A double displacement reaction (also called a metathesis reaction) is the bimolecular mechanism, whereby two compound reactants AB and CD result in products of AC and BD. This is typically caused by nucleophilic attack of one group onto another, followed by the release of another group to form a different compound. A common example of this reaction is olefin metathesis, which is the rearrangement of fragments between alkenes about double bonds. Double displacement reactions occur when one or more products are released before all the substrates bind to the enzyme. Another name for double displacement reactions is "Ping-Pong" reactions. The name comes from the fact that substrates appear to bounce on and off the enzyme just like a ping-pong ball bouncing up and down on a table. An example of double displacement reaction can be seen with the enzyme, aspartate aminotransferase. This enzyme catalyzes the transfer of an amino group from aspartate to alpha-ketoglutarate to form oxaloacetate and glutamate. However, this reaction does not occur sequentially. Aspartate first binds to the enzyme and donates it's amino group to the enzyme, thus modifying the enzyme into what is called a substituted enzyme intermediate. After donating the amino group, aspartate becomes oxaloacetate and is then released. The second substrate, alpha-ketoglutarate, then binds to the enzyme and receives the amino group from the substituted enzyme intermediate. The reaction follows with the release of the final product, glutamate. (1) Aspartate (2) α-Ketoglutarate (3) Oxaloacetate (4) Glutamate
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