Double displacement reactions ping-pong mechanism

The steady state kinetics of arsenite oxidoreductase from A. faecalis indicate a so-called double displacement (or ping-pong ) mechanism (15) in which the enzyme cycles between oxidized and reduced forms in its reaction with arsenite and azurin (or cytochrome c). This overall kinetic scheme is common in redox-active proteins. Arsenite must bind, the oxygen atom transfer chemistry take place, and arsenate dissociate before the subsequent reaction of a second molecule of substrate. Since arsenate is not an inhibitor of arsenite oxidoreductase (43), product dissociation must be effectively irreversible. The turnover number (kcai) of 27 sec and for arsenite of 8 pM are reasonable parameters for the detoxification of arsenite, especially since A. faecalis is able to survive in at least 80 mM (1%) sodium arsenite. The considerable catalytic power of the enzyme is reflected by the kinetic parameter k JK of 3.4 X 10 M sec , which is fairly close to the diffusion-controlled maximum of 10 -10 M sec for proteins in... 

In double-displacement, or Ping-Pong, reactions, one or more products are released before all substrates bind the enzyme. The defining feature of double-displacement reactions is the existence of a substituted enzyme intermediate, in which the enzyme is temporarily modified. Reactions that shuttle amino groups between amino acids and a-keto acids are classic examples of double-displacement mechanisms. The enzyme aspartate aminotransferase (Section 23.3.1) catalyzes the transfer of an amino group from aspartate to a-ketoglutarate. 

Steady-state kinetics have been used to determine the kinetic mechanisms of many of these enzymes. The questions that have been primarily addressed are the sequence of steps that occur in substrate binding prior and subsequent to the catalytic reaction and the potential formation of covalent enzyme intermediates. Classical interpretation of kinetic analyses has been the determination of the relevant reactions occurring via a random or an ordered sequential reaction, or if the reaction is a double-displacement or Ping-Pong reaction. In the former case, phosphoryl transfer occurs in the ternary complex that contains enzyme, phosphoryl donor, and phosphoryl acceptor. In the latter case, enzyme reacts with... 

Reactions that fit this model are called ping-pong or double-displacement reactions. Two distinctive features of this mechanism are the obligatory formation of a modified enzyme intermediate, E, and the pattern of parallel lines obtained in double-reciprocal plots (Figure 14.19). 

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.

Although this reaction is fully reversible, the relatively high [ATP]/[ADP] ratio in cells normally drives the reaction to the right, with the net formation of NTPs and dNTPs. The enzyme actually catalyzes a two-step phosphoryl transfer, which is a classic case of a double-displacement (Ping-Pong) mechanism (Fig. 13-12 see also Fig. 6-13b). First, phosphoryl group transfer from ATP to an active-site His residue produces a phosphoenzyme... 

Dopamine P-monooxygenase 887 Double displacement reactions 595 ping-pong mechanism 595 in ribonuclease 647 Double helices 201,213-218... 

Enzymes catalyzing transfer of the a-amino group of an amino acid to the a-carbon of an a-keto acid (often a-ketoglutarate) are known as transaminases or aminotransferases. Important intermediates in such reactions are a series of Schiff bases that can be trapped by reduction with sodium borohydride (Fischer et al., 1958). These enzymes provide typical examples of double-displacement (ping-pong) mechanisms, whereby the pyridoxa-mine phosphate form is a discrete intermediate, and reaction with an a-keto acid is necessary to regenerate the pyridoxal form (Scheme 2). Although the... 

Figure 2.14 Reaction pathway for a bi-bi double-displacement (ping-pong) reaction mechanism.

PING PONG HALF-REACTIONS. Many enzymes operate by double-displacement mechanisms involving covalent enzyme-substrate intermediates as shown in the following scheme ... 

The double-displacement mechanism with its covalent intermediate can often be distinguished from single displacement reactions by kinetic experiments in which an appropriate nudeophilic spedes is added to reaction solutions of enzyme and substrate and allowed to compete with water for the covalent intermediate. The dependence of initial velocity on the concentrations of substrate and added nucleophile will produce a distinctive kinetic pattern (i.e., ping-pong kinetics). Observation of such a pattern constitutes strong evidence suggesting that the hydrolase under study effects catalysis through a double-displacement mechanism. 

The catalytic reaction of lipases follow the so called ping-pong bi-bi mechanism, a double displacement mechanism. This is a special multisubstrate reaction in which, for a two-substrate, two-product (i.e., bi-bi) system, an enzyme reacts with one substrate to form a product and a modified enzyme, the latter then reacting with a second substrate to form a second, final product, and regenerating the original enzyme (ping-pong). 

Despite the facts cited above, which are undisputed, the mechanism of reaction (16) is no longer believed to involve a double displacement or a phosphoenzyme. The apparent Ping-Pong kinetics, the rates of the exchange reactions (16a) and (16b), and the isolated phosphoenzyme fail to meet the most rigorous criteria for the double-displacement mechanism. And the exchange re-... 

Despite the fact that reaction kinetics cannot be Ping-Pong, the enzyme is phosphorylated by substrates and the phosphoenzyme reacts with ADP to form ATP. Is this process a part of the mechanism of reaction (16) The stereochemical analysis of phospho transfer argues strongly against the importance of the phosphoenzyme in the reaction mechanism. The phospho transfer proceeds with inversion of configuration at phosphorus, which is consistent with a single displacement at phosphorus and inconsistent with a double-displacement mechanism (64). Therefore, the phosphoenzyme does not appear to be on the main catalytic pathway. 

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