The Kinetics of Enzyme Catalysis
1. Meaning of Enzyme Kinetics
Enzyme kinetics is the study of the rate of enzyme-catalyzed reactions and how this rate changes in response to various conditions — such as substrate concentration, enzyme concentration, temperature, pH, and inhibitors.
It helps us understand how enzymes work, how fast they convert substrates to products, and how drugs or poisons can modify these reactions.
The overall reaction can be written as:
Where:
- E = enzyme
- S = substrate
- ES = enzyme–substrate complex
- P = product
The rate of reaction depends mainly on how much ES complex is formed and converted into product per unit time.
2. Velocity of Enzyme Reactions
The velocity (v) or rate of an enzyme reaction is the amount of substrate converted into product per unit time.
At the beginning of the reaction, when the substrate concentration is high and product accumulation is minimal, the reaction rate is called the initial velocity (v₀).
The relationship between substrate concentration and velocity (v) follows a hyperbolic curve, known as the Michaelis–Menten curve.
3. Effect of Substrate Concentration
When substrate concentration increases:
- At very low [S]:
Few substrate molecules are available; enzyme active sites are mostly unoccupied, so reaction rate is low and directly proportional to [S].
- At moderate [S]:
More substrate molecules bind to enzyme active sites; velocity increases rapidly but starts to level off.
- At high [S]:
All active sites become occupied — the enzyme is saturated.
Increasing [S] further does not increase reaction rate.
The reaction reaches its maximum velocity (Vmax).
The substrate concentration at which the reaction velocity is half of Vmax is called the
Michaelis constant (Km) — it represents the affinity of an enzyme for its substrate.
A low Km means high affinity, as the enzyme reaches half-maximal velocity even at low [S].
4. Effect of Enzyme Concentration
At constant substrate concentration (in excess), the reaction velocity is directly proportional to enzyme concentration.
If the enzyme concentration doubles, the number of active sites doubles, so the rate of product formation doubles as well.
This relationship is valid only when there is enough substrate to occupy all available enzyme molecules.
5. Other Factors Affecting Reaction Rate
Though not the focus of this topic, keep in mind that:
- Temperature: Velocity increases with temperature up to an optimum (around 37–40°C for most human enzymes), after which denaturation reduces activity.
- pH: Each enzyme has an optimal pH (e.g., pepsin ~2, trypsin ~8).