首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 15 毫秒
1.
2.
The focus of the present study was to investigate the mechanisms for the alleviation of Cu toxicity in plants by coexistent cations (e.g. Al(3+), Mn(2+), Ca(2+), Mg(2+), H(+), Na(+), and K(+)) and the development of an electrostatic model to predict 50% effect activities (EA50s) accurately. The alleviation of Cu(2+) toxicity was evaluated in several plants in terms of (i) the electrical potential at the outer surface of the plasma membrane (PM) (Ψ(0)(°)) and (ii) competition between cations for sites at the PM involved in the uptake or toxicity of Cu(2+), the latter of which is invoked by the Biotic Ligand Model (BLM) as the sole explanation for the alleviation of toxicity. The addition of coexistent cations into the bulk-phase medium reduces the negativity of Ψ(0)(°) and hence decreases the activity of Cu(2+) at the PM surface. Our analyses suggest that the alleviation of toxicity results primarily from electrostatic effects (i.e. changes in both the Cu(2+) activity at the PM surface and the electrical driving force across the PM), and that BLM-type competitive effects may be of lesser importance in plants. Although this does not exclude the possibility of competition, the data highlight the importance of electrostatic effects. An electrostatic model was developed to predict Cu(2+) toxicity thresholds (EA50s), and the quality of its predictive capacity suggests its potential utility in risk assessment of copper in natural waters and soils.  相似文献   

3.
Gan HH  Schlick T 《Biophysical journal》2010,99(8):2587-2596
Characterizing the ionic distribution around chromatin is important for understanding the electrostatic forces governing chromatin structure and function. Here we develop an electrostatic model to handle multivalent ions and compute the ionic distribution around a mesoscale chromatin model as a function of conformation, number of nucleosome cores, and ionic strength and species using Poisson-Boltzmann theory. This approach enables us to visualize and measure the complex patterns of counterion condensation around chromatin by examining ionic densities, free energies, shielding charges, and correlations of shielding charges around the nucleosome core and various oligonucleosome conformations. We show that: counterions, especially divalent cations, predominantly condense around the nucleosomal and linker DNA, unburied regions of histone tails, and exposed chromatin surfaces; ionic screening is sensitively influenced by local and global conformations, with a wide ranging net nucleosome core screening charge (56-100e); and screening charge correlations reveal conformational flexibility and interactions among chromatin subunits, especially between the histone tails and parental nucleosome cores. These results provide complementary and detailed views of ionic effects on chromatin structure for modest computational resources. The electrostatic model developed here is applicable to other coarse-grained macromolecular complexes.  相似文献   

4.
A microscopic model of an amphotericin B channel is proposed. The structure of the pores is generated using the atomic coordinates of the molecule in the structure determined experimentally by X-ray diffraction. The net charges of the atoms are determined by Mulliken analysis. With these charges the electrostatic energy profiles are calculated for a monovalent ion passing through the channels formed by different number of antibiotic molecules having different radii. The water inside the channel was considered through a continuum medium using the dielectric constant of the bulk, and the membrane contribution was included using the virtual images of the pore in a dielectric slab of epsilon = 3. The model satisfactorily explains the permeability and selectivity characteristics as well as other observations yet unexplained. The electrostatic profiles obtained reinforce the hypothesis of the existence of channels formed by a variable number of units.  相似文献   

5.
Microtubules (MTs) are important cytoskeletal polymers engaged in a number of specific cellular activities including the traffic of organelles using motor proteins, cellular architecture and motility, cell division and a possible participation in information processing within neuronal functioning. How MTs operate and process electrical information is still largely unknown. In this paper we investigate the conditions enabling MTs to act as electrical transmission lines for ion flows along their lengths. We introduce a model in which each tubulin dimer is viewed as an electric element with a capacitive, inductive and resistive characteristics arising due to polyelectrolyte nature of MTs. Based on Kirchhoff’s laws taken in the continuum limit, a nonlinear partial differential equation is derived and analyzed. We demonstrate that it can be used to describe the electrostatic potential coupled to the propagating localized ionic waves. An erratum to this article can be found at  相似文献   

6.
7.
A screened electrostatic potential model of hydration which could avoid the time consuming Monte-Carlo procedures is derived from the study of the dielectric effect of a water molecule in the vicinity of two charges. It is shown that the molecular dielectric constant depends on the distance, sizes and signs of the interacting charges. The model is used to evaluate the hydration energy of the tetrapeptide (TYR-GLY-GLY-PHE) conformations. Results are in good agreement with Monte-Carlo calculations.  相似文献   

8.
9.
A two-channel microarray measures the relative expression levels of thousands of genes from a pair of biological samples. In order to reliably compare gene expression levels between and within arrays, it is necessary to remove systematic errors that distort the biological signal of interest. The standard for accomplishing this is smoothing "MA-plots" to remove intensity-dependent dye bias and array-specific effects. However, MA methods require strong assumptions, which limit their general applicability. We review these assumptions and derive several practical scenarios in which they fail. The "dye-swap" normalization method has been much less frequently used because it requires two arrays per pair of samples. We show that a dye-swap is accurate under general assumptions, even under intensity-dependent dye bias, and that a dye-swap removes dye bias from a single pair of samples in general. Based on a flexible model of the relationship between mRNA amount and single-channel fluorescence intensity, we demonstrate the general applicability of a dye-swap approach. We then propose a common array dye-swap (CADS) method for the normalization of two-channel microarrays. We show that CADS removes both dye bias and array-specific effects, and preserves the true differential expression signal for every gene under the assumptions of the model.  相似文献   

10.
Majeux N  Scarsi M  Caflisch A 《Proteins》2001,42(2):256-268
A method is presented for the fast evaluation of the binding energy of a protein-small molecule complex with electrostatic solvation. It makes use of a fast preprocessing step based on the assumption that the main contribution to electrostatic desolvation upon ligand binding originates from the displacement of the first shell of water molecules. For a rigid protein, the precomputation of the energy contributions on a set of grids allows the estimation of the energy in solution of about 300 protein-fragment binding modes per second on a personal computer. The docking procedure is applied to five rigid binding sites whose size ranges from 17 residues to a whole protein of 107 amino acids. Using a library of 70 mainly rigid molecules, known micromolar inhibitors or close analogs are docked and prioritized correctly. The docking based rank-ordering of the library requires about 5 h and is proposed as a complementary approach to structure-activity relationships by nuclear magnetic resonance. Proteins 2001;42:256-268.  相似文献   

11.
The model is based upon an ion channel with an electric dipolar structure. With simplifying assumptions it is possible to calculate that a typical channel, 1 nm in diameter and 5 nm long, could contain at most two or three univalent cations at a time. The channel ion binding sites have an effective affinity for ions from the fluid bathing the negative end of the channel, several orders of magnitude higher than their affinity for ions from the fluid bathing the positive end of the channel. The approach of an external, positively charged body to the negative end of the channel, is sufficient to convert the two- or three-channel ion sites with high affinity for ions from the fluid bathing this end into very low affinity sites for the same ions that now have access only to the fluid bathing the other end of the channel. The change in affinity and fluid access requires no molecular or electrical change in the channel structure other than the passive superposition of the electrostatic potential of the dipolar channel and that of the charged body. An oscillating electric field externally applied to an electric dipolar channel is shown to result in the unidirectional pumping of cations in the direction of the channel dipole even against large adverse ion concentration gradients. The energy required must be supplied by the sources of the electric field. By using two such channels in close proximity, one selective for K+ ions with its dipole moment pointing into a cell and the other selective for Na+ ions with its dipole moment pointing out from the cell, it is possible to construct a model pump with calculated properties that simulate many of those measured for Na+-K+-ATPase, with both physiological and artificial ionic concentrations.  相似文献   

12.
Zemlin C  Storch E  Herzel H 《Bio Systems》2002,66(1-2):1-10
ECG alternans is commonly held to be an indicator of electrical instability of the heart, but the development of alternans has not yet been fully understood theoretically. We investigate the onset of alternans and 2:1 rhythms for stimulation at increasing frequencies in the Beeler-Reuter model, a simple ionic model of cardiac tissue. We find hysteresis and bistability at the onset of alternans; well-timed stimuli can switch between the two limit cycles. We determine quantitatively the effect of blocking specific ionic currents. Moreover, we find that calcium buffers generally promote alternans.  相似文献   

13.
14.
15.
We propose the hypothesis that extracellular regulation of cell division and differentiation acts through just two communications channels. These channels consist of a series of redundant components: extracellular messenger hormones; these hormones' receptors; cytoplasmic proteins activated by the hormone-receptor complex; and trans-activating nuclear regulatory proteins. One channel, here labeled "D" ("differentiate"), includes transforming growth factor-beta as one of its hormones; the other, labeled "G" ("growth") includes epidermal growth factor. We postulate that signal reception occurs as a result of competition between different actuating proteins for equilibrium-controlled binding to critical DNA sites. Stem cells commit to mitosis when some high proportion of critical sites is occupied by actuating proteins of the G class, and to terminal differentiation when a high proportion is occupied by "D" actuators. Intermediate occupancy can either lead to division into one differentiated and one stem cell, or to maintenance of cells in the reference state, quiescence. Equilibrium control of binding implies that critical site occupancy will be proportional to the relative concentrations of "D" and "G" actuating proteins in the nuclear fluid. These concentrations depend on the external hormone concentrations, the numbers of receptors on the cell membrane, and the coefficients of the rate-determining steps between internalization of the hormone-receptor complexes and activation of the actuating proteins. All of these quantities can be affected by various factors, including endocrine hormones. This model is consistent with most reported behavior of various growth factors, interferons, etc, toward a variety of cells in culture. It predicts that under otherwise constant conditions, high relative concentrations of a D-hormone (e.g. transforming growth factor-beta) will induce commitment to terminal differentiation, while high relative concentrations of a "G" hormone (e.g. epidermal growth factor) will induce mitosis. We have seen no report of an experiment which adequately tests this prediction. The model implies that cancer causing mutations are those which increase the relative intensity of the "G" signal; this can occur via changes in components of either channel. Such mutant cells should be both more likely to divide and less likely to differentiate than normal stem cells. Conversely, mutations which increase relative sensitivity to the "D" signal during embryonal development can lead to premature differentiation, cessation of growth, and structural abnormalities (terata).  相似文献   

16.
Multiple-equilibrium equations were solved to investigate the individual and separate effects of Mg2+, Mn2+, Ca2+, ATP4–, and their complexes on the kinetics of brain adenylate cyclase. The effects of divalent metals and/or ATP4– (in excess of their participation in complex formation) were determined and, from the corresponding apparent affinity values, the following kinetic constants were obtained:K m(MgATP)=1.0 mM,K i(ATP4–)=0.27 mM,K m(MnATP)=0.07 mM, andK i(CaATP)=0.015 mM. MgATP, MnATP, ATP4–, and CaATP were shown to compete for the active site of the enzyme. Hence, it is proposed that endogenous metabolites with a strong ligand activity for divalent metals, such as citrate and some amino acids, become integrated into a metabolite feedback control of the enzyme through the release of ATP4– from MgATP. Ca2+ fluxes may participate in the endogenous regulation of adenylate cyclase by modifying the level of CaATP. The free divalent metals show an order of affinityK 0.5(Ca2+)=0.02 mM,K 0.5(Mn2+)=3.8 mM,K 0.5(Mg2+)=4.7 mM, and an order of activity Mn2+>Mg2+>Ca2+. The data indicate that Mn2+ and Mg2+ ions may compete for a regulatory site distinct from the active site and increaseV m without changingK m(MgATP),K m(MnATP), orK i(ATP4–). The interactions of ATP4– and CaATP, which act as competitive inhibitors of the reaction of the enzyme with the substrates MgATP and MnATP, and Mg2+ and Mn2+, which act as activators of the enzyme in the absence of hormones, are shown to follow the random rapid equilibrium BiBi group-transfer mechanism of Cleland with the stipulation that neither Mg2+ nor Mn2+, in excess of their respective participation in substrate formation, are obligatorily required for basal activity. ATP4– and CaATP are involved in dead-end inhibition. For MgCl2 saturation curves at constant total ATP concentration, the computer-generated curves based on the RARE BiBi model predict a change in the Hill cooperativityh from a basal value of 2.6, when Mg2+ is not obligatorily required, to 4.0 when the addition of hormones or neurotransmitters induces an obligatory requirement for Mg2+.Abbreviations used: Me, divalent metal; MeT (MgT or MnT), total Me (Me2+ and its complexes); ATPT, total ATP (ATP4– and its complexes).  相似文献   

17.
18.
Numerous reports have described biological effects in animals exposed to electrostatic fields. Present equilibrium theory does not envision such effects because the bulk conductivity of biological tissue is generally held to prevent penetration of the applied electric field. Employing a two-layer mathematical model of an animal exposed to an electrostatic field we show that if the transient response of the animal is considered, then electric fields of significant strength and periodicity can occur inside the animal. We show also that the total energy available to an animal in an electrostatic field is extraordinarily small, and therefore that the biological effects associated with such exposure are not energetically driven by the applied field.  相似文献   

19.
Electrical alternans, a beat-to-beat alternation in the electrocardiogram or electrogram, is frequently seen during the first few minutes of acute myocardial ischemia, and is often immediately followed by malignant cardiac arrhythmias such as ventricular tachycardia and ventricular fibrillation. As ischemia progresses, higher-order periodic rhythms (e.g., period-4) can replace the period-2 alternans rhythm. This is also seen in modelling work on a two-dimensional (2-D) sheet of regionally ischemic ventricular muscle. In addition, in the experimental work, ventricular arrhythmias are overwhelmingly seen only after the higher-order rhythms arise. We investigate an ionic model of a strand of ischemic ventricular muscle, constructed as a 3-cm-long 1-D cable with a centrally located 1-cm-long segment exposed to an elevated extracellular potassium concentration ([K(+)](o)). As [K(+)](o) is raised in this "ischemic segment" to represent one major effect of ongoing ischemia, the sequence of rhythms {1:1-->2:2 (alternans)-->2:1} is seen. With further increase in [K(+)](o), one sees higher-order periodic 2N:M rhythms {2:1-->4:2-->4:1-->6:2-->6:1-->8:2-->8:1}. In a 2N:M cycle, only M of the 2N action potentials generated at the proximal end of the cable successfully traverse the ischemic segment, with the remaining ones being blocked within the ischemic segment. Finally, there is a transition to complete block {8:1-->2:0-->1:0} (in an n:0 rhythm, all action potentials die out within the ischemic segment). Changing the length of the ischemic segment results in different rhythms and transitions being seen: e.g., when the ischemic segment is 2 cm long, the period-6 rhythms are not seen; when it is 0.5 cm long, there is a 3:1 rhythm interposed between the 2:1 and 1:0 rhythms. We discuss the relevance of our results to the experimental observations on the higher-order rhythms that presage reentrant ischemic ventricular arrhythmias.  相似文献   

20.
In single molecule experiments Smith et al. (Science 271 (1996) 795) have unwound the B-DNA helix by stretching it in an aqueous salt solution. They found that the stretching force required for the transition decreases significantly with lowering of the salt concentration. We show that the observed salt effect is consistent with a uniformly charged cylinder model of DNA surrounded by a Poisson-Boltzmann ionic atmosphere. We also derive a simple connection between the sharpness of the center part of the transition and its cooperativity in terms of an average block size of base pairs that unwinds or rewinds.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号