Solvent effect on binding thermodynamics of biopolymers

The indirect solvent-induced effect on the free energy of binding of biopolymers is examined within the framework of classical statistical mechanics. We focus specifically on the role of the solute-solvent hydrogen bonding. In particular, we have estimated the first order solvent effect on the indirect interaction between two biopolymers. We find that the solvent-induced interactions between two hydrophilic groups through water-bridged hydrogen bonds could significantly enhance the binding free energy. Some preliminary estimates indicate that this effect is significant and perhaps could be crucial in molecular recognition processes. Furthermore, we have calculated, from crystal structure data, the distance distribution between all the oxygens and nitrogens on the surface of some proteins that do not belong to the binding domain. In most cases we found an enhanced peak in the range of 4-5 A, which is where we expect to find strong solvent-induced interactions.     

Measurement of membrane potential inBacillus subtilis: A comparison of lipophilic cations,rubidium ion,and a cyanine dye as probes

Summary Two of the commonly used probes for measuring membrane potential—lipophilic cations and the cyanine dye diS-C₃(5)—indicated nominally opposite results when tetraphenylarsonium ion was added as a drug to suspensions of metabolizingBacillus subtilis cells. [³H]-Triphenylmethylphosphonium uptake was enhanced by the addition, indicating hyperpolarization, yet fluorescence of diS-C₃(5) was also enhanced, indicating depolarization. Evidence is presented that both effects are artifactual, and can occur without any change in membrane potential, as estimated by⁸⁶Rb⁺ uptake in the presence of valinomycin. The fluorescence studies suggest that tetraphenylarsonium ion displaces the cyanine dye from the cell envelope, or other binding site, into the aqueous phase.The uptake characteristics of the radiolabeled lipophilic cations were quite unusual: At low concentrations (e.g., less than 10 m for triphenylmethylphosphonium) there was potential-dependent uptake of the label to a stable level, but subsequent addition of nonradioactive lipophilic cation caused further uptake of label to a new stable level. Labeled triphenylmethylphosphonium ion taken up to the first stable level could be displaced by 10mm magnesium ion, whereas⁸⁶Rb⁺ uptake was unperturbed. Association of the lipophilic cations with the surface of de-energized cells was concentration-dependent, but there was no evidence for cooperative binding. This phenomenon of stimulated uptake inB. subtilis (which was not seen inEscherichia coli cells or vesicles) is consistent with a two-compartment model with access to the second compartment only being possible above a critical cation concentration. We tentatively propose such a model, in which these compartments are the cell surface and the cytoplasm, respectively.Triphenylmethylphosphonium up to 0.5mm exhibited linear binding to de-energized cells; binding of tetraphenylphosphonium and tetraphenylarsonium was nonlinear but was not saturated at the highest concentration tested (1mm). The usual assumption, that association of the cation with cell surfaces is saturated and so can be estimated on de-energized cells, therefore leads to undercorrected estimates of cytoplasmic uptake inB. subtilis, and hence to overestimates of membrane potential. We describe a more realistic procedure, in which the estimate of extent of binding is based on a mean aqueous concentration related both to the external concentration and to the much higher internal concentration that exists in energized cells. Using this procedure we estimate the membrane potential inB. subtilis to be 120 mV, inside-negative. The procedure is of general applicability, and should yield more accurate estimates of membrane potential in any system where there is significant potential-dependent binding.Work performed while on sabbatical leave from Department of Biology, Ben-Gurion University of the Negev, Beer-Sheva, Israel.     

DNA synthesis in Escherichia coli during a nutritional shift-up

Summary DNA synthesis in a thymine-requiring Escherichia coli K12 strain was studied by exploiting deoxyguanosine, so simulating the behaviour of Thy⁺ strains. DNA synthesis is inhibited during the first 25 min after a nutritional shift-up. The new DNA/mass is lower than that predicted by current models for initiation control.Dedicated to the memory of Shmuel Zabrovitz, whose high spirits and good humor enabled him to complete the work while struggling with his lethal diseaseDeceased     

Crystal structure studies and inhibition kinetics of tripeptide chloromethyl ketone inhibitors with Streptomyces griseus protease B

The bacterial serine protease, SGPB, was inhibited by two specific tripeptide chloromethyl ketones, N-t-butyloxycarbonyl-l-alanylglycyl-l-phenylalanine chloromethyl ketone (BocAGFCK) and N-t-butyloxycarbonyl-glycyl-l-leucyl-l-phenylalanine chloromethyl ketone (BocGLFCK). Crystals of the inhibited complexes were grown and examined by X-ray crystallographic methods. The peptide backbone of each inhibitor is bound by three hydrogen bonds to the main chain of residues Ser214 to Gly216. There are two well-characterized hydrophobic pockets, S₁ and S₂, on the surface of SGPB which accommodate the P₁ and P₂ side-chains of the BocGLFCK inhibitor. A conformational change of Tyr171 is induced by the binding of this inhibitor. Both inhibitors make two covalent bonds to the SGPB enzyme. The imidazole ring of His57 is alkylated at the N^?2 atom and O^γ of Ser195 forms a hemiketal bond with the carbonyl-carbon atom of the inhibitor. Comparison of the binding modes of the two tripeptides in conjunction with the differences in their inhibition constants (K_I) allows one to estimate the binding energy of the leucyl side-chain as ?2.6 kcal mol^?1. The importance of an electrophilic component in the serine protease mechanism, which involves the polarization of the susceptible carbonyl bond of a substrate or inhibitor by the peptide NH groups of Gly193 and Ser195 is discussed.     

Light Suppresses Sporulation and Epidemics of Peronospora belbahrii

Peronospora belbahrii is a biotrophic oomycete attacking sweet basil. It propagates asexually by producing spores on dichotomously branched sporophores emerging from leaf stomata. Sporulation occurs when infected plants are incubated for at least 7.5h in the dark in moisture-saturated atmosphere at 10-27°C. Exposure to light suppresses spore formation but allows sporophores to emerge from stomata. Incandescent or CW fluorescent light of 3.5 or 6 µmoles.m².s^-1 respectively, caused 100% inhibition of spore formation on lower leaf surface even when only the upper leaf surface was exposed to light. The inhibitory effect of light failed to translocate from an illuminated part of a leaf to a shaded part of the same leaf. Inhibition of sporulation by light was temperature-dependent. Light was fully inhibitory at 15-27°C but not at 10°C, suggesting that enzyme(s) activity and/or photoreceptor protein re-arrangement induced by light occur at ≥15°C. DCMU or paraquat could not abolish light inhibition, indicating that photosystem I and photosystem II are not involved. Narrow band led illumination showed that red light (λmax 625 nm) was most inhibitory and blue light (λmax 440 nm) was least inhibitory, suggesting that inhibition in P. belbahrii, unlike other oomycetes, operates via a red light photoreceptor. Nocturnal illumination of basil in the field (4-10 µmoles.m².s^-1 from 7pm to 7am) suppressed sporulation of P. belbahrii and reduced epidemics of downy mildew, thus reducing the need for fungicide applications. This is the first report on red light inhibition of sporulation in oomycetes and on the practical application of light for disease control in the field.     

162 Dominant forces in protein folding

In the first part of this talk, I will discuss the need for a paradigm shift from hydrophobic (HφO) to a hydrophilic ((HφI) based theory of protein folding. Next, I will discuss the various types of solvent-induced forces that are exerted on various groups on the protein. It is argued, both theoretically and by simulations, that the HφI–HφI solvent-induced forces are likely to be the strongest. Therefore, it is suggested that these forces are also the forces that force the protein to fold, in a short time, along a narrow range of pathways. This paradigm shift also answers Levinthal’s question about the factors that “speed” and “guide” the folding of proteins.     

Exploring the origin of the ion selectivity of the KcsA potassium channel

The availability of structural information about biological ion channels provides an opportunity to gain a detailed understanding of the control of ion selectivity by biological systems. However, accomplishing this task by computer simulation approaches is very challenging. First, although the activation barriers for ion transport can be evaluated by microscopic simulations, it is hard to obtain accurate results by such approaches. Second, the selectivity is related to the actual ion current and not directly to the individual activation barriers. Thus, it is essential to simulate the ion currents and this cannot be accomplished at present by microscopic MD approaches. In order to address this challenge, we developed and refined an approach capable of evaluating ion current while still reflecting the realistic features of the given channel. Our method involves generation of semimacroscopic free energy surfaces for the channel/ions system and Brownian dynamics (BD) simulations of the corresponding ion current. In contrast to most alternative macroscopic models, our approach is able to reproduce the difference between the free energy surfaces of different ions and thus to address the selectivity problem. Our method is used in a study of the selectivity of the KcsA channel toward the K+ and Na+ ions. The BD simulations with the calculated free energy profiles produce an appreciable selectivity. To the best of our knowledge, this is the first time that the trend in the selectivity in the ion current is produced by a computer simulation approach. Nevertheless, the calculated selectivity is still smaller than its experimental estimate. Recognizing that the calculated profiles are not perfect, we examine how changes in these profiles can account for the observed selectivity. It is found that the origin of the selectivity is more complex than generally assumed. The observed selectivity can be reproduced by increasing the barrier at the exit and the entrance of the selectivity filter, but the necessary changes in the barrier approach the limit of the error in the PDLD/S-LRA calculations. Other options that can increase the selectivity are also considered, including the difference between the Na+...Na+ and K+...K+ interaction. However, this interesting effect does not appear to lead to a major difference in selectivity since the Na+ ions at the limit of strong interaction tend to move in a less concerted way than the K+ ions. Changes in the relative binding energies at the different binding sites are also not so effective in changing the selectivity. Finally, it is pointed out that using the calculated profiles as a starting point and forcing the model to satisfy different experimentally based constraints, should eventually provide more detailed understanding of the different complex factors involved in ion selectivity of biological channels.     

Ovine placental lactogen specifically binds to endometrial glands of the ovine uterus

A hormonal servomechanism has been proposed to regulate differentiation and function of the endometrial glandular epithelium (GE) in the ovine uterus during pregnancy. This mechanism involves sequential actions of estrogen, progesterone, ovine interferon tau (IFNtau), placental lactogen (oPL), and placental growth hormone (oGH). The biological actions of oPL in vitro are mediated by homodimerization of the prolactin receptor (oPRLR) and heterodimerization of the oPRLR and oGH receptor. The objectives of the study were to determine the effects of intrauterine oPL, oGH, and their combination on endometrial histoarchitecture and gene expression and to localize and characterize binding sites for oPL in the ovine uterus in vivo using an in situ ligand binding assay. Intrauterine infusion of oPL and/or oGH following IFNtau into ovariectomized ewes treated with progesterone daily differentially affected endometrial gland number and expression of uterine milk proteins and osteopontin. However, neither hormone affected PRLR, insulin-like growth factor (IGF)-I, or IGF-II mRNA levels in the endometrium. A chimeric protein of placental secretory alkaline phosphatase (SEAP) and oPL was used to identify and characterize binding sites for oPL in frozen sections of interplacentomal endometrium from pregnant ewes. Specific binding of SEAP-oPL was detected in the endometrial GE on Days 30, 60, 90, and 120 of pregnancy. In Day 90 endometrium, SEAP-oPL binding to the endometrial GE was displaced completely by oPL and prolactin (oPRL) but only partially by oGH. Binding experiments using the extracellular domain of the oPRLR also showed that iodinated oPL binding sites could be competed for by oPRL and oPL but not by oGH. Collectively, results indicate that oPL binds to receptors in the endometrial glands and that oPRL is more effective than oGH in competing for these binding sites. Thus, effects of oPL on the endometrial glands may be mediated by receptors for oPRL and oGH.     

Computer simulation studies of the fidelity of DNA polymerases

Computer simulations can provide in principle quantitative correlation between the structures of DNA polymerases and the replication fidelity. This paper describes our progress in this direction. Using several theoretical approaches, including the free energy perturbation (FEP), linear response approximation (LRA), and the empirical valence bond (EVB) methods, we examined the stability of several mismatched base pairs in DNA duplex in aqueous solution, the contribution of binding energy to the fidelity of DNA polymerases beta and T7, and the mechanism and energetics of the polymerization reaction catalyzed by T7 DNA polymerase.     

Subcloning,expression, purification,and characterization of recombinant human leptin-binding domain

A subdomain of the human leptin receptor encoding part of the extracellular domain (amino acids 428 to 635) was subcloned, expressed in a prokaryotic host, and purified to homogeneity, as evidenced by SDS-PAGE, with over 95% monomeric protein. The purified leptin-binding domain (LBD) exhibited the predicted beta structure, was capable of binding human, ovine, and chicken leptins, and formed a stable 1:1 complex with all mammalian leptins. The binding kinetics, assayed by surface plasmon resonance methodology, showed respective k(on) and k(off) values (mean +/- S.E.) of 1.20 +/- 0.23 x 10(-5) mol(-1) s(-1) and 1.85 +/- 0.30 x 10(-3) s(-1) and a K(d) value of 1.54 x 10(-8) m. Similar results were achieved with conventional binding experiments. LBD blocked leptin-induced, but not interleukin-3-induced, proliferation of BAF/3 cells stably transfected with the long form of human leptin receptor. The modeled LBD structure and the known three-dimensional structure of human leptin were used to construct a model of 1:1 LBD.human leptin complex. Two main residues, Phe-500, located in loop L3, and Tyr-441, located in L1, are suggested to contribute to leptin binding.