献   
996.
Hydrogenase of Desulfovibrio vulgaris shows nonlinear kinetics in hydrogen production with both the natural electron carrier, cytochrome c3, and the artificial donor, methyl viologen semiquinone. Increasing concentrations of salt progressively inhibit the hydrogen production, as do increasing amounts of dimethylsulfoxide (Me2SO). Hydrogen consumption activity does not change up to 30% (v/v) of Me2SO. Preincubation in Me2SO up to 55% (v/v) does not affect the hydrogen uptake or production. The production activity of the enzyme shows an optimum around pH 6. When plotted as a function of redox potential the activity can be fitted to a Nernst equation with n = 1. Midpoint potentials calculated at various values follow approximately the hydrogen electrode to pH 6. Thereafter, there is a shift of about 40 mV to higher redox potentials.  相似文献   
997.
N-glycosidically-linked glycans released by hydrazinolysis of human factor VIII/von Willebrand factor (FVIII/vWf) were separated by high-voltage electrophoresis. Five fractions were obtained, one of them representing 60% of the total amount of the N-glycosidically-linked glycans of FVIII/vWf. On the basis of the carbohydrate composition, methylation analysis and 500 MHz 1H-NMR spectroscopy, we describe the primary structure of this major glycan which is of the monosialylated and monofucosylated biantennary N-acetyllactosaminic type.  相似文献   
998.
The amino-acid sequence of bovine myelin lipophilin (proteolipid apoprotein, Folch-protein) has been completed. Lipophilin is a 276 amino acid residues containing, extremely hydrophobic membrane protein with molecular mass 30,000 Da. The sequence determination was based on automated Edman degradation of four tryptophan and four cyanogen bromide fragments and of proteolytic peptides of complete lipophilin as well as the fragments obtained by chemical cleavage. Four additional sequences were determined which led to the completion of the primary structure. Lipophilin is esterified at threonine-198 by long chain fatty acids (palmitic, stearic and oleic acid). The attachment site has been established at the same threonine residue in three different peptides isolated from thermolysinolytic, papainolytic and chymotrypsinolytic hydrolysates. This threonine residue is part of a hydrophilic segment of lipophilin. The covalent fatty acyl bond is being discussed together with important structural and functional properties of this membrane protein which can be derived from sequence information. New separation and purification methods of hydrophobic and hydrophilic polypeptides for this sequence determination (fractional solubilization, silica gel exclusion, high-performance liquid chromatography) had to be elaborated as indispensable tools. They are generally applicable to the structural analysis of hydrophobic membrane proteins. Four long (26, 29, 40 and 36 residues) and one medium long (12 residues) hydrophobic segments are separated by four predominantly positively and one negatively charged hydrophilic segments. On the basis of structural data a model for the membrane integration of lipophilin is proposed.  相似文献   
999.
The bulk rheology of close-packed red blood cells in shear flow   总被引:1,自引:0,他引:1  
T W Secomb  S Chien  K M Jan  R Skalak 《Biorheology》1983,20(3):295-309
A theoretical analysis is made of the dynamical behavior and bulk rheology of close-packed red blood cell suspensions subjected to simple shear flow. The model for the polyhedral cell shapes and tank-treading membrane motion developed in the companion paper (1) is used. The flow in the thin lubricating plasma layers between cells is analyzed taking into account the mechanical properties of the membrane at the corner regions of sharp membrane curvature. This leads to predictions for the apparent viscosity as a function of hematocrit and shear rate. Good agreement with experimental results is obtained at moderate and high shear rates (above 20 s-1). At lower shear rates, a rapid rise in apparent viscosity has been found experimentally, and the mechanisms leading to this behavior are examined.  相似文献   
1000.
A method is presented for the analysis of fluorescence photobleaching recovery curves. Based on the simplified kinetic expression of Yguerabide, J., J.A. Schmidt, and E.E. Yguerabide (1982, Biophys. J., 40:69-75), a linearization procedure is described that permits unequivocal determination of all diffusion parameters. The presence of additional membrane flow or multiple diffusion coefficients can easily be detected by this method, and simple corrections for the presence of these alternative recovery processes can be made by the use of a regular mini-computer. The validity of the method is tested on simulated recovery curves, varying the contribution of flow, multiple diffusion coefficients, and statistical noise due to counting error.  相似文献   
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