Aminotransferase ping pong mechanism

Aspartate aminotransferase, and all other enzymes possessing a ping-pong mechanism, are described by Eq. 2.32 ... 

Fig. 14-4 "Ping Pong mechanism of transamination via aminotransferase enzymes.

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... 

One class of enzymes that follow a ping-pong-type mechanism are aminotransferases (previously known as transaminases). These enzymes catalyze the transfer of an amino group from an amino acid to an a-keto acid. The products are a new amino acid and the keto acid corresponding to the carbon skeleton of the amino donor ... 

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

Figure 8-11. Representations of three classes of Bi-Bi reaction mechanisms. Horizontal lines represent the enzyme. Arrows indicate the addition of substrates and departure of products. Top An ordered Bi-Bi reaction, characteristic of many NAD(P)H-dependent oxidore-ductases. Center A random Bi-Bi reaction, characteristic of many kinases and some dehydrogenases. Bottom A ping-pong reaction, characteristic of aminotransferases and serine proteases.

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. 

Figure 8-6 Double-reciprocal plots of Mv against l/[S,] for two-substrate reactions, showing the effect of falling concentration of the second substrate, [S2]. A, In a dehydrogenase reaction in which a ternary complex is formed. B, In a ping-pong bi-bi reaction mechanism (e.g aminotransferase) in which no ternary complex is formed, (From Moss DW. Measurement of enzymes. In Enzymes in cardiology Diagnosis and research. Hearse DJ, de Leiris j, Eds. New York John Wiley Sons Inc, 1979. Reprinted by permission of John Wiley Sons, Inc.)...

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