Kinetics of Enzymes Catalyzing Two-Substrate Reactions

Most enzymatic reactions involving two-substrate reactions show more complex kinetics than do one-substrate reactions. Examples are catalyzed by dehydrogenases and aminotransferases. Hydrolytic reactions are bisubstrate reactions in which water is one of the substrates. The change in water concentration is negligible and has no effect on the rate of reaction. A two-suhstrate reaction can be written as 

The enzyme—substrate interactions can proceed by either single-displacement or double-displacement reactions (commonly known as ping-pong reactions). A substrate reaction proceeding by way of a single-displacement reaction can be shown by 

Note that the ternary complex EAB can be formed in two different ways. If the formation of EAB can occur with either substrate binding first, the reaction is known as a random single-displacement reaction. Many reactions catalyzed by phosphotransferases are of this type. If a particular substrate must bind first with the enzyme before the second substrate can bind, the reaction is known as an ordered single-displacement reaction. Many reactions catalyzed by dehydrogenases are of this type. The values for Km and Vmax for each substrate can be obtained from experiments in which the concentration of one substance is held constant at saturating levels while the concentration of the second substrate is varied. Kinetic analyses can distinguish between these types of reactions. 

In a double-displacement reaction, at first only one substrate is bound the release of one product and covalently modified enzyme (E ) follows. E then combines with the second substrate to form a second product. Reactions catalyzed by aminotransferases are of this type. In this mechanism, no ternary complex EAB is formed. The double displacement reaction sequence is shown below  

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