Ping pong, enzymatic reaction

As mentioned above, rat liver cytosol contains one or more factors that stimulate microsomal iodothyronine deiodinase activity. It has been realized for more than a decade now that the enzymatic deiodination of iodothyronines is a reductive process which is supported by different synthetic and natural SH compounds [48]. Most investigations of the catalytic mechanism of the deiodinase have utilized artificial cofactors such as the dithiol DTT. The results have demonstrated that both ORD and IRD follow ping-pong type reaction kinetics, indicating that the enzyme exists in two alternating forms induced by the reactions with substrate and cofactor [7,8]. 

Example 8.5. Ping-pong transfer reactions. Some enzymatic transfer reactions proceed by so-called ping-pong mechanisms [40,41]. In these, the conversion of a reactant to a product leaves the enzyme in a different form. The modified enzyme then converts a second reactant to another product while itself being restored to its original form. Enzymatic transaminase reactions interconverting amino and keto acids provide a typical example [40] ... 

Transamination, often also referred to as aminotransfer, is applied to those enzymatic reactions in which an amino group is exchanged between an amino acid and an a-keto acid. This type of reaction is catalyzed by a group of transferases called transaminases or aminotransferases. They are active in both the cytosol and the mitochondria of most cells. An essential prosthetic group of such enzymes is pyridoxal phosphate, and the reaction is generally of the ping-pong type. 

The discussion here is limited to CuZnSOD for a number of reasons. First, in this system the ping-pong mechanism described in reactions (22) and (23) is operative as written whereas both MnSOD and FeSOD carry out catalysis by mechanisms that involve observable enzyme-substrate complexes under certain conditions. Secondly, this enzymatic system has been studied as a model to look at such factors as electrostatic guidance of substrate (see below). Finally, the link that was demonstrated between over 100 point mutations in CuZnSOD and the inherited version of amyotrophic lateral sclerosis (Lou Gehrig s Disease) has made underscored the importance in understanding details of enzyme function. ... 

Enzymes are more commonly involved in the reaction of two substrates to form products. In this problem, analyze the specific case of the ping pong hi hi mechanism [W. W. Cleland, Biochim. Biophys. Acta, 67 (1963) 104] for the irreversible enzymatic conversion of... 

In conclusion, all enzymatic methyl transfer reactions studied so far proceed with net inversion of configuration of the methyl group. All these methyl transferases therefore involve an uneven number of transfers of the methyl group, most likely a single, direct transfer from the sulfur of AdoMet to the acceptor atom in an Sj.2-type reaction. Ping-pong mechanisms in which a group in the enzyme active site is transiently methylated can be excluded. The two substrates must be oriented in the enzyme active site such that in the transition state the sulfur, the methyl carbon and the acceptor atom form a linear array. 

TABLE 11.5 Cleland nomenclature for bisubstrate reactions exemplified. Three common kinetic mechanisms for bisubstrate enzymatic reactions are exemplified. The forward rate equations for the order bi bi and ping pong bi hi are derived according to the steady-state assumption, whereas that of the random bi bi is based on the quasi-equilibrium assumption. These rate equations are first order in both A and B, and their double reciprocal plots (1A versus 1/A or 1/B) are linear. They are convergent for the order bi bi and random bi bi but parallel for the ping pong bi bi due to the absence of the constant term (KiaKb) in the denominator. These three kinetic mechanisms can be further differentiated by their product inhibition patterns (Cleland, 1963b)... 

Fig. I. (a) Mechanism of enzymatic transamination, showing conversion of pyridoxal-phosphate to pyridoxamine phosphate in the presence of the amino acid substrate, followed by release of keto acid product, (b) Mechanism of transamination, according to Ping Pong Bi Bi reaction mechanism, where the cofactor oscillates between the aminated and unaminated form. Abbreviations AA, amino acid substrate E, pyridoxal-P form of the enzyme OA, keto acid product E, pyridoxamine-phosphate form of the enzyme OAj, keto acid substrate AAj, amino acid product.

The zinc ion is responsible for the structure role in CuZnSOD, whereas the copper is the essential for the enzymatic process. Replacing the zinc by Co(II), Cu(II), or Cd(II) indicated no or very limited loss of activity (42). This dismuta-tion reaction occurs by a two-step process via a classical ping-pong mechanism (eqs. (3) and (4)). First, O2 reduces the active site copperdD ion and generates and releases dio gen (eq. (3)). Second, another O2 obtains one electron from the reduced active site and two protons to form hydrogen peroxide (eq. (4)). 

Multiple substrate reactions are more frequently occurred than single substrate reactions. Thus, efforts have been made to develop two or more substrate enzymatic reaction mechanisms. One of the most important examples is lipid hydrolysis, where lipid and water molecules act as two substrates to produce two products namely fatty acids and glycerol. Cleland (1963) proposed three types of mechanism for two substrate reactions based on the order of adding the substrates and products release from the active site within the reaction sequence. These are ordered-sequential, random-sequential, and Ping-Pong, shown in Figure 4.1. 

The proeess of bioeleetrocatal3d ic redox mediation can be generally described as a ping pong mechanism that occurs in three discrete reactions the enzymatic oxidation of a substrate, electron transfer between the enzyme and the artificial redox mediator, and diffusion of the oxidized mediator to the electrode surface (Scheme 1). ... 

In enzymatic reactions where functional group transfers are involved, as a rule only binary enzyme-substrate complexes are formed by the so-called ping pong mechanism ... 

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