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Slow Step (slow + step)
Selected AbstractsStructure,activity relationships of wheat flavone O -methyltransferase , a homodimer of convenienceFEBS JOURNAL, Issue 9 2008Jack A. Kornblatt Wheat flavone O -methyltransferase catalyzes three sequential methylations of the flavone tricetin. Like other flavonoid O -methyltransferases, the protein is a homodimer. We demonstrate, using analytical ultracentrifugation, that perchlorate dissociates the dimer into monomers. The resulting monomers retain all their catalytic capacity, including the ability to catalyze the three successive methylations. We show, using isothermal titration calorimetry, that the binding constant for S -adenosyl- l -methionine does not change significantly as the protein dissociates. The second substrate, tricetin, binds to the dimers but could not be tested with the monomers. CD, UV and fluorescence spectroscopy show that there are substantial changes in the structure of the protein as it dissociates. The fact that there are differences between the monomers and dimers even as the monomers maintain activity may be the result of the very low catalytic capacity of this enzyme. Maximal turnover numbers for the dimers and monomers are only about 6,7 per minute. Even though the binding pockets for S -adenosyl- l -methionine, tricetin, selgin and tricin are intact, selection of a catalytically competent structure may be a very slow step during catalysis. [source] Kinetics and mechanism of oxidation of aurate(I) by peroxydisulphate in aqueous hydrochloric acidINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 10 2002R. M. Babshet The reaction between Au(I), generated by reaction of thallium(I) with Au(III), and peroxydisulphate was studied in 5 mol dm,3 hydrochloric acid. The reaction proceeds with the formation of an ion-pair between peroxydisulphate and chloride ion as the Michealis,Menten plot was linear with intercept. The ion-pair thus formed oxidizes AuCl2, in a slow two-electron transfer step without any formation of free radicals. The ion-pair formation constant and the rate constant for the slow step were determined as 113 ± 20 dm,3 mol,1 and 5.0 ± 1.0 × 10,2 dm3 mol,1 s,1, respectively. The reaction was retarded by hydrogen ion, and formation of unreactive protonated form of the reductant, HAuCl2, causes the rate inhibition. From the hydrogen ion dependence of the reaction rate, the protonation constant was calculated to be as 0.6 ± 0.1 dm3 mol,1. The activation parameters were determined and the values support the proposed mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 589,594, 2002 [source] Kinetic study of the ruthenium(VI)-catalyzed oxidation of benzyl alcohol by alkaline hexacyanoferrate(III)INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2002A. E. Mucientes The kinetics of the Ru(VI)-catalyzed oxidation of benzyl alcohol by hexacyanoferrate(III), in an alkaline medium, has been studied using a spectrophotometric technique. The initial rates method was used for the kinetic analysis. The reaction is first order in [Ru(VI)], while the order changes from one to zero for both hexacyanoferrate(III) and benzyl alcohol upon increasing their concentrations. The rate data suggest a reaction mechanism based on a catalytic cycle in which ruthenate oxidizes the substrate through formation of an intermediate complex. This complex decomposes in a reversible step to produce ruthenium(IV), which is reoxidized by hexacyanoferrate(III) in a slow step. The theoretical rate law obtained is in complete agreement with all the experimental observations. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 421,429, 2002 [source] Mechanism of lidocaine release from carbomer,lidocaine hydrogelsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 1 2002Alvaro Jimenez-Kairuz Abstract Rheology, acid-base behavior, and kinetics of lidocaine release of carbomer,lidocaine (CL) hydrogels are reported. A series of (CL)x (x,=,mol% of L,=,25, 50, 75, 100) that covers a pH range between 5.33 and 7.96 was used. Concentrations of ion pair ([R-COO,LH+]) and free species (L) and (LH+) were determined by the selective extraction of (L) with cyclohexane (CH) together with pH measurements, i.e., CH in a ratio CH/hydrogel 2:1 extracted 48% of the whole concentration of lidocaine [LT] of a (CL)100, {[LT],=,([R-COO,LH+]),+,(L),+,(LH+)}. The remaining species in the aqueous phase were distributed as: (L) 3.82%, (LH+) 14.5%, and [R-COO, LH+] 81.7%. Rheology and pH as a function of (CL) concentration are also reported. Delivery rates of free base L were measured in a Franz-type bicompartmental device using water and NaCl 0.9% solution as receptor media. (CL) hydrogels behave as a reservoir that releases the drug at a slow rate. pH effects on rate suggest that, under the main conditions assayed, dissociation of [R-COO,LH+] is the slow step that controls releasing rates. Accordingly, release rate was increased upon addition of a second counterion (i.e., Na+), or through the diffusion of neutral salts such as NaCl, into the matrix of the gel. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:267,272, 2002 [source] Reactivity of isobutane in fluorosulfonic based superacids,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2002Alain Goeppert Abstract The behavior of isobutane in DSO3F containing various amounts of SbF5 has been studied in relation to the acid strength of the superacid system. In contrast to the DF,SbF5 system, H/D exchange occurs only in the weakest superacid via deprotonation of the t -butyl ion intermediate formed by an oxidative process. Kinetic isotope effect determination shows that the slow step is hydride transfer. At higher acidity, the increasing stability of this intermediate impedes isotopic exchange. Copyright © 2002 John Wiley & Sons, Ltd. [source] Visual guidance of the human foot during a stepTHE JOURNAL OF PHYSIOLOGY, Issue 2 2005Raymond F. Reynolds When the intended foot placement changes during a step, either due to an obstacle appearing in our path or the sudden shift of a target, visual input can rapidly alter foot trajectory. However, previous studies suggest that when intended foot placement does not change, the path of the foot is fixed after it leaves the floor and vision has no further influence. Here we ask whether visual feedback can be used to improve the accuracy of foot placement during a normal, unperturbed step. To investigate this we measured foot trajectory when subjects made accurate steps, at fast and slow speeds, to stationary floor-mounted targets. Vision was randomly occluded in 50% of trials at the point of foot-off. This caused an increase in foot placement error, reflecting lower accuracy and higher variability. This effect was greatest for slow steps. Trajectory heading analysis revealed that visually guided corrections occurred as the foot neared the target (on average 64 mm away). They occurred closer to the target for the faster movements thus allowing less time and space to execute corrections. However, allowing for a fixed reaction time of 120 ms, movement errors were detected when the foot was approximately halfway to the target. These results suggest that visual information can be used to adjust foot trajectory during the swing phase of a step when stepping onto a stationary target, even for fast movements. Such fine control would be advantageous when environmental constraints place limitations on foot placement, for example when hiking over rough terrain. [source] | ||||||||||