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ChemWiki
Biological Chemistry
Enzymes
Enzymatic Kinetics
The "Ping-Pong" Mechanism
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The "Ping-Pong" Mechanism
Modified 01:06, 5 Mar 2013 by Delmar
Table of Contents
1.
2.
1.
2.
3.
4.
5.
6.
7.
1. Introduction
2. Examples
2.1. Chymotrypsin
2.2. Pyruvate Carboxylase
3. Further Reading
4. Sample Questions
5. Answers
6. References
7. Contributors
In order to understand the ping-pong mechanism, it is important to have an underlying
knowledge of enzymes. Particularly, focus on enzyme and substrate chemistry. The simplest
of enzymes will involve one substrate binding to the enzyme and producing a product plus
the enzyme. However, the majority of enzymes are more complex and catalyze reactions
involving multiple substrates. Binding of two substrates can occur through two mechanisms:
sequential mechanism and non-sequential mechanism. In sequential mechanisms both
substrates bind the enzyme and the reaction proceeds to form products which are then
released from the enzyme. This mechanism can be further subdivided into random and
ordered reactions. For random reactions the order in which the substrates bind does not
matter. In ordered reactions one substrate must bind the enzyme before the second substrate
is able to bind. Non-Sequential mechanism does not require both substrates to bind before
releasing the first product. This page will focus on the nonsequential mechanism, which is
also known as the "ping-pong" mechanism. It is called this because the enzyme bounces back
and forth from an intermediate state to its standard state.The enzyme acts like a ping-pong
ball, bouncing from one state to another.
1.
2.
1.
2.
3.
4.
5.
6.
7.
1. Introduction
2. Examples
2.1. Chymotrypsin
2.2. Pyruvate Carboxylase
3. Further Reading
4. Sample Questions
5. Answers
6. References
7. Contributors
Introduction
Ping-pong mechanism also called a double-displacement reaction is characterized by the
change of the enzyme into an intermediate form when the first substrate to product reaction
occurs. It is important to note the term intermediate indicating that this form is only
temporary. At the end of the reaction the enzyme MUST be found in its original form. An
enzyme is defined by the fact that it is involved in the reaction and is not consumed. Another
key characteristic of the ping-pong mechanism is that one product is formed and released
before the second substrate binds. The figure below explains the Ping Pong mechanism
through an enzymatic reaction.
This image shows that as substrate A binds to the enzyme, enzyme-substrate complex
EA forms. At this point, the intermediate state, E* forms. P is released from E* , then B binds
to E*. B is converted to Q, which is released as the second product. E* becomes E, and the
process can be repeated. Often times, E* contains a fragment of the original substrate A.This
fragment can alter the function of the enzyme, gets attached to substrate B, or both.
Here is another diagram showing this same reaction:
Examples
Chymotrypsin
An example of the ping-pong mechanism would be the action of chymotrypsin. When
reacted with p-nitrophenyl acetate (A), the reaction of chymotrypsin is seen to occur in two
steps. In the first step, the substrate reacts extremely fast with the enzyme, leading to the
formation of a small amount of p-nitrophenolate (P). In the second step, the substrate-enzyme
interaction results in the formation of acetate ion (Q). The action of chymotrypsin is a pingpong reaction because the binding of the two substrates causes the enzyme to switch back and
forth between two states. Please refer to the section Chymotrypsin and pre-steady-state
enzyme kinetics for more details on the action of chymotrypsin.
Pyruvate Carboxylase
Another example of an enzyme that exhibits a ping-pong mechanism is pyruvate carboxylase.
This enzyme catalyzes the addition of carbon dioxide to pyruvate in order to form
oxaloacetate. (leads to gluconeogenesis) This biotin-containing enzyme works by binding
CO2 (A) to form carboxybiotin (EA). The biotin swings over towards pyruvate (E*P) and
releases CO2. (P, due to the fact that it had been moved from its original binding site)
Pyruvate (B), in close proximity to CO2, attacks the partial positive of Carbon in CO2 (E*B).
Oxaloacetate is formed within the enzyme (EQ) and gets released (Q). While this attack is
occuring, biotin swings back to its initial position, (E* --> E) and is ready to bind another
CO2.
Further Reading
An important factor to understand about the ping-pong mechanism is that when plotting a 1/v
and 1/[A] plot at varying concentrations of B, a series of parallel lines are seen. In this case A
is the first substrate and B being the second substrate.
Refer to these sections on enzyme kinteics and michaelis-menten kinetics to get a better
understanding of what this type of plot means.
Sample Questions
1. The form in which the ping pong mechanism binds substrates is identified as which
type of mechanism?
2. What are two characteristics of an enzyme that catalyzes a reaction through the pingpong mechanism?
3. The following diagram shows the mechanism of glutamate-aspartate
aminotransferase: Would this mechanism be considered a ping-pong/doubledisplacement reaction? Why or why not?
Answers
1) The ping-pong mechanism is a non-sequential mechanism. A product is released after the
first substrate is bound.
2) One, a product is seen before the second substrate is bound.
Two, binding of the first substrate causes the enzyme to change into an imtermediate form
that will bind the second substrate.
Three, the plot of 1/v vs. 1/[A] as [B] changes will be parallel lines.
3) Yes! This would definitely be considered a ping-pong mechanism. First, we can see that
there is an E' state which is indicative of a ping-pong mechanism. Pyridoxal is a coenzyme
bound to glutamate-aspartate aminotransferase that accepts an amino group from glutamate
and becomes pyridoxamine while releasing alpha-ketoglutarate. Pyridoxamine bound to the
enzyme will then donate its amino group to oxaloacetate to regenrate pyridoxal as well as
aspartate.
References
1. Chang, Raymond. 2005. Physical Chemistry for the Biosciences. Sausalito (CA):
University Science Books. p. 372-377.
2. Cleland, Wallace. "Derivation of Rate Equations for Multisite Ping-Pong Mechanisms
with Ping-Pong Reactions at One or More Sites." Journal of Biological Chemistry
248.24 (1973): 8353-355. Print.
3. Garrett, R., and Charles M. Grisham. "Gluconeogenesis, Glycogen Metabolism, and
the Pentose Phosphate Pathway." Biochemistry. Belmont, CA: Brooks/Cole, Cengage
Learning, 2010. 664-66. Print.
4. Garrett, Reginald and Charles M. Grisham. "Enzymes-Kinetics and Specificity".
Biochemistry, Fourth Edition. Belmont, CA: Brooks/Cole, Cengage Learning, 2010.
p. 406-407.
Contributors

Melissa Hill, Laura Lan, Andrew Jilwan
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