Saturation kinetic behavior

The rate law for covalent modification shows saturation kinetic behavior if the second step is rate-determining ... 

Criteria for calling a compound a synthetic enzyme are (i) completion of at least one catalytic cycle (ii) its presence after the catalytic cycle in unchanged form and (iii) a saturation kinetics behavior such as is manifested by Michaelis-Menten kinetics. There is a tetrameric helical peptide that catalyzes the decarboxylation of oxaloacetate with Michaelis-Menten kinetics and accelerates the reaction 103-104-fold faster than n-butylamine as control, a record for a chemically derived artificial enzyme. 

To elucidate the mechanism of silver-catalyzed silylene transfer, kinetic studies were performed by Woerpel and coworkers (Scheme 7.17).83 The reaction of cyclohexene silacyclopropane 58 and styrene in the presence of 5mol% of (Ph3P)2AgOTf was followed using 1H NMR spectroscopy. The kinetic order in cyclohexene silacyclopropane 58 was determined to be 1. In contrast to the rate acceleration observed with increasing the concentration of 58, inhibition of the rate of the reaction was observed when styrene, cyclohexene, or triphenylphosphine concentrations were increased. Saturation kinetic behavior in catalyst concentration was observed. Activation parameters were determined to be A// = 30(1) kcal/mol and A A 31(7) eu (entropy units). Similar activation parameters were observed in... 

As binding sites for certain hydrophobic moieties, P CD has been attached to PEI. The PEI derivative (CD-PEI 28) containing P-CD possesses several amino groups (represented by gray circles in 28) located above the CD cavity. Deacylation of 29 and 30 in the presence of CD-PEI manifested saturation kinetic behavior. 

Rate data were collected by varying the amount of the catalyst (CJ. Here, was expressed as the concentration of the Cu(II) complex of cyclen obtainable when the resin is assumed to be dissolved. As illustrated in Figure 3 for the dependence of kg on Cq, saturation kinetic behavior was observed at pH 4.5-7 for the cleavage of both the heavy and light chains of Gbn by 39. On the other hand, was proportional... 

Saturation kinetic behavior was manifested by 39 at pH 4.5-7, indicating much stronger complexation of Gbn to 39 compared with 38. Thus, the guanidinium moieties of 39 acted as the binding site for Gbn recognizing the carboxylate ions of Gbn. The reactivity of the Cu(II) complex of cyclen toward Gbn was enhanced by > 10 times upon attachment to PCD containing guanidinium ions. The microenvironment of Cu(II)-cyclen moieties in the gel phase of the polymer surface contains both hydrophobic and ionic characters. The enhanced reactivity of the Cu(II) center may be related to the unique medium properties. 

Hiickel expression. The rates of electron transfer from bovine liver cyt to a series of Fe(lII) and Cu(III) chelate complexes have been measured, with prior binding of the Cu(II) species to the protein indicated from saturation kinetic behavior. " ... 

Quite complex kinetic behavior has been identified on some surfaces. For instance, on Ir(100), the TPD data from NO-saturated surfaces display two N2 desorption peaks, one at 346 K from the decomposition of bridge-bonded NO, and a second at 475 K from the decomposition of atop-bonded NO molecules [13], Interestingly, the first feature is quite narrow, indicating an autocatalytic process for which the parallel formation of N20 appears to be the crucial step. An additional complication arises from the fact that this Ir(100) surface undergoes a (1x5) reconstruction, and that NO adsorbed on the metastable unreconstructed (lxl) phase leads to N2 desorption at lower temperatures. In another example, on the reconstructed hexagonal Pt(100) surface, when a mixed NO + CO adsorbed layer is heated, a so-called surface explosion is observed where the reaction products (N2, C02 and N20) desorb simultaneously in the form of sharp peaks with half-widths of only 7 to 20 K. The shape of the TPD spectra suggests again an autocatalytic mechanism [14],... 

As a simple model for the enzyme penicillinase, Tutt and Schwartz (1970, 1971) investigated the effect of cycloheptaamylose on the hydrolysis of a series of penicillins. As illustrated in Scheme III, the alkaline hydrolysis of penicillins is first-order in both substrate and hydroxide ion and proceeds with cleavage of the /3-lactam ring to produce penicilloic acid. In the presence of an excess of cycloheptaamylose, the rate of disappearance of penicillin follows saturation kinetics as the cycloheptaamylose concentration is varied. By analogy to the hydrolysis of the phenyl acetates, this saturation behavior may be explained by inclusion of the penicillin side chain (the R group) within the cycloheptaamylose cavity prior to nucleophilic attack by a cycloheptaamylose alkoxide ion at the /3-lactam carbonyl. The presence of a covalent intermediate on the reaction pathway, although not isolated, was implicated by the observation that the rate of disappearance of penicillin is always greater than the rate of appearance of free penicilloic acid. 

Mathematically, the Michaelis-Menten equation is the equation of a rectangular hyperbola. Sometimes you ll here reference to hyperbolic kinetics, this means it follows the Michaelis-Menten equation. A number of other names also imply that a particular enzyme obeys the Michaelis-Menten equation Michaelis-Menten behavior, saturation kinetics, and hyperbolic kinetics. 

Campbell and coworkers269 also published a kinetics study of the reverse water-gas shift over Cu(110) in 1992, and the results were cast in terms of the redox mechanism (reverse of Scheme 60, left side). A hydrogen-induced surface phase transition was suggested to impact the rate at high H2/C02 ratios, as the rate was found to exhibit a saturation-like behavior with increasing P(H2) when 5 Torr of C02 was used, but continued on a log-linear trend when 150 Torr of C02 was... 

The kinetic behavior of drugs in the body can generally be accounted for by first-order kinetics that are saturable, i.e., Michaelis-Menten kinetics. A brief review of the principles of Michaelis-Menten kinetics is given next (4). 

Figure 11.1 illustrates the behavior of Equation 11.6. By the assumption of rapid equilibrium the rate determining step is the unimolecular decomposition. At high substrate composition [S] KM and the rate becomes zero-order in substrate, v = Vmax = k3 [E0], the rate depends only on the initial enzyme concentration, and is at its maximum. We are dealing with saturation kinetics. The most convenient way to test mechanism is to invert Equation 11.6... 

This behavior is sometimes referred to as saturation kinetics. When ft [B] < 1, the observed second-order is easily understood (rate constant = a). When 6 [B] 1 there is a mixed-order... 

It is relatively easy to spot behavior (c). Plots of k vs [B] are non-linear with [B] > [A] but will always remain linear with [A] > [B]. Saturation kinetics will arise with (a) and (b)... 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

Similar kinetic behavior was observed for the reaction of 4-methyhuercaptoacetophe-none with MeiMg in diethyl ether. Two absorption maxima at 308 nm (free ketone) and 337 nm (complex) showed saturation kinetics, which fitted with the scheme Ketone -F Me2Mg Complex Product. From a linear plot of l/k vs. l/[Me2Mg], ki and K were calculated where ki is the rate constant of the second step and k is the equilibrium constant of the first step. 

The presence of 4e as the predominant species during the catalysis is also in accord with the observed kinetic behavior of this catalyst with 1-octene and styrene as the substrates. The observation of this saturated acyl rhodium complex is in line with the positive dependence of the reaction rate on the hydrogen concentration and the zero order in alkene concentration. It was concluded previously that this saturated acyl complex is an unreactive resting state [18]. Before the final hydro-genolysis reaction step can occur, a CO molecule has to dissociate in order to form... 

As a result of saturation of protein binding glucocorticosteroids may exhibit a dose-dependent kinetic behavior with increases of both distribution volume and half-life with increased doses. 

A further manifestation of an initial binding step is saturation kinetics. For example, in the presence of excess catalyst, (26) predicts a hyperbolic relation between fcobs and concentration of nucleophilic sites C0(=nP0). Such behavior is actually what is observed. 

Non Michaelis-Menten behavior (i.e., no saturation kinetics in presence of excess HC03) was observed and the second-order rate constants for the catalyzed decarboxylation (/ = k k<>j(k i / 2) for... 

In contrast with our results with the flavopapains III and V, we have found that flavopapain IV can act as an effective catalyst for the oxidation of dihydronicotinamides (13, 14). Using dihydronicotinamide in excess, the oxidation reactions of N-benzyl-, N-ethyl-, N-propyl- and N-hexyl-l,4-dihydronicotinamide as well as that of NADH by flavopapain IV exhibit saturation kinetics at relatively low substrate concentrations. This behavior contrasts with the oxidation by model flavins or flavopapains III and V, where saturation... 

Can a phosphorylation-dephosphorylation switch be more sensitive to the level of kinase concentration than n = 1 as given in Equation 5.12 We note that the kinetic scheme in Equation (4.7) is obtained under the assumption of no Michaelis-Menten saturation. Since this assumption may not be realistic, let us move on to study the enzyme kinetics in Figure (5.2) in terms of saturable Michaelis-Menten kinetics. The mechanism by which saturating kinetics of the kinase and phosphatase leads to sensitive switch-like behavior is illustrated in Figure 5.4. The reaction fluxes as a function of / (the ratio [S ]/Sc) for two cases are plotted. The first case (switch off)... 

Such behavior is generally referred to as Michaelis-Menten kinetics or saturation kinetics. As will be seen, it is a rather common feature of trace-level catalysis, not restricted to cycles as simple as 8.14. 

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