PH dependence of activity

Nevertheless, the pH dependence of activity does not appear to reflect a rate-limiting proton transfer since but not is pH dependent. The increase in Am at high pH can be accounted for by two active site pKs. In the oxidized state, OH binds to Fe in competition with other anions and likely 02, with a pK of 8.5, and the reduced state displays a pK of... 

The first hint that two active-site carboxyl groups—one proto-nated and one ionized—might be involved in the catalytic activity of the aspartic proteases came from studies of the pH dependence of enzymatic activity. If an ionizable group in an enzyme active site is essential for activity, a plot of enzyme activity versus pH may look like one of the plots at right. 

The pH dependence of HIV-1 protease has been assessed by measuring the apparent inhibition constant for a synthetic substrate analog (b). The data are consistent with the catalytic involvement of ionizable groups with pK values of 3.3 and 5.3. Maximal enzymatic activity occurs in the pH range between these two values. On the basis of the accumulated kinetic and structural data on HIV-1 protease, these pK values have been ascribed to the... 

The pH dependence of nitrogenase activity has been interpreted in terms of a group with a pi a = 6.3 that must he deprotonated for activity and another group with a pi a = 9 that must be protonated for activity 128). The pi a of the latter group was moved about 0.5 pH units more acid in the presence of acetylene and carbon monoxide and the group with the pi of 6.3 was moved about 0.4 pH units more acid by acetylene. The behavior of the group with the pZa of 9 is fully consistent with earlier observations (50) on the effect of acetylene on... 

The pH-dependence of the inactivation rate indicated the participation of both a basic and an acidic group in the reaction with 40. The latter could be explained by the formation at the active site of the highly reactive epoxide 1,2-anhydroconduritol F (42) which is subsequently activated by the acidic... 

A series of alkyl esters (Fig. 10.1) of/ (4)-hydroxybenzoic acid was originally prepared to overcome the marked pH-dependence on activity of the acids. 

Damjanovic A, SepaDB. 1990. An analysis of the pH dependence of enthalpies and Gibbs energies of activation for O2 reduction at Pt electrodes in acid solutions. Electrochim Acta 35 1157-1162. 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

The pH dependence of the reaction of m-tolyl acetate with cyclohexa-amylose implies a pAa of 12.1 for the catalytically active secondary hydroxyl group (Van Etten et al., 1967b). Although this pK at first appears low for the ionization of an aliphatic alcohol, it is consistent with the value of 12.35 determined thermodynamically for the ionization of the secondary hydroxyl groups of the ribose moiety of adenosine (Izatt et al., 1966 Christensen et al., 1966), and with the value of 12.2 reported by Lach for the... 

There have been two independent mutagenesis studies that have been directed toward probing the role of E4 in the PLCB(. reaction [36, 94]. In the first of these, the kinetic parameters kcM and Km of the E4L, E4D, and E4Q mutants, which each gave CD spectra similar to wild-type, were determined by the choline quantitation method [33], and these mutants were found to retain 6-60% of the catalytic efficiency (i.e., kcat/iCm) of wild-type [36, 64]. Furthermore, the pH-dependence with activity curve of the E4L mutant was virtually identical with that for E146Q (Fig. 13) and similar to that of wild-type. In the other inves-... 

The effect of non-participating ligands on the copper catalyzed autoxidation of cysteine was studied in the presence of glycylglycine-phosphate and catecholamines, (2-R-)H2C, (epinephrine, R = CH(OH)-CH2-NHCH3 norepinephrine, R = CH(OH)-CH2-NH2 dopamine, R = CH2-CH2-NH2 dopa, R = CH2-CH(COOH)-NH2) by Hanaki and co-workers (68,69). Typically, these reactions followed Michaelis-Menten kinetics and the autoxidation rate displayed a bell-shaped curve as a function of pH. The catecholamines had no kinetic effects under anaerobic conditions, but catalyzed the autoxidation of cysteine in the following order of efficiency epinephrine = norepinephrine > dopamine > dopa. The concentration and pH dependencies of the reaction rate were interpreted by assuming that the redox active species is the [L Cun(RS-)] ternary complex which is formed in a very fast reaction between CunL and cysteine. Thus, the autoxidation occurs at maximum rate when the conditions are optimal for the formation of this species. At relatively low pH, the ternary complex does not form in sufficient concentration. 

A kinetic model was developed to describe the pH-dependent uncoupling activity of substituted phenols in bacterial photosynthetic membranes [2]. In this model, the overall uncoupling activity is quantitatively separated into the contribution of membrane concentration, which can be estimated by the Kmw, and of intrinsic activity. The intrinsic activity of an uncoupler is influenced not only by the hydrophobicity and acidity, but also by steric effects and by the charge distribution within the molecule [2]. 

The extent of surface coordination and its pH dependence can again be explained by considering the affinity of the surface sites for metal ion or ligand and the pH dependence of the activity of surface sites and ligands. The tendency to form surface complexes may be compared with the tendency to form corresponding (inner-sphere) solute complexes (Fig. 2.7), e.g.,... 

The component waves in the voltammograms (a lower potential catalytic wave and a higher potential switch wave associated with the activation/inactivation of the enzyme) can be seen in Fig. 5.13. The positions of their inflection points were obtained as local extrema in the first derivative with respect to potential (Fig. 5.13, inset), switch corresponds to the potential of the reductive reactivation process. Figure 5.13 shows that as pH is increased Eswitch and both decrease. Furthermore, the pH dependence of Eswitch could be fitted to a 1H+ le stoichiometry with an apparent pK value of 7.7 and a potential at the alkaline limit of -166 mV. 

Consistent with the two-electron donor nature of H2, the reaction behaved as an n=2 Nernst redox reaction. It showed a pH dependence of 66mV per pH unit, so again one proton was taken up for each electron. It is not known where all incoming protons are localized in the enzyme. The reaction shows that in addition to the light-sensitive hydrogen species bound to the active site in the Nia-C " state, a second hydrogen can react at the active site and deliver its two electrons to the enzyme. We hence proposed that the active site of the A. vinosum enzyme has two sites where hydrogen can bind. If H2 is completely removed, the Nia-C state persists for hours this is unlike the situation in redox titrations in the presence of redox mediators. As the active site in the Nig-SR state has one electron more than that in the Nia-C state, an Fe-S cluster has to be involved in this reaction with H2. 

From the known pH dependence of the activity of other enzymes, a domain of approximately 4 pH units should be sufficient to bracket the optimum, yet not be so wide as to seriously invalidate a parabolic approximation to the true behavior of the system. The chosen treatment combinations will therefore be pH = 8, 9, 10, 11, and 12. We will code them as —2, —1, 0, +1, and +2 (c, = 10, = 1 see Section 8.5). 

Figure 1. pH-dependence of the half-life for loss of activity in native and glutaraldehyde-modified j9-D-glucosidase upon preincubation at the indicated pH values and at a range of temperatures from 55o to 70 oC, with subsequent assay at pH 5.0, 45 oC. A,B Linear-ordinate and logarithmic-ordinate plots, respectively, for the glutaraldehyde-modified enzyme. 

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