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1.
G. C. Grigg F. Seebacher L. A. Beard D. Morris 《Proceedings. Biological sciences / The Royal Society》1998,265(1407):1793-1799
We monitored behaviour and environmental and body temperatures (Tb) in summer and winter in 11 salt-water crocodiles (Crocodylus porosus), of body mass 32 to 1010kg, free-ranging in naturalistic captivity in northern Australia. We found pronounced daily cycles in air and water temperatures in both winter (16 to 33 degrees C and 20 to 31degrees C, respectively) and summer (21 to 45 degrees C and 24 to 36 degrees C, respectively). In winter, crocodiles exposed themselves to the sun during the day and stayed in the water at night. In summer, they remained in the water during the day and emerged onto land at night. Body temperature showed a daily cycle the amplitude of which decreased with increasing mass, from 3.5 degrees C (mass 32kg) to 1.0 degrees C (660kg) in summer, and from 3.5 degrees C (42kg) to 1.4 degrees C (1010kg) in winter. Underlying the daily cycles in Tb were intermediate (10 to 13 day, tidal?) and seasonal cycles. Overall, values of modal Tb ranged from 25.1 to 28.7 degrees C in winter and from 28.4 to 33.6 degrees C in summer, trending upwards with body size. This pattern of continuous oscillations in Tb, with no daily plateau, is conspicuously different from that seen in crocodilians of small sizes and from the pattern usually regarded as typical of reptiles in general. 相似文献
2.
PurposeThis study aimed to assess the noise characteristics of ultra-high-resolution computed tomography (UHRCT) with deep learning-based reconstruction (DLR).MethodsTwo different diameters of water phantom were scanned with three different resolution acquisition modes. Images were reconstructed by filtered back projection (FBP), hybrid iterative reconstruction (hybrid-IR), and DLR. Image noise analysis was performed with noise magnitude, peak frequency (fp) of the noise power spectrum (NPS), and the square root of the area under the curve (√AUCNPS) for the NPS curve.ResultsThe noise magnitude was up to 3.30 times higher for the FBP acquired in SHR mode than that for the NR mode.The fp values of the FBP were 0.20–0.21, 0.34–0.36, and 0.34–0.37 cycles/mm for normal resolution (NR), high resolution (HR), and super high resolution (SHR) mode, respectively. The fp of hybrid-IR was 0.16–0.19, 0.21–0.26, and 0.23–0.26 cycles/mm for NR, HR, and SHR mode, respectively. The fp of DLR was 0.21–0.32 and 0.22–0.33 cycles/mm for HR and SHR mode, respectively.√AUCNPS showed that the highest value in FBP images of the SHR mode was up to 1.89 times that of the NR mode. DLR in the HR and SHR modes showed high noise reduction while suppressing fp shift with respect to FBP.ConclusionsThe new DLR algorithm could be a solution to the noise increase due to the high-definition detector elements and the small reconstruction matrix element size. 相似文献
3.
Fábio P. Gomes Marco A. Oliva Marcelo S. Mielke Alex-Alan F. de Almeida Helio G. Leite Leonardo A. Aquino 《Environmental and Experimental Botany》2008,62(3):195-204
Young plants of two Brazilian Green Dwarf coconut ecotypes from contrasting regions in relation to climate (UGD, from a hot and humid climate and JGD, from a hot and dry climate) were submitted to three consecutive drying/recovery cycles, under greenhouse conditions to determine the photosynthetic limitations encountered during the recovery phase of water deficiency. Three hypotheses were tested: (1) non-stomatal factors contribute to the incomplete recovery of net photosynthesis rate (A), (2) photochemical impairment is an important component of the non-stomatal limitations to A and (3) the two coconut ecotypes respond differently to the drought recovery. In each drying/recovery cycle, irrigation was suppressed until A reached zero. Then the plants were rewatered and the recovery was accompanied for 8 days. After 4 days of rewatering, CO2 and light response curve parameters were used to discriminate the stomatal and non-stomatal factors contributing to the incomplete recovery of A. Upon rewatering for 4 days, the pre-dawn leaf water potential of the stressed plants increased from ?1.20 to ?0.15 MPa on average. Only small differences were detected in the intercellular to atmospheric CO2 concentration ratio but A did not recover completely to the control values. In both ecotypes, light and CO2-saturated A, carboxylation efficiency, maximum quantum yield of PSII and maximum electron transport rate were significantly lower in stressed plants in comparison with the well-watered ones. Relative stomatal limitation (LS) ranged from 16 to 20% without significant differences between treatments and between ecotypes. The relative mesophyll limitation (Lm) was significantly higher in JGD in cycle 1 but this changed after repeated drought stress, decreasing to values significantly lower than those in UGD in the other two cycles. The results showed that after 4 days of rewatering stressed plant stomatal limitations were comparable to that of well-watered control plants (~20%). However A did not recover completely after drought stress due to non-stomatal factors. Important differences between the ecotypes were identified. The ecotype JGD presented smaller Lm, faster recovery of the apparent quantum efficiency of CO2 assimilation, adjustment of some carbon balance components and signs of photosynthetic capacity acclimation after repeated drying/recovery cycles. 相似文献
4.
Photosynthetic assimilation of CO2 in plants results in the balance between the photochemical energy developed by light in chloroplasts, and the consumption of that energy by the oxygenation processes, mainly the photorespiration in C3 plants. The analysis of classical biological models shows the difficulties to bring to fore the oxygenation rate due to the photorespiration pathway. As for other parameters, the most important key point is the estimation of the electron transport rate (ETR or J), i.e. the flux of biochemical energy, which is shared between the reductive and oxidative cycles of carbon. The only reliable method to quantify the linear electron flux responsible for the production of reductive energy is to directly measure the O2 evolution by 18O2 labelling and mass spectrometry. The hypothesis that the respective rates of reductive and oxidative cycles of carbon are only determined by the kinetic parameters of Rubisco, the respective concentrations of CO2 and O2 at the Rubisco site and the available electron transport rate, ultimately leads to propose new expressions of biochemical model equations. The modelling of 18O2 and 16O2 unidirectional fluxes in plants shows that a simple model can fit the photosynthetic and photorespiration exchanges for a wide range of environmental conditions. Its originality is to express the carboxylation and the oxygenation as a function of external gas concentrations, by the definition of a plant specificity factor Sp that mimics the internal reactions of Rubisco in plants. The difference between the specificity factors of plant (Sp) and of Rubisco (Sr) is directly related to the conductance values to CO2 transfer between the atmosphere and the Rubisco site. This clearly illustrates that the values and the variation of conductance are much more important, in higher C3 plants, than the small variations of the Rubisco specificity factor. The simple model systematically expresses the reciprocal variations of carboxylation and oxygenation exchanges illustrated by a “mirror effect”. It explains the protective sink effect of photorespiration, e.g. during water stress. The importance of the CO2 compensation point, in classical models, is reduced at the benefit of the crossing points Cx and Ox, concentration values where carboxylation and oxygenation are equal or where the gross O2 uptake is half of the gross O2 evolution. This concept is useful to illustrate the feedback effects of photorespiration in the atmosphere regulation. The constancy of Sp and of Cx for a great variation of P under several irradiance levels shows that the regulation of the conductance maintains constant the internal CO2 and the ratio of photorespiration to photosynthesis (PR/P). The maintenance of the ratio PR/P, in conditions of which PR could be reduced and the carboxylation increased, reinforces the hypothesis of a positive role of photorespiration and its involvement in the plant-atmosphere co-evolution. 相似文献
5.
T. A. Shtam E. Yu. Varfolomeeva E. V. Semenova M. V. Filatov 《Cell and Tissue Biology》2008,2(5):463-467
The RAD 51 protein, a eukaryotic homologue of Escherichia coli RecA, plays a significant role in both mitotic and meiotic homologous recombination. Here, we demonstrate that short-term silencing of the Rad51 gene by specific small interfering RNA (siRNA) that inhibits cell proliferation and reduces the viability of most cells. Cells with suppressed expression of Rad51 gene have altered cell cycles and accumulate in the S and G2 phases. Our findings show that the disruption of homologous recombination leads to cell death. However, some cells, e.g., MCF-7 cells, are insensitive to the suppression of Rad51 gene expression. 相似文献
6.
Periodicity in population dynamics is one of the fundamental issues in ecology. In addition to species-specific analyses, allometric studies facilitate understanding of limit cycles amongst different species. So far, body-size regressions have been derived for the oscillation period of warm-blooded species, in particular herbivores. Here, oscillations expected from a one-species (delayed logistic) and a two-species (Rosenzweig–MacArthur) model were compared to cycles observed in laboratory experiments and field surveys for a wide range of invertebrates and vertebrates. Supplemented by historical original studies, 759 oscillation periods were derived from the ‘Global Population Dynamics Database’ (GPDD) to cover a broad range of species and environmental conditions. The parameters in the equations were linked to body mass, using a consistent set of allometric relationships that was calibrated on 230 log–log linear regressions. Oscillation period and amplitude predicted by the models were validated with available data. The one-species model produced cycle times that increase with species’ body mass to the power 1/4 if the delay was set equal to the size-dependent age at maturity. If the delay was set on 1 year, the delayed logistic model yielded oscillations with a size-independent period of 4.7 years. Cycle times calculated by the two-species model scaled less than expected to the 1/4 power of mass m. The intercepts expected from the two-species were generally higher than those for the one-species model and increased with decreasing consumer-resource mass mi/mi−1 ratios. Amplitudes turned out to be size-independent according to both models. With exception of aquatic herbi-detritivores, intercepts were observed at the level calculated by the two-species model. Remarkably, oscillation periods were size-independent for predatory metazoans. Average cycles were of 4–5 years, similar to those predicted by the one-species model with a size-independent delay of one year. The consistent difference between lower trophic levels (i.e. herbivores) and higher trophic levels (i.e. carnivores) could be explained by the models from the small parameter space for consumer-resource cycles in generalist predators. Amplitudes recorded in the field did not scale to size and observed oscillation periods were about a factor of 2. This demonstrates that one allometric setting for age and density applicable to a wide range of species at lower trophic levels allows a reasonable estimate of independently measured cycles. 相似文献
7.
Robert W. Snow Eliud Kibuchi Stella W. Karuri Gilbert Sang Caroline W. Gitonga Charles Mwandawiro Philip Bejon Abdisalan M. Noor 《PloS one》2015,10(6)
Background
Progress toward reducing the malaria burden in Africa has been measured, or modeled, using datasets with relatively short time-windows. These restricted temporal analyses may miss the wider context of longer-term cycles of malaria risk and hence may lead to incorrect inferences regarding the impact of intervention.Methods
1147 age-corrected Plasmodium falciparum parasite prevalence (PfPR2-10) surveys among rural communities along the Kenyan coast were assembled from 1974 to 2014. A Bayesian conditional autoregressive generalized linear mixed model was used to interpolate to 279 small areas for each of the 41 years since 1974. Best-fit polynomial splined curves of changing PfPR2-10 were compared to a sequence of plausible explanatory variables related to rainfall, drug resistance and insecticide-treated bed net (ITN) use.Results
P. falciparum parasite prevalence initially rose from 1974 to 1987, dipped in 1991–92 but remained high until 1998. From 1998 onwards prevalence began to decline until 2011, then began to rise through to 2014. This major decline occurred before ITNs were widely distributed and variation in rainfall coincided with some, but not all, short-term transmission cycles. Emerging resistance to chloroquine and introduction of sulfadoxine/pyrimethamine provided plausible explanations for the rise and fall of malaria transmission along the Kenyan coast.Conclusions
Progress towards elimination might not be as predictable as we would like, where natural and extrinsic cycles of transmission confound evaluations of the effect of interventions. Deciding where a country lies on an elimination pathway requires careful empiric observation of the long-term epidemiology of malaria transmission. 相似文献8.
Most methods for analyzing real-time quantitative polymerase chain reaction (qPCR) data for single experiments estimate the hypothetical cycle 0 signal y0 by first estimating the quantification cycle (Cq) and amplification efficiency (E) from least-squares fits of fluorescence intensity data for cycles near the onset of the growth phase. The resulting y0 values are statistically equivalent to the corresponding Cq if and only if E is taken to be error free. But uncertainty in E usually dominates the total uncertainty in y0, making the latter much degraded in precision compared with Cq. Bias in E can be an even greater source of error in y0. So-called mechanistic models achieve higher precision in estimating y0 by tacitly assuming E = 2 in the baseline region and so are subject to this bias error. When used in calibration, the mechanistic y0 is statistically comparable to Cq from the other methods. When a signal threshold yq is used to define Cq, best estimation precision is obtained by setting yq near the maximum signal in the range of fitted cycles, in conflict with common practice in the y0 estimation algorithms. 相似文献
9.
Christine Breynaert Tom Dresselaers Clémentine Perrier Ingrid Arijs Jonathan Cremer Leentje Van Lommel Kristel Van Steen Marc Ferrante Frans Schuit Séverine Vermeire Paul Rutgeerts Uwe Himmelreich Jan L. Ceuppens Karel Geboes Gert Van Assche 《PloS one》2013,8(7)
Introduction
Chronically relapsing inflammation, tissue remodeling and fibrosis are hallmarks of inflammatory bowel diseases. The aim of this study was to investigate changes in connective tissue in a chronic murine model resulting from repeated cycles of dextran sodium sulphate (DSS) ingestion, to mimic the relapsing nature of the human disease.Materials and Methods
C57BL/6 mice were exposed to DSS in drinking water for 1 week, followed by a recovery phase of 2 weeks. This cycle of exposure was repeated for up to 3 times (9 weeks in total). Colonic inflammation, fibrosis, extracellular matrix proteins and colonic gene expression were studied. In vivo MRI T 2 relaxometry was studied as a potential non-invasive imaging tool to evaluate bowel wall inflammation and fibrosis.Results
Repeated cycles of DSS resulted in a relapsing and remitting disease course, which induced a chronic segmental, transmural colitis after 2 and 3 cycles of DSS with clear induction of fibrosis and remodeling of the muscular layer. Tenascin expression mirrored its expression in Crohn’s colitis. Microarray data identified a gene expression profile different in chronic colitis from that in acute colitis. Additional recovery was associated with upregulation of unique genes, in particular keratins, pointing to activation of molecular pathways for healing and repair. In vivo MRI T2 relaxometry of the colon showed a clear shift towards higher T2 values in the acute stage and a gradual regression of T2 values with increasing cycles of DSS.Conclusions
Repeated cycles of DSS exposure induce fibrosis and connective tissue changes with typical features, as occurring in Crohn’s disease. Colonic gene expression analysis revealed unique expression profiles in chronic colitis compared to acute colitis and after additional recovery, pointing to potential new targets to intervene with the induction of fibrosis. In vivo T2 relaxometry is a promising non-invasive assessment of inflammation and fibrosis. 相似文献10.
Osmotic Adjustment in Cotton (Gossypium hirsutum L.) Leaves and Roots in Response to Water Stress
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The relative magnitude of adjustment in osmotic potential (ψs) of water-stressed cotton (Gossypium hirsutum L.) leaves and roots was studied using plants raised in pots of sand and grown in a growth chamber. One and three water-stress preconditioning cycles were imposed by withholding water, and the subsequent adjustment in solute potential upon relief of the stress and complete rehydration was monitored with thermocouple psychrometers. Both leaves and roots exhibited a substantial adjustment in ψs in response to water stress with the former exhibiting the larger absolute adjustment. The osmotic adjustment of leaves was 0.41 megapascal compared to 0.19 megapascal in the roots. The roots, however, exhibited much larger percentage osmotic adjustments of 46 and 63% in the one and three stress cycles, respectively, compared to 22 and 40% in the leaves in similar stress cycles. The osmotically adjusted condition of leaves and roots decreased after relief of the single cycle stress to about half the initial value within 3 days, and to the well-watered control level within 6 days. In contrast, increasing the number of water-stress preconditioning cycles resulted in significant percentage osmotic adjustment still being present after 6 days in roots but not in the leaves. The decrease in ψs of leaves persisted longer in field-grown cotton plants compared to plants of the same age grown in the growth chamber. The advantage of decreased ψs in leaves and roots of water-stressed cotton plants was associated with the maintenance of turgor during periods of decreasing water potentials. 相似文献
11.
We recently developed a display method for the directed evolution of integral membrane proteins in the inner membrane of Escherichia coli for higher expression and stability. For the neurotensin receptor 1, a G-protein-coupled receptor (GPCR), we had evolved a mutant with a 10-fold increase in functional expression that largely retains wild-type binding and signaling properties and shows higher stability in detergent-solubilized form. We have now evolved three additional human GPCRs. Unmodified wild-type receptor cDNA was subjected to successive cycles of mutagenesis and fluorescence-activated cell sorting, and functional expression could be increased for all three GPCR targets. We also present a new stability screening method in a 96-well assay format to quickly identify evolved receptors showing increased thermal stability in detergent-solubilized form and rapidly evaluate them quantitatively. Combining the two methods turned out to be very powerful; even for the most challenging GPCR target—the tachykinin receptor NK1, which is hardly expressed in E. coli and cannot be functionally solubilized—receptor mutants that are functionally expressed at 1 mg/l levels in E. coli and are stable in detergent solution could be quickly evolved. The improvements result from cumulative small changes in the receptor sequence. This combinatorial approach does not require preconceived notions for designing mutations. Our results suggest that this method is generally applicable to GPCRs. Existing roadblocks in structural and biophysical studies can now be removed by providing sufficient quantities of correctly folded and stable receptor protein. 相似文献
12.
Relationship between Sublethal Injury and Microbial Inactivation by the Combination of High Hydrostatic Pressure and Citral or tert-Butyl Hydroquinone
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The aim was to investigate (i) the occurrence of sublethal injury in Listeria monocytogenes, Escherichia coli, and Saccharomyces cerevisiae after high hydrostatic pressure (HHP) treatment as a function of the treatment medium pH and composition and (ii) the relationship between the occurrence of sublethal injury and the inactivating effect of a combination of HHP and two antimicrobial compounds, tert-butyl hydroquinone (TBHQ) and citral. The three microorganisms showed a high proportion of sublethally injured cells (up to 99.99% of the surviving population) after HHP. In E. coli and L. monocytogenes, the extent of inactivation and sublethal injury depended on the pH and the composition of the treatment medium, whereas in S. cerevisiae, inactivation and sublethal injury were independent of medium pH or composition under the conditions tested. TBHQ alone was not lethal to E. coli or L. monocytogenes but acted synergistically with HHP and 24-h refrigeration, resulting in a viability decrease of >5 log10 cycles of both organisms. The antimicrobial effect of citral depended on the microorganism and the treatment medium pH. Acting alone for 24 h under refrigeration, 1,000 ppm of citral caused a reduction of 5 log10 cycles of E. coli at pH 7.0 and almost 3 log10 cycles of L. monocytogenes at pH 4.0. The combination of citral and HHP also showed a synergistic effect. Our results have confirmed that the detection of sublethal injury after HHP may contribute to the identification of those treatment conditions under which HHP may act synergistically with other preserving processes. 相似文献
13.
14.
Rabbit heart mitochondria were used as a source of enzymes for the synthesis of phosphoruslabeled creatine phosphate. This method is based on the coupled reaction between mitochondrial oxidative phosphorylation and mitochondrial-bound creatine kinase. It is possible to convert more than 90% of the inorganic phosphate (Pi) to creatine phosphate. The method used only small amounts of adenine nucleotides which led to a product with only slight nucleotide contamination. This could be removed by activated charcoal extraction. For further purification, a method for the removal of residual Pi is described. 相似文献
15.
Philippa D. Darbre 《Analytical biochemistry》1980,108(2):274-278
Carboxymethyl (CM)-cellulose chromatography of globins is the technique used most frequently in analysis of hemoglobin synthesis. However, if this method is to be reliable in cases where only small amounts of fetal hemoglobin (α2γ2) compared to adult hemoglobin (α2β2A) are synthesized, it is important to obtain a clean separation of γ chains from pre-βA chains. In the past, it has been found that small amounts of pre-βA chains tend to elute with the γ chains. Radioactively labeled γ chains can be completely and reproducibly separated from small amounts of labeled pre-βA chains by the addition of unlabeled βJ chains (Hb J Baltimore = β16 Gly → Asp) which elute at the same position as the pre βA chains, thus increasing the quantity of this peak and allowing a clean separation from the γ chains. 相似文献
16.
《Inorganica chimica acta》1986,125(2):107-110
The conformation of N-acetyl alanine methyl ester CH3CONHCH(CH3)COOCH3 is determined by CNDO/2 and ab initio calculations with minimal GLO basis sets. The binding sites of small monovalent cations to the ligand are investigated by the ab initio method. The chelate geometry involving peptide and ester carbonyl groups was found to be the most preferential conformation. 相似文献
17.
Vinod Kumar Firdaus Jahan Shailendra Raghuwanshi Richi V. Mahajan Rajendra Kumar Saxena 《Biotechnology and Bioprocess Engineering》2013,18(4):787-795
A novel and simple method was developed for the preparation of magnetic Fe3O4 nanoparticles by chemical co-precipitation method and subsequent coating with 3-aminopropyltrimethoxysilane (APTMS) through silanization process. Magnetic Fe3O4-chitosan particles were prepared by the suspension cross-linking and covalent technique to be used in the application of magnetic carrier technology. The synthesized immobilization supports were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). Using glutaraldehyde as the coupling agent, the lipase from R. oryzae was successfully immobilized onto the functionalized magnetic Fe3O4-chitosan beads. The results showed that 86.60% of R. oryzae lipase was bound on the synthesized immobilization support. This immobilized lipase was successfully used for the esterification of phenolic acid which resulted in esterification of phenolic acid in isooctane solvent reaction system for 8 consecutive cycles (totally 384 h), 72.6% of its initial activity was retained, indicating a high stability in pharmaceutical and industrial applications. 相似文献
18.
《Journal of steroid biochemistry》1984,20(4):963-970
In order to study the biological effects of estriol in women 20 mg estriol was administered daily to 7 young women. Plasma luteinizing hormone (LH), estradiol (E2), progesterone (Pg) and prolactin (Prl) were measured during a treatment and a control cycle every second or third day. Further 3, 6 or 20 mg estriol was administered in a single dose to 5 women and plasma Prl, unconjugated and conjugated estriol (E3) measured over 24 h at 2–3 h intervals. In 2 experiments with 20 mg E3, blood samples were taken more frequently, over 6h.When 20 mg E2 was administered daily, 2 of the 7 young women had anovulatory cycles. The mean plasma E2 was lower during the follicular and ovulatory phases (P < 0.025) and mean plasma LH was higher (P < 0.005) during the luteal phase, when E3 was given. Because of the 2 anovulatory cycles the mean Pg value during the luteal phase was lower (P < 0.05) during treatment. There was a slight decrease in mean Prl in 5 out of 6 women (P < 0.0005), but in only 1 woman was this decrease substantial (from a mean value of 27.6–18.9 ng/ml; P < 0.01). When 6 or 20 mg E3 was administered orally in the morning a significant negative correlation (P < 0.01) between plasma Prl and unconjugated E3 was found. The correlation coefficient was highest (r = −0.74) with 6mg E3. When 3mg was administered no obvious effect on Prl section was seen. However, when results from all experiments with identical time schedules were pooled (two with 3 mg, two with 6 mg and one with 20 mg E3) and the mean values for plasma Prl calculated and compared with the mean values obtained in 7 control experiments, it was found that E3 administration in the morning almost abolishes the Prl rise during the following night. There was a statistically significant (P < 0.0125) decrease in the difference between the maximum value during the night and the minimum value during the day. The minimum value was significantly higher (P < 0.01) and the maximum value significantly lower (P < 0.025) after e3) treatment, compared to the control values.It is concluded that long-term administration of 20 mg e3 usually has only a slight but significant decreasing effect on mean plasma Prl concentration measured in the morning, before the next dose is taken. However, E3 has an inhibiting effect on Prl secretion during the night and a stimulating effect during the day, if administered in the morning, resulting in a considerable reduction in the amplitude of the circadian rhythm. Continuous administration of high doses (20 mg/day) of E3 may result in anovulatory cycles, but has little effect on plasma hormone values in the cycles which remain ovulatory. 相似文献
19.
Nanogels are promising materials as supports for enzyme immobilization. A new hydrogel comprising of methacrylic acid (MAAc) and N-vinyl pyrrolidone (N-VP) and ethyleneglycol dimethacrylate (EGDMA) was synthesized and converted to nanogel by an emulsification method. Nanogel was further functionalized by Curtius azide reaction for use as support for the covalent immobilization of invertase (Saccharomyces cerevisiae). As-prepared or invertase-immobilized nanogel was characterized by FTIR, XRD, TEM and nitrogen analysis. The characterization of both free and the immobilized-invertase were performed using a spectrophotometric method at 540 nm. The values of Vmax, maximum reaction rate, (0.123 unit/mg), km, Michaelis constant (7.429 mol/L) and Ea, energy of activation (3.511 kj/mol) for the immobilized-invertase are comparable with those of the free invertase at optimum conditions (time 70 min, pH 6.0 and temperature 45 °C). The covalent immobilization enhanced the pH and thermal stability of invertase. The immobilized biocatalyst was efficiently reused up to eight cycles. 相似文献
20.
The dynamic behaviour of an open futile cycle composed of two enzymes has been investigated in the vicinity of a steady-state. A necessary condition required for damped or sustained oscillations of the system is that enzyme E2, which controls recycling of the substrate S2, be inhibited by an excess of this substrate. In order for the system to be neutrally stable and therefore to exhibit sustained oscillations, it is not necessary for antagonist enzyme E1 to be activated by its product S2. If it is enzyme E1 which is inhibited by an excess of its substrate S1, the system has a saddle point. Other conditions for stability or instability of the system have been determined. If the enzyme E1, which is not inhibited by the substrate, exhibits a slow conformational transition of the mnemonical type, this transition dramatically alters the stability behavior of the system. If the mnemonical enzyme E1 were exhibiting a positive kinetic co-operativity, decreasing the rate of the conformational transition of the mnemonical enzyme will increase the stability of the whole system and will tend to damp the oscillations in the vicinity of the steady-state. If conversely the mnemonical enzyme E1 were exhibiting a negative kinetic co-operativity, decreasing the rate of the enzyme conformational transition will decrease the stability of the system and will tend to create or amplify oscillations of the system taken as a whole. If these results may be extended to more complex metabolic cycles, involving more than two enzymes, it may be tentatively considered that positive co-operativity associated with slow transition has emerged in the course of evolution in order to limit temporal instabilities of metabolic cycles. Alternatively one may speculate that the “biological function” of negative co-operativity is to create or amplify these temporal instabilities. 相似文献