Mutations that suppress the defects introduced into T4 lysozyme by single amino acid substitutions were isolated and characterized. Among 53 primary sites surveyed, 8 yielded second-site revertants; a total of 18 different mutants were obtained. Most of the restorative mutations exerted global effects, generally increasing lysozyme function in a number of primary mutant contexts. Six of them were more specific, suppressing only certain specific deleterious primary substitutions, or diminishing the function of lysozymes bearing otherwise nondeleterious primary substitutions. Some variants of proteins bearing primary substitutions at the positions of Asp 20 and Ala 98 are inferred to have significantly altered structures. 相似文献
Chaperone proteins demonstrate the paradoxical ability to bind ligands rapidly and with high affinity but with no apparent sequence specificity. To learn more about this singular property, we have mapped the binding frame of the chaperone SecB from E. coli on the oligopeptide-binding protein. Similar studies performed on the maltose-binding and galactose-binding proteins revealed centrally positioned binding frames of approximately 160 aminoacyl residues. The work described here shows that OppA, which is significantly longer than the previously studied ligands, has a binding frame that covers 460 amino acids, nearly the entire length of the protein. We propose modes of binding to account for the data. 相似文献
The 13C resonances of Nα,N-[13C]dimethylserine of partially 13C reductively methylated glycophorin AM were monitored as a function of pH at 45°C. For comparison, limited data are also presented for the pH dependence of the 13C resonances of Nα,N- [13C]dimethylserine of fully 13C reductively methylated deglycosylated glycophorin AM. The ‘major’ component of Nα,N- [13C]dimethylserine of glycophorin AM did not titrate, whereas the ‘minor’ component titrated with a pKa of 7.80 (Hill coefficient of 0.95). Similar results are also indicated for the Nα,N- [13C]dimethylserine resonances of 13C reductively methylated deglycosylated glycophorin AM. 相似文献
Recommendations vary on whether blood pressures should be measured in the right or in the left arm because no frequency distributions for a pressure difference between the arms exist. We took a total of 12 blood pressure determinations in both arms of 174 elderly persons and analyzed the data by a least-squares components of variance method. The mean difference between the arms (right minus left) was 0.93 mm of mercury for systole and 0.70 mm of mercury for diastole. For systole the proportion of persons having arm pressure differences exceeding 10 mm of mercury is 1.4% and that exceeding 7.5 mm of mercury is 6.5%. For most people, the pressure difference between the arms is small. 相似文献
Heat evolved by isolated soybean (Glycine max cv Clark) nodules was measured to estimate more directly the metabolic cost associated with the symbiotic N2 fixation system. A calorimeter constructed by modifying standard laboratory equipment allowed measurement on 1 gram of detached nodules under a controlled gas stream. Simultaneous gas balance and heat output determinations were made.
There was major heat output by nodules for all of the nitrogenase substrates tested (H+, N2, N2O, and C2H2) further establishing the in vivo energy inefficiency of biological N2 fixation. Exposure to a short burst of 100% O2 partially inactivated nitrogenase to permit calculations of heat evolved per mole of substrate reduced. The specific rate of heat evolution for H+ reductions was 171 ± 6 kilocalories per mole H2 evolved in an Ar-O2 atmosphere, that for N2 fixation was 784 ± 26 kilocalories per mole H2 evolved and N2 fixed, and that for C2H2 reduction was 250 ± 12 kilocalories/mole C2H4 formed. When the appropriate thermodynamic parameters are taken into account for the different substrates and products, a ΔH′ of −200 kilocalories per mole 2e− is shown to be associated with active transfer of electrons by the nitrogenase system. These values lead to a calculated N2 fixation cost of 9.5 grams glucose per gram N2 fixed or 3.8 grams C per gram N2, which is in close agreement with earlier calculations based on nodular CO2 production.
When the outer surface of short-circuited frog skin was penetrated with microelectrodes, stable negative potentials that averaged near -100 mV were recorded consistently, confirming the results of Nagel (W. Nagel. 1975. Abstracts of the 5th International Biophysics Congress, Copenhagen. P-147.). The appearance of these stable potentials, V(O), concurrent with the observations that (a) a high resistance outer barrier R(O) accounting for approximately 75 percent or more of the transcellular resistance of control skins had been penetrated and that (b) 10(-5) M amiloride and reduced [Na] outside caused the values of V(O) to increase towards means value near -130 mV while the values of percent R(O) increased to more than 90 percent. It was of relationships were the same as the values of E(1) observed in studies of the current-voltage relationships were the same as the values of E’(1) defined as the values of voltage at the inner barrier when the V(O) of the outer barrier was reduced to zero by voltage clamping of the skins. Accordingly, these data are interpreted to mean that the values of E(1), approximately 130 mV, represent the E(Na) of the sodium pump at the inner barrier. 2,4-DNP was observed to decrease the values of transepithelial voltage less than E(1) the V(O) was negative. These data can be interpreted with a simple electrical equivalent circuit of the active sodium transport pathway of the frog skin that includes the idea that the outer membrane behaves as an electrical rectifier for ion transport. 相似文献
Summary The extracellular Ca2+ requirement for antidiuretic hormone (ADH) stimulation of water permeability in the toad urinary bladder has been critically examined. The polarity of the tissue was maintained with 1mm Ca2+ in the mucosal bathing medium and a serosal bath nominally free of Ca2+. Under these condition, ADH-induced osmotic water flow was inhibited by more than 60% while enhancement of the diffusional permeability to water was unaffected. Structural studies revealed that low serosal Ca2+ led to parallel alterations in epithelial architecture that amounted to a significant distorition of the osmotic water pathway. Prevention of these alterations, or restoration of normal cell-cell contact showed that the reduction of serosal Ca2+ did not restrict hormonal action,per se, but that it resulted in a weakening of cell-cell junctions such that intercellular space distension during water flow occurred to a point where the geometric conditions for maintenance of osmotic flow were compromised. We conclude that extracellular Ca2+ is not a requirement for the molecular aspects of ADH action but that, in its absence, a direct measurement of ADH-induced osmotic flow proves to be an inaccurate index of the hormone-generated changes in epithelial transport characteristics. Under certain conditions the ADH-effect on the tissue's hydraulic permeability is probably best assessed by measurement of the diffusional permability to water; although accuracy in this determination is difficult, it is not as strongly dependent on tissue geometry. 相似文献
The endocytic vacuoles induced in white ghosts were found to be depleted of spectrin and therefore it was proposed that they arose from spectrin-free areas in the erythrocyte membrane. To follow changes in spectrin distribution during endocytosis, affinity-purified rabbit antispectrin antibodies were produced. Quantitative techniques were developed for the use of a highly specific 125I-F(ab')2 antispectrin, and these showed that before the appearance of vacuoles, as assessed by phase microscopy, there was a reproducible decrease in immunoreactive spectrin. To determine whether this spectrin decrease represented a local or diffuse spectrin loss or a spectrin rearrangement, morphologic studies were undertaken using transmission electron microscopy on samples treated with rabbit antispectrin and ferritin-conjugated goat anti-rabbit immunoglobulin. These studies showed that endocytosis was preceded by the creation of extensive spectrin-free areas separated by discrete spectrin-containing zones. Pretreatment of ghosts with alkaline phosphatase blocked all forms of endocytosis and prevented the creation of spectrin-free areas. Therefore, it is proposed that under the impetus of endocytosis inducers, phosphorylated spectrin is redistributed so that spectrin-free zones are created, and that endocytic vacuoles form and fuse in spectrin-free areas. 相似文献