8.7. Kinetics of Multisubstrate Reaction
In the enzyme kinetics, simple reactions involve one substrate binding to an enzyme and undergoing catalytic reactions. This condition is not common. A majority of biochemical reactions catalyzed by two or more substrates taking part in the reactions. For example, an enzyme E, catalyzed the reaction involving two substrates A and B and yield the product P and Q.
This type of reaction is called as Bi-Substrate reaction . These reaction can proceed in two ways:
Both the substrates A and B, bind to the enzyme E, and then reactions proceeds to yield products P and Q
This type of reaction is called as sequential or simple-displacement reactions which are further divided into following groups.
Ordered substrate binding or ordered sequential mechanism – In this type, one substrate must bind before a second substrate.
This reaction indicates the sequential binding of substrates as well as sequential release of product. This type of mechanism is observed in the reactions catalyzed by lactate dehydrogenes involving NAD+ and lactate.
8.7.2. Random substrate binding- In this type either A or B may bind to the enzyme first, followed by the other substrate and the release of the product.
This type of mechanism is observed in reactions catalyzed by transferases enzyme as hexokinase catalyzed phosphorylation of glucose by ATP.
8.7.3. Theorell-Chance Sequential mechanism
It is a type of ordered sequential bisubstrate reaction in which the ternary complex does not accumulate.
8.7.4. Ping pong mechanism
The other possibility in bi-substrate reaction is that one substrate, A, binds to the enzyme and on reacting with it a product, P, is released and enzyme turns into a modified form, E′. The second substrate, B, comes in and binds with modified enzyme to yield second product, Q and regenerate the enzyme, E.
The reactions following the above mechanism are called Ping-Pong or double-displacement reactions. This type of mechanism is observed in reactions catalyzed by aminotransferases.
These enzymes catalyze the transfer of an amino group from an amino acid to an α-keto acid.The products formed are a new amino acid corresponding to keto acid and a new keto acid corresponding to carbon skeleton of amino acid such as:-
Another example of ping pong reaction is phosphoglycerate mutase. The enzyme get phosphate from one substrate and after phosphorylation of enzyme, the phosphate is transferred to second substrate.