Enzyme Catalysis and Michaelis Constant

Enzyme catalysis is the catalysis of chemical reactions by specialized proteins known as enzymes. Catalysis of biochemical reactions in the cell is vital due to the very low reaction rates of the uncatalysed reactions.

The mechanism of enzyme catalysis is similar in principle to other types of chemical catalysis. By providing an alternative reaction route and by stabilizing intermediates the enzyme reduces the energy required to reach the highest energy transition state of the reaction. The reduction of activation energy (Ea) increases the number of reactant molecules with enough energy to reach the activation energy and form the product.

Michaelis-Menten kinetics (occasionally also referred to as Michaelis-Menten-Henri kinetics) approximately describes the kinetics of many enzymes. It is named after Leonor Michaelis and Maud Menten. This kinetic model is relevant to situations where very simple kinetics can be assumed, (i.e. there is no intermediate or product inhibition, and there is no allostericity or cooperativity).

The reaction rate V is the number of reactions per second catalyzed per mole of the enzyme. The reaction rate increases with increasing substrate concentration [S], asymptotically approaching the maximum rate V_max. There is therefore no clearly-defined substrate concentration at which the enzyme can be said to be saturated with substrate. A more appropriate measure to characterize an enzyme is the substrate concentration at which the reaction rate reaches half of its maximum value (V_max/2). This concentration can be shown to be equal to the Michaelis constant (K_M);

For enzymatic reactions which exhibit simple Michaelis-Menten kinetics, the Michaelis constant is defined as

Note that the unit of k_-1 and k₂ is time^-1 , while the unit of k₁ is concentration^-1 times time^-1 . This can be seen from eq. (2) above, sincek₁ is multiplied by the product of two concentrations ([E] times [S]) but k_-1 and k₂ are multiplied by a single concentration ([ES]). For example, if the unit of concentration is millimolars (mM) and the unit of time is seconds, the unit of k_-1 and k₂ is sec^-1 and the unit of k₁ is mM^-1 sec^-1 . Thus the unit of K_M in the equation above is "concentration", or in this example, mM.