Reaction inactivation rate constant

Boltzmann universal constant first-order inactivation rate constant Planck universal constant catalytic rate constant catalytic rate constant at infinite dilution reaction rate constants according to reaction scheme molar concentration of product molar concentration of competitive inhibitor product molar concentration of non-competitive inhibitor product... 

Reversible collapse of enzyme IPEC species can also be used to control enzyme stability with respect to denaturation processes. Thus, thermal stability of PAase in soluble IPEC (PMANa/PAase Q-P4VP) is at pH > 6.0 practically equal to that of native PAase. However, compacting of the complex species at pH < 6.0 (see Figs. 10.3 and 10.5a) results in more than a 10-fold decrease in the inactivation rate constant (Kjn), i.e. in considerable enhancement of thermal stability, as it is seen in Fig. 10.8. At the same time a sharp decrease of the apparent catalytic activity (F/Xm, where V is the reaction rate) is also observed (Fig. 10.6, curve 2). Dissolving of enzyme - IPEC species at pH < 2.9 is followed by immediate loss of the additional stabilization effect, so that Kin becomes equal again to that of native enzyme [39] (Fig. 10.8, compare curves 1 and 2). Conformation transitions of enzyme IPEC species with variation of pH are represented schematically in Fig. 10.9. Stabilization of the enzyme globule... 

The inactivation is normally a first-order process, provided that the inhibitor is in large excess over the enzyme and is not depleted by spontaneous or enzyme-catalyzed side-reactions. The observed rate-constant for loss of activity in the presence of inhibitor at concentration [I] follows Michaelis-Menten kinetics and is given by kj(obs) = ki(max) [I]/(Ki + [1]), where Kj is the dissociation constant of an initially formed, non-covalent, enzyme-inhibitor complex which is converted into the covalent reaction product with the rate constant kj(max). For rapidly reacting inhibitors, it may not be possible to work at inhibitor concentrations near Kj. In this case, only the second-order rate-constant kj(max)/Kj can be obtained from the experiment. Evidence for a reaction of the inhibitor at the active site can be obtained from protection experiments with substrate [S] or a reversible, competitive inhibitor [I(rev)]. In the presence of these compounds, the inactivation rate Kj(obs) should be diminished by an increase of Kj by the factor (1 + [S]/K, ) or (1 + [I(rev)]/I (rev)). From the dependence of kj(obs) on the inhibitor concentration [I] in the presence of a protecting agent, it may sometimes be possible to determine Kj for inhibitors that react too rapidly in the accessible range of concentration. ... 

The inactivation of enzymes containing the zinc-thiolate moieties by peroxynitrite may initiate an important pathophysiological process. In 1995, Crow et al. [129] showed that peroxynitrite disrupts the zinc-thiolate center of yeast alcohol dehydrogenase with the rate constant of 3.9 + 1.3 x 1051 mol-1 s-1, yielding the zinc release and enzyme inactivation. Later on, it has been shown [130] that only one zinc atom from the two present in the alcohol dehydrogenase monomer is released in the reaction with peroxynitrite. Recently, Zou et al. [131] reported the same reaction of peroxynitrite with endothelial NO synthase, which is accompanied by the zinc release from the zinc-thiolate cluster and probably the formation of disulfide bonds between enzyme monomers. The destruction of zinc-thiolate cluster resulted in a decrease in NO synthesis and an increase in superoxide production. It has been proposed that such a process might be the mechanism of vascular disease development, which is enhanced by diabetes mellitus. 

For Aspergillus niger extracellular endo-D-galacturonanase, the role of histidine in the enzyme reaction was investigated by the method of photo-oxidative inactivation, catalyzed by Methylene Blue.140 The inactivation of the enzyme was paralleled by the decomposition of histidine. The similarity of pH profiles, as well as the values of the rate constants of enzyme inactivation (4.0 X 10-2 min-1) and of decomposition of histidine (3.9 X 10-2 min-1), indicate that one of the five histidine residues present in the molecule of the enzyme141 is essential for its activity. 

Selected entries from Methods in Enzymology [vol, page(s)] Sulfonylation reaction, 11, 706 reaction kinetics, 11, 707 second-order rate constants for inactivation of chymotrypsin, trypsin, and acetylcholine esterase by PMSE and related sulfonylat-ing agents, 11, 707 reactivation of PMS-chymotrypsin, 11, 710 as inhibitor [of calcium-activated factor, 80, 674 of cathepsin G, 80, 565 of crayfish trypsin, 80, 639 of elastase, 80, 587 of pro-lylcarboxypeptidase, 80, 465 of protease Re, 80, 691 of protease So, 80, 695 of protein C, 80, 329] proteolysis, 76, 7. 

The concentration of impurities which inactivate a catalyst, C, usually becomes obvious from a plot of the rate constant of the catalysed reaction against [C], such as is shown in Fig. 4.7. As the purification of the solvent, the monomer, and the hardware progresses, not only does the intercept diminish, but the slope of the line (usually) increases. It has been shown that if the dependence of on monomer concentration, [M], at [C] = constant and on [C] at [M] = constant is available, the [Imp] originating from the solvent and the [Imp] originating from the monomer can be measured separately (Holdcroft and Plesch, 1984). 

Other peroxidases have a similar mechanism, but peroxidases vary significantly in terms of their rate constants and their susceptibility to side reactions that may cause temporary or permanent inactivation. 

Comparing Equation 29 with Equation 9 shows that the two expressions for kobe differ only in the constants in the numerators of the right-hand sides. Both mechanisms predict first-order kinetics for the loss of site activity and identical dependence of the observed first-order rate constant, kobBy on the [R]. The similarity of Equations 9 and 29 demonstrates that the documentation of saturation kinetics as evidenced by linear Kitz-Wilson or Eadie-Hofstee plots or by the critria of the direct linear plot does not prove that true affinity labeling is involved necessarily in a site-inactivating reaction. 

This expression for E emphasizes the fact that the most specific reagents will have the largest values for k2/Kr. The expression /c2/KR is the parameter which is the most fundamental determinant of labeling specificity. It has a very simple kinetic significance reference to Equation 7 shows that it is the second-order rate constant for the inactivation reaction which is obtained at low [R]. 

Affinity labeling agents are intrinsically reactive compounds that initially bind reversibly to the active site of the enzyme then undergo chemical reaction (generally an acylation or alkylation reaction) with a nucleophile on the enzyme (Scheme 8). To differentiate a reversible inhibitor from an irreversible one, often the dissociation constant is written with a capital i, K (65), instead of a small i, K, which is used for reversible inhibitors. The K denotes the concentration of an inactivator that produces half-maximal inactivation. Note that this kinetic Scheme is similar to that for substrate turnover except instead of the catalytic rate constant, kcat for product formation, kmact is used to denote the maximal rate constant for inactivation. 

The scheme summarized here is certainly a simplified representation of reality since it neglects the breaking of bonds involving primary chains and cyclization reactions. However, the overall vulcanization process can be described by two rate constants, i.e. kj and kf (Table 18), the former referring to the inactivation reactions and the latter to the reactions yielding the polymer network. 

Figure 4-S9 The activation energy, for a reaction can be determined by measuring the reaction rate constant at different temperatures and plotting log k versus 1 /T. For enzyme-catalyzed reactions, log Vms /[E]( or just log can be plotted. Curve A The usual plot. Curve B Sometimes the plot will show a definite change in slope if at some temperature a different step becomes rate-limiting. Curve C A sudden drop in the plot indicates enzyme inactivation.

The role of mass transfer effects, whether occurring accidentally or by design, is ambivalent, causing Trevan to ask the question Diffusion limitation - friend or foe [115]. Lower activity as a result of low efficiency indicates that only a minor portion of enzyme is active during operation. The other unused portion may, in simple terms, replace the enzyme as it is inactivated step by step. In other words, mass transfer controlled reactions appear to be much less sensitive to decay of enzyme activity, thus falsely creating an impression of stabilization. Under harsh reaction conditions it may be advantageous to operate under these conditions to keep the reaction rate constant until the diffusion limitation disappears [82,115,116]. 

FIGURE 4.6 Dependence of (pseudo) first-order reaction rate constants (k) on temperature (T). Approximate examples for heat inactivation of alkaline phosphatase and plasmin, for killing of Clostridium botulinum spores, and for the formation of a certain small amount of Maillard products. t 0A is the time needed for the reaction to proceed for 0.1 times the final value (not for the Maillard reaction). 

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