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1.
Incubation of the submersed aquatic macrophyte, Hydrilla verticillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO2/1) and low (6 to 8 μl CO2/l) CO2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C3 and C4 species, respectively. Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO3 ?) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg2+) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively. Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C4 acid inhibition and protection were not reliable methods to distinguish C3 and C4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from hight Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C3 and C4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C4 type. 相似文献
2.
Incubation of the submersed aquatic macrophyte, Hydrilla vertieillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO2/l) and low (6 to 8 μl CO2/l) CO2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C3 and C4 species, respectively. Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO3 -) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg2+) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively. Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C4 acid inhibition and protection were not reliable methods to distinguish C3 and C4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from high Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C3 and C4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C4 type. 相似文献
3.
Chang‐Ge Zheng Chun‐Lei Ma Xiao‐Wei Yu Qian‐Li Qian Yang Song Jun Kong Yan Xu 《化学与生物多样性》2011,8(8):1486-1496
Five ruthenium(II) complexes, i.e., [Ru(bpy)2(TIP)]2+ (bpy=2,2′‐bipyridine; TIP=2‐thiophenimidazo[4,5‐f] [1,10]phenanthroline; 1 ), [Ru(bpy)2(5‐NTIP)]2+ (5‐NTIP=2‐(5‐nitrothiophen)imidazo[4,5‐f] [1,10]phenanthroline; 2 ), [Ru(bpy)2(5‐MOTIP)]2+ (5‐MOTIP=2‐(5‐methoxythiophen)imidazo[4,5‐f] [1,10]phenanthroline; 3 ), [Ru(bpy)2(5‐BTIP)]2+ (5‐BTIP=2‐(5‐bromothiophen)imidazo[4,5‐f] [1,10]phenanthroline; 4 ), and [Ru(bpy)2(4‐BTIP)]2+ (4‐BTIP=2‐(4‐bromothiophen)imidazo[4,5‐f] [1,10]phenanthroline; 5 ), were synthesized and characterized by elemental analysis and UV/VIS, IR, and 1H‐NMR spectroscopic methods. The photophysical and DNA‐binding properties were investigated by means of UV and fluorescence spectroscopic methods and viscosity measurements, respectively. The results suggest that all five complexes can bind to CT‐DNA with various binding strength. Complexes 2 and 3 showed the strongest and the weakest binding affinity, respectively, among these five complexes. Due to the substituent position of the Br‐atom in the ligand, complex 5 interacted stronger with CT‐DNA than complex 4 . The binding affinities of the complexes decreased in the order 2, 5, 4, 1 , and 3 . 相似文献
4.
The binding ratio, Γa, for several long-chain amines to calf-thymus DNA was measured as function of the ligand concentration, C, using the equilibrium dialysis method. The different amines used in the binding experiments at constant temperature were dodecyl trimethyl ammonium bromide (DTAB), myristyl trimethyl ammonium bromide (MTAB), cetyl trimethyl ammonium bromide (CTAB), and cetyl pyridinium chloride (CPCL). The formation and dissociation of the saturated DNA–amine complex were reversible. The initial slope of the binding isotherm decreased sharply with the reduction of the electrostatic effect as a result of the increase of the ionic strength of the medium. A sharp inflexion region was noted in the binding isotherm where the ligands bound in significant numbers may undergo hydrophobic interactions with each other. Γa increased with C until a maximum value, Γam, was reached, beyond which binding slowly decreased with an increase of concentration. Both Γam and Γa increased significantly with the increase of the hydrocarbon chain length of a ligand. The free energy change ΔGm for each saturated DNA–amine complex was evaluated on the basis of a thermodynamic relation and the standard state for binding was defined. The average free energy change for the binding per CH2 group of the amine was found to be ?1550 cal/mol. The difference between ΔGm for CTAB and CPCL was examined on the basis of the structural difference of their head groups. The binding isotherms for MTAB and CPCL were obtained from the binding data at 15, 30, and 45°C. The binding increased with increasing temperature. From the plot of ΔGm/T vs 1/T, the changes in enthalpy and entropy due to the binding were evaluated for MTAB and CPCL. The binding reactions in these two cases were driven primarily by the entropy change due to the hydrophobic interaction. Standard free energy changes ΔG0m for the unsaturated complexes were close to ΔGm for the saturated complexes. The binding isotherms also depended on the nature of the neutral salt of the medium. At a given salt concentration, the order of the binding of the inorganic salts was as follows: KCl > NaCl > LiCl > Na2SO4 > MgCl2. The effect of pH on binding was also examined. The importance of these results on the formation of the reconstituted and natural nucleohistone complexes is discussed. 相似文献
5.
(i) A factor, EIF-2, that binds methionyl-tRNAfMet in the presence of GTP has been isolated from pig liver. (ii) Dodecylsulfate-gel electrophoresis and sedimentation equilibrium centrifugation indicate that the factor has a molecular weight of 122,000 and that it consists of three unequal subunits. (iii) The apparent KD for binding of methionyl-tRNAfMet varies with factor concentration. GTP participates in the binding with a KD of 0.5 μm. β,γ-Methylene-guanosine triphosphate supports 40% of the binding observed with GTP. GDP is a competitive inhibitor with a Ki of 0.2 μm. The optimal, free Mg2+ concentration is approximately 50 μm. GTP and Mg2+ stabilize the factor against thermal inactivation and inactivation by N-ethyl maleimide. (iv) The factor is required for the formation of a sucrose gradient-stable complex between methionyl-tRNAfMet and the 40S ribosomal subunit. The presence of template is not necessary, but poly(A,U,G) increases the binding observed 1.5-fold. (v) The factor markedly stimulates synthesis in a reconstituted protein-synthesizing system with globin messenger RNA as template. 相似文献
6.
109Cd transport was studied in the highly differentiated TC7 clone of the enterocytic-like Caco-2 cells grown on filters. Accumulation curves for 0.3 μM 109Cd over 12 h from the apical (AP) or the basal (BL) sides revealed a three-step mechanism involving: 1) a zero-time accumulation A0; 2) a fast process Af (t1/2 ≤ 10 min); and 3) a slow process of uptake AS (5 h ≤ t1/2 ≤ 10 h) responsible for the major cellular levels of 109Cd. The relative contribution of adsorption to total accumulation is greater for short exposure times (≤35%), but is no longer significant after the exposure times needed to reach equilibrium. Transepithelial transport was less than 4% of the cellular level at 12 h. A negligible but specific binding onto the BL surface of the filters was characterized. Saturable systems of accumulation with comparable affinities (Km = 2.5 ± 0.5 and 5.4 ± 0.4 μM) but distinct capacities (Vmax = 8.9 ± 1.2 and 312 ± 22 pmol/min/mg protein) were identified at the AP and BL cell membranes, respectively. Efflux studies revealed that Cd accumulation is only partially reversible, with an exclusive metal release at the same side. A 2-h exposure on both sides simultaneously failed to demonstrate any competition for cellular accumulation: uptake was additive relative to AP and BL uptake values. These data suggest that Af leads to an accumulation of loosely bound Cd, whereas AS represents irreversible intracellular binding processes. We conclude that Cd transport occurs exclusively by a transcellular route and that saturation of the intracellular high-capacity binding sites is the rate-limiting step in Cd absorption. J. Cell. Physiol. 180:285–297, 1999. © 1999 Wiley-Liss, Inc. 相似文献
7.
Δ53β hydroxysteroid dehydrogenase activity transforms biologically inactive Δ53β hydroxy steroids into the active Δ43-keto products (e.g. pregnenolone to progesterone). Using a cytochemical procedure which allows for the continuous microdensitometric monitoring of an enzyme reaction as it proceeds and a well described cytochemical assay for Δ53β HSD we have analysed the initial velocity rates (Vo) for dehydroepiandrosterone (DHEA) binding to this enzyme in regressing (i.e. 20α hydroxy steroid dehydrogenase positive) corpus luteum (CL) cells in unfixed tissue sections (5 μm) of the dioestrous and proestrous rat ovary. The results are mean ± S.E.M. The relationship between DHEA concentration (0 to 50 μM) and Δ53β HSD activity in the dioestrous corpora lutea was sigmoidal and had an atypical 1/Vo versus 1/S plot, the x intercept being positive. Using a 1/Vo versus 1/S2 plot the Vmax was determined to be 1·0 ± 0·08 μmol min?1 mg?1 CL (n = 6). The Hill constant was 2·7 ± 0·02 (n = 6) suggesting a high degree of positive co-operativity for DHEA binding. The S concentration for half maximal activity was 17 ± 1 μmoles (n = 6). In the corpora lutea cells of the proestrous ovary, the Vmax for DHEA transformation was unchanged (0·95 ± 0·04 μmol min?1 mg?1, n = 3) whilst the S0·5 was significantly increased to 27 ± 0·1 (p < 0·01, n = 3). The Hill constant remained positive being 2·9 ± 0·2 (n = 3). NAD+ binding to 3β HSD in regressing corpora lutea of the proestrous ovary has been demonstrated previously to be hyperbolic and fit the classical Michaelis-Menten model.1 Extending the analysis of NAD+ binding to the regressing corpus luteum of the dioestrous rat ovary revealed similar kinetic characteristics to that seen with the proestrous enzyme, the apparent Vmax and Km being 0·84 ± 0·04 μmol min?1 mg?1 CL (n = 3) and 27 ± 7 μmol 1?1 (n = 3) respectively. The Hill constant was 1·1 ± 0·03 (n = 3), indicating no co-operativity of co-factor binding. 相似文献
8.
Characterization of an Adenylate Cyclase-Linked Serotonin (5-HT1 ) Receptor in a Neuroblastoma × Brain Explant Hybrid Cell Line (NCB-20) 总被引:5,自引:5,他引:0
Clonal cell line NCB-20 (a hybrid of mouse neuroblastoma N18TG2 and Chinese hamster 18-day embryonic brain expiant) expressed both high- (KD 180 nM) and low-affinity (>3000 nM) binding sites for [3H]serotonin (5-HT) which were absent from the parent neuroblastoma. The low-affinity binding site was eliminated by 1 μM spiperone. The order of drug potency for inhibition of high-affinity [3H]5-HT binding was consistent with a 5-HT1 receptor (5,6 - dihydroxytryptamine = 5-HT = methysergide = 5-methoxytryptamine > cyproheptadine = clozapine = mianserin > spiperone > dopamine = morphine = ketanserin = norepinephrine). [3H]5-HT binding was inhibited by guanine nucleotides (e.g., GTP and Gpp(NH)p), whereas antagonist binding was not; as-corbate was also inhibitory. A 30-min exposure of cells to 1—2 μM 5-HT or other agonists produced a three- to fivefold stimulation of cyclic AMP levels. The order of potency for 5-HT agonist stimulation of basal cyclic AMP levels and 5-HT antagonist reversal of agonist-stimulated levels was the same as the order of drug potency for inhibition of high-affinity [3H]5-HT binding, suggesting linkage of the 5-HT1 receptor to adenylate cyclase in NCB-20 cells. 相似文献
9.
In the acridine orange–dermatan sulfate system, free and bound dye can be distinguished from each other spectroscopically. This permits the use of fluorometric methods to study the binding of acridine orange to the acid mucopolysaccharide dermatan sulfate. Experiments were conducted at 24°C in 10?3 M citrate/phosphate buffer at pH = 7.0. The binding of the dye is highly cooperative, as evidenced by considerable interaction between adjacent bound dye molecules. Analysis of the data indicates that dermatan sulfate binds 2.3 ± 0.3 mol of acridine orange per dermatan sulfate uronic acid residue with a cooperative binding constant, Kq ranging from 4.9 to 6.0 × 105 M?1 which corresponds to a free energy of 7.74 ? ΔG° ? 7.86. The cooperativity parameter q apparently increases with increasing polymer-to-dye ratio. 相似文献
10.
Henri B. Weems Elzbieta Chalecka-Franaszek Thomas E. Ct 《Journal of neurochemistry》1996,66(3):1042-1050
Abstract: High-affinity μ-opioid receptors have been solubilized from rat brain membranes. In most experiments, rats were treated for 14 days with naltrexone to increase the density of opioid receptors in brain membranes. Occupancy of the membrane-associated receptors with morphine during solubilization in the detergent 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate appeared to stabilize the μ-opioid receptor. After removal of free morphine by Sephadex G50 chromatography and adjustment of the 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate concentration to 3 mM, the solubilized opioid receptor bound [3H][d -Ala2,N-Me-Phe4,Gly-ol5]-enkephalin ([3H]DAMGO), a μ-selective opioid agonist, with high affinity (KD = 1.90 ± 0.93 nM; Bmax = 629 ± 162 fmol/mg of protein). Of the membrane-associated [3H]-DAMGO binding sites, 29 ± 7% were recovered in the solubilized fraction. Specific [3H]DAMGO binding was completely abolished in the presence of 10 µM guanosine 5′-O-(3-thiotriphosphate). The solubilized receptor also bound [3H]diprenorphine, a nonselective opioid antagonist, with high affinity (KD = 1.4 ± 0.39 nM, Bmax = 920 ± 154 fmol/mg of protein). Guanosine 5′-O-(3-thiotriphosphate) did not diminish [3H]diprenorphine binding. DAMGO at concentrations between 1 nM and 1 µM competed with [3H]diprenorphine for the solubilized binding sites; in contrast, [d -Pen2,d -Pen5]-enkephalin, a δ-selective opioid agonist, and U50488H, a κ-selective opioid agonist, failed to compete with [3H]diprenorphine for the solubilized binding sites at concentrations of <1 µM. In the absence of guanine nucleotides, the DAMGO displacement curve for [3H]diprenorphine binding sites better fit a two-site than a one-site model with KDhigh = 2.17 ± 1.5 nM, Bmax = 648 ± 110 fmol/mg of protein and KDlow = 468 ± 63 nM, Bmax = 253 ± 84 fmol/mg of protein. In the presence of 10 µM guanosine 5′-O-(3-thiotriphosphate), the DAMGO displacement curve better fit a one- than a two-site model with KD = 815 ± 33 nM, Bmax = 965 ± 124 fmol/mg of protein. 相似文献
11.
Humayun Pervez Zahid Hussain Chohan Muhammad Ramzan Faiz-Ul-Hassan Nasim Khalid Mohammed Khan 《Journal of enzyme inhibition and medicinal chemistry》2013,28(2):437-446
A new series of 12 N4-substituted isatin-3-thiosemicarbazones 2a-l has been synthesized, characterized and screened for in vitro cytotoxic, phytotoxic and urease inhibitory effects. All the compounds proved to be active in the brine shrimp bioassay; 2a, 2b, 2d, 2f and 2h-l exhibited a high degree of cytotoxic activity (LD50 = 1.10 × 10? 5 M–3.10 × 10? 5 M). In urease-inhibition assay, compounds 2a, 2b, 2e, 2f, 2h-j and 2l proved to be potent inhibitors displaying relatively much greater inhibition of the enzyme with IC50 values ranging from 20.6 μM to 50.6 μM. Amongst these, 2a and 2f were found to be the most potent ones exhibiting pronounced inhibition with IC50 value 20.6 μM. All the synthetic compounds showed weak to moderate (10–40%) phytotoxicity at the highest tested concentration (500 μg/mL) indicating their usefulness as inhibitors of soil ureases. 相似文献
12.
Abstract: High-affinity μ-opioid receptors have been solubilized from 7315c cell membranes. Occupancy of the membrane-associated receptors with morphine before their solubilization in the detergent 3-[(3-cholamidopropyl) dimethyl]-1-propane sulfonate was critical for stabilization of the receptor. The solubilized opioid receptor bound [3H]-etorphine with high affinity (KD= 0.304 ± 0.06 nM; Bmax= 154 ± 33 fmol/mg of protein). Of the membrane-associated [3H]etorphine binding sites, 40 ± 5% were recovered in the solubilized fraction. Both μ-selective and non-selective enkephalins competed with [3H]etorphine for the solubilized binding sites; in contrast, 5- and K-opioid enkephalins failed to compete with [3H]etorphine for the solubilized binding sites at concentrations of <1 μM.The μ-selective ligand [3H][D-Ala2,A/-Me-Phe4,Gly5-ol]enkephalin also bound with high affinity (KD= 0.79 rM; Bmax= 108±17 fmol/mg of protein) to the solubilized material. Of the membrane-associated [3H][D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin binding sites, 43 ± 3% were recovered in the solubilized material. Guanosine 5′-O-(3-thiotriphosphate), GTP, and guanosine 5′-O-(2-thiodiphosphate), but not adenylylimidodiphosphate, diminished [3H][D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin binding in a concentration-dependent manner. Finally, μ-opioid receptors from rat brain membranes were also solubilized in a high-affinity, guanine nucleotide-sensitive state if membrane-associated receptors were occupied with morphine before and during their solubilization with the detergent 3-[(3-cholamidopropyl) dimethyl]-1-propane sulfonate. 相似文献
13.
The interaction between DNA and ionen polymers, -[N+(CH3)2(CH2)mN+(CH3)2(CH2)n], with m-n of 3–3, 6–6, and 6–10 were examined in order to know how the binding behavior of cationic polymers with DNA depends on the charge density of polycation. The ionen polymer has no bulky side chain and the binding forces with DNA would be attributed mainly to electrostatic interaction. When 3–3 ionen polymers were added to DNA solution, precipitable complexes with the ratio of cationic residue to DNA phosphate (+/?) of 1/1 and the free DNA molecules were segregated, while 6–6 and 6–10 ionen polymers formed soluble complexes with DNA molecules up to (+/?) = 0.5. This suggests that 3–3 ionen polymers bind cooperatively with DNA while 6–6 and 6–10 ionen polymers bind noncooperatively. The cooperative binding of 3–3 ionen polymer and the noncooperative binding of 6–6 ionen polymer were also supported by the thermal melting and recooling profiles from the midpoint between first and second meltings. It was concluded that the charge density of DNA phosphate is a critical value determining whether the ionen polymers bind to DNA by a cooperative or by a noncooperative binding, since the distance between successive cationic charges of 3–3 ionen polymer is shorter than that between successive phosphate charges on DNA double helix and those of 6–6 and 6–10 ionen polymers are longer. 相似文献
14.
Influences of dithiothreitol (DTT), p-chloromercuriphenyl sulfonate (PCMPS) and ascorbate on CuCl2-induced elevation of [3H]cimetidine binding were investigated in brain membranes of rats. CuCl2 (10–500 μM) elevated specific [3H]cimetidine binding in a concentration-dependent manner. There were two types of [3H]cimetidine binding in the presence of 50 μM CuCl2: high affinity binding with Kd = 1.97 nM and low affinity with Kd = 21.6 nM. PCMPS (10 and 100 μM) reduced the binding in both media with and without CuCl2. DTT (1–30 μM) or ascorbate (0.1 and 1.0 mM) markedly elevated the binding in the presence of CuCl2 but showed no effect and ascorbate rather inhibited the binding in the absence of CuCl2. DTT (0.1 mM) diminished the binding in the presence and absence of CuCl2. CuCl2 (50 μM) significantly (P < 0.01) increased the IC50 of histamine for [3H]cimetidine binding and the effect was greater than that from 100 μM GTP. It is suggested that sulfhydryl groups sensitive to PCMPS could interact with Cu2+ and thus be involved in an elevation of cimetidine binding. Cu2+ seems to regulate affinity of agonist binding for cimetidine binding sites presumably by acting on cimetidine binding sites and/or GTP binding regulatory proteins. 相似文献
15.
The tumor promoter 20-3H-phorbol 12,13-dibutyrate bound in a specific manner to particulate preparations from both whole mouse skin and mouse epidermis. The binding, which was comparable in both whole skin and epidermal preparations, occurred rapidly, was reversible upon addition of non-radioactive ligand and showed high affinity (KD = 2.4 × 10?8 M). The potencies of phorbol esters for inhibiting binding of 3H-PDBu corresponded to their biological and tumor-promoting activities: phorbol 12-myristate 13-acetate, KI = 0.74 nM; phorbol 12,13-didecanoate, KI = 16 nM; phorbol 12,13-dibenzoate, KI = 82 nM; mezerein, KI = 98 nM; phorbol 12,13-diacetate, KI = 3 μM; phorbol 12,13,20-triacetate, KI = 5.6 μM; phorbol 13-acetate, KI = 64 μM. The biologically inactive derivatives phorbol (0.88 mM) and 4α-phorbol 12,13-didecanoate (15 μM) did not inhibit binding. Likewise, 3H-PDBu binding was only weakly inhibited by phorbol-related diterpenes which are highly inflammatory but nonpromoting. These structure-activity relationships suggest that the 3H-PDBu binding activity mediates phorbol ester tumor promotion. 3H-PDBu binding was not inhibited by the nonphorbol promoters examined. Similarly, it was not blocked by compounds which antagonize (dexamethasone acetate, 2 μM; retinoic acid, 10 μM) or mimic (epidermal growth factor, 100 ng/ml; melittin, 25 μg/ml; PGE2, 1 μM) some of the effects of the phorbol esters in vivo or in vitro. 相似文献
16.
JIANHONG CHEN AGRIPINA G. DEACIUC LINDA P. DWOSKIN PETER A. CROOKS DONGLU BAI 《Journal of enzyme inhibition and medicinal chemistry》2013,28(6):667-680
A series of N, N– disubstituted piperazines and homopiperazines were prepared and evaluated for binding to natural α4β2* and α7* neuronal nicotinic acetylcholine receptors (nAChRs) using whole brain membrane. Some compounds exhibited good selectivity for α4β2* nAChRs and did not interact with the α7* nAChRs subtype. The most potent analogs were compounds 8-19 (Ki = 10.4 μM), 8–13 (Ki = 12.0 μM), and 8–24 (Ki = 12.8 μM). Thus, linking together a pyridine π-system and a cyclic amine moiety via a homopiperazine ring affords compounds with low affinity but with good selectivity for α4β2* nAChRs. 相似文献
17.
Abstract The crystal structure of the dehydro octapeptide Boc-Val-ΔPhe-Phe-Ala-Leu-Ala-ΔPhe-Leu-OH has been determined to atomic resolution by X-ray crystallographic methods. The crystals grown by slow evaporation of peptide solution in methanol/water are orthorhombic, space group P212121. The unit cell parameters are a= 8.404 (3), b= 25.598(2) and c = 27.946(3) Å, Z=4. The agreement factor is R= 7.58% for 3636 reflections having (IF0I) ≥ 3σ (IF0I). The peptide molecule is characterised by a 310-helix at the N-terminus and a π-turn at the C-terminus. This conformation is exactly similar to the helix termination features observed in proteins. The π-turn conformation observed in the octapeptide is in good agreement with the conformational features of π-turns seen in some proteins. The αL-position in the π-turn of the octapeptide is occupied by ΔPhe7, which shows that even bulky residues can be accommodated in this position of the π-turns. In proteins, it is generally seen that aL- position is occupied by glycine residue. No intermolecular head-to-tail hydrogen bonds are observed in solid state structure of the octapeptide. A water molecule located in the unit cell of the peptide molecule is mainly used to hold the peptide molecule together in the crystal. The conformation observed for the octapeptide might be useful to understand the helix termination and chain reversal in proteins and to construct helix terminators for denovo protein design. 相似文献
18.
Polynucleotides. XI. Thermodynamics of (A) N -2(U) infinity from the dependence of T m N on oligomer length 总被引:3,自引:0,他引:3
The variation in the helix-coil transition temperature, TmN, with oligomer length, N, for the system ((I)) has been examined. The results for N = 4-13, measured in 0.2M Na+, have been analyzed in terms of the expression of Blake (1972): ((II)) where cm is the free oligomer concentration at TmN, and Vrf is the thermodynamic free volume available to a helical base-triplet residue. The correlation coefficient for the fit to expression (II) of data obtained over a 50° temperature range is 0.997 when ΔHr = ?12.6 kcal/mole of base-triplets (independent of oligomer length (N ? 4) or temperature), the value previously obtained from both calorimetry of (A)∞·2(U)∞ and (A)4 concentration dependence of Tm. It is found that Vrf = 8.0 × 10?4 1/mole (± 30%) or 1.33 Å3 per helical base-triplet, and is constant with temperature. A maximum value for Vrf of 21.0 × 10?4 1/M (± 1.3%), equivalent to 3.54 Å3 per helical basetriplet is obtained by the same treatment of the helix-coil transition data for the three-stranded helix formed by adenosine (N = 1) and 2(U)∞ obtained by Davies and Davidson (1971). 相似文献
19.
Abstract: We have suggested recently the existence of three subtypes of B2 bradykinin receptors in tissues of guinea pigs. We have classified these B2 bradykinin receptors into B2a, B2b, and B2c subtypes depending on their affinity for various bradykinin antagonists. Because the actions of bradykinin in different cell systems appear to be both dependent on and independent of G proteins, we sought to determine whether the binding of [3H]bradykinin to the B2 subtypes is sensitive to guanine nucleotides and, therefore, possibly coupled to G proteins. In the ileum, where we have demonstrated B2a and B2b subtypes, specific [3H]bradykinin binding was reduced with GDP (100 μM) and the nonmetabolized analogue of GTP, guanyl-5′-yl-imidodiphosphate (GppNHp; 100 μM). Competition studies with bradykinin and with [Hyp3]-bradykinin, which shows approximately 20-fold greater selectivity for the B2a subtype than bradykinin, were performed in the presence or absence of GppNHp (100 μM). The competition experiments demonstrated that binding to the B2a subtype, which has higher affinity for [Hyp3]-bradykinin and bradykinin than the B2b subtype, was lost in the presence of GppNHp, whereas binding to the B2b subtype was unaffected. In contrast, GppNHp (100 μM) and GDP (100 μM) failed to alter specific [3H]bradykinin binding to B2b and B2c subtypes in lung. [3H]Bradykinin binding was unaffected by AMP, ADP, ATP, and GMP (100 μM each). Based on this evidence, we suggest that the B2a bradykinin subtype is coupled to G proteins. The B2b and B2c subtypes are either not coupled to G proteins, or may be coupled to the Go-type GTP binding proteins, which have been suggested to be less sensitive to guanine nucleotides. These data provide further evidence for three subtypes of B2-type bradykinin receptors in guinea pig. 相似文献
20.
《Journal of enzyme inhibition and medicinal chemistry》2013,28(5):331-341
AbstractThree new mono-pyridinium compounds were prepared: 1-phenacyl-2-methylpyridinium chloride (1), 1-benzoylethylpyridinium chloride (2) and 1-benzoylethylpyridinium-4-aldoxime chloride (3) and assayed in vitro for their inhibitory effect on human blood acetylcholinesterase (EC 3.1.1.7, AChE). All the three compounds inhibited AChE reversibly; their binding affinity for the enzyme was compared with their protective effect (PI) on AChE phosphonylation by soman and VX. Compound 1 was found to bind to both the catalytic and the allosteric (substrate inhibition) sites of the enzyme with estimated dissociation constants of 6.9 μM (Kcat) and 27 μM (Kall), respectively. Compound 2 bound to the catalytic site with Kcat= 59 μM and compound 3 only to the allosteric site with Kall = 328 μM. PI was evaluated from phosphonylation measured in the absence and in presence of the compounds applied in a concentration corresponding to their Kcat or Kall value, and was also calculated from theoretical equations deduced from the reversible inhibition of the enzyme. Compounds 1 and 3 protected the enzyme from phosphonylation by soman and VX, whereas no protection was observed in the presence of compound 2 under the same conditions. Irrespective of the binding sites to AChE, PI for compounds 1 and 3 evaluated from phosphonylation agreed with PI calculated from reversible inhibition. Compound 3 was found to be a weak reactivator of methylphosphonylated AChE with kr = 1.1 × 102Lmol-1 min-1. 相似文献