Mechanisms ping pong

Figure 7-4. Ping-pong mechanism for transamination. E—CHO and E—CHjNHj represent the enzyme-pyridoxal phosphate and enzyme-pyridoxamine complexes, respectively. (Ala, alanine Pyr, pyruvate KG, a-ketoglutarate Glu, glutamate).

Several mechanisms have been proposed for lipase-catalyzed reactions. Kinetic studies of hydrolysis [14,15] and esterification [50] catalyzed by Pseudomonas cepecia lipase, demonstrate that the enzyme has a ping-pong mechanism. 

Figure 2.15 Double recipcrocal plots for a bi-bi enzyme reactions that conform to (A) a ternary complex mechanism and (B) a double-displacement (ping-pong) mechanism.

Two-Step (Push-Pull, Ping-Pong) Mechanisms Two-step mechanisms are typical of chemical catalytic processes, as opposed to redox catalysis processes, that are discussed and exemplified in Section 6.2. The first step following the generation at the electrode of the active form of the catalyst, Q, is the formation of an adduct, C, with the substrate A (Scheme 2.11). C requires an additional electron transfer to regenerate the initial catalyst, P. There are then two main possibilities. One is when C is easier to reduce (or oxidize in oxidative processes) than P. The main route is then a homogeneous electron... 

The Ping-Pong Mechanism. Kinetic Control by Substrate and/or Cosubstrate... 

FIGURE 5.1. Ping-pong mechanism, normalized catalytic wave. From left to right log c — oo,2, 1, 0, —oo. Adapted from Figure 3 of reference 9, with permission from Elsevier. 

FIGURE5.3. Ping-pong mechanism. Variation of the peak or plateau current with the kinetic parameter from no catalysis and the pure kinetic conditions leading to plateau-shaped responses for several values of the competition parameter s from top to hottom 0, 0.31, 0.725, 1.25, 2.5, 5, 10, 20, oo. Adapted from Figure 2 in reference 10, with permission from the American Chemical Society. 

The Ping-Pong Mechanism with an Immobilized Enzyme and the Cosubstrate in Solution... 

The Ping-Pong Mechanism in Homogeneous Enzymatic Catalysis... 

Enzymatic catalysis mechanisms may well be considerably more complex than the basic ping-pong mechanism, involving inhibition and hysteresis phenomena. Horseradish peroxidase offers a remarkable example... 

Figure 8. The most common enzyme mechanisms, represented by their corresponding Cleland plots The order in which substrates and products bind and dissociate from the enzyme is indicated by arrows, (a) The Random Bi Bi Mechanism-. Both substrates bind in random order, (b) The Ordered Sequential Bi Bi Mechanism-. The substrates bind sequentially, (c) The Ping Pong Mechanism-. The enzyme exists in different states E and E. A substrate may transfer a chemical group to the enzyme. Only upon release of the first substrate, the chemical group is transferred to the second substrate.

Certain classes of enzymes tend to have characteristic mechanisms. (Examples transaminases often have ping-pong mechanisms, kinases usually do not). 

Figure 4.10 are representative for the case when both reactions in the basic system are reversible, and also when both directions follow the ping-pong mechanism. This figure presents the effect of A eq on the concentration of B when A eq is defined as the ratio between the maximal reaction rate in the reverse direction Vm-2) and the maximal reaction rate in the forward... 

Figure 4.13 Effect of reaction mechanism on the concentrations of Sj and B in the basic system when operated as a fed-batch reactor. The kinetic mechanism and the values of the parameters Ka and Ki, are indicated on top of each section —indicates that the parameter is not applicable for the ping-pong mechanism. The values used for all other parameters are given in Table 4.1, set I.

From Figure 4.54 it can he seen that the family of reciprocal plots obtained at different fixed concentrations of NADP are essentially parallel to one another. This is also indicated in Figure 4.55, where the value of the slopes of the tines seem to be approximately constant. These results imply that the velocity equation for the ping-pong mechanism [146] can be used to describe the rate of the reaction catalyzed by G6PDH. Although initial velocity studies alone cannot define the exact kinetic mechanism [146,147], we are more interested in the appropriate rate equation that describes the reaction progress. 

Fig. 2. The Bonnichsen, Chance, and Theorell 34) mechanism for the dismutation of hydrogen peroxide by catalase. (A) The simple ping-pong mechanism (ferric-peroxide compound (ycle) involves only the successive formation and decomposition of the compound 1 intermediate by two successive molecules of H2O2. (B) Reversible ES(Fe -H202) and ternary (compound I-H2O2]) complexes are added to the mechanism in A.

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