Enzyme Reaction Mechanisms and Kinetic Regulation

Enzymatic Reactions: Fundamentals

Chemical reactions begin with the breaking of bonds between atoms in reactant molecules, followed by the formation of new links that yield product molecules.

Transition State

This state occurs when the bonds in the reactants are weakened or broken, but new bonds have not yet fully formed.

Activation Energy (Ea)

The energy required for reactants to reach the transition state, allowing the chemical reaction to proceed.

Spontaneous Reactions (Low Ea)

Reactions where the activation energy is very low, often obtained from the kinetic energy of molecules or incident light.

Non-Spontaneous Reactions (High Ea)

Reactions where the activation energy is so high that they require external heat to proceed.

The catalytic action lowers the activation energy, making it easier to reach the transition state. Catalysts act upon the reactants but are not consumed in the reaction.

Stages of a Catalyzed Reaction

A catalyzed reaction occurs in three stages:

1. Formation of the Enzyme-Substrate complex (ES).
2. The reaction takes place (often facilitated by a cofactor), yielding the final product.
3. The product is released from the active center, and the apoenzyme is freed.

Enzyme-Substrate Binding

• Binding is specific for each substrate and reaction, often explained by the Key-Lock Theory.
• The binding occurs in the active site of the apoenzyme.
• Induced Fit Model: In some enzymes, the active site is capable of changing its shape to adapt to the substrate (analogous to a hand-glove).
• Amino acid (AA) radicals of the active center bind to the substrate, weakening its energy links to achieve the transition state.
• The binding is reversible.
• If no cofactor is present, the reaction may be carried out by certain amino acids in the active site.

Enzyme Kinetics

As substrate concentration increases, the reaction rate increases, provided free enzyme exists. Maximum velocity (V_max) is achieved when all enzyme molecules are forming the ES complex.

In enzymatic reactions, the limiting step is the formation of ES, as this step is reversible.

Equilibrium Constant (K_e)

K_e = [E][S] / [ES]

The reaction rate is half the maximum velocity (V_max) when the substrate concentration [S] equals the Michaelis constant (K_m).

Michaelis-Menten Equation

This equation allows calculation of the speed (V) of an enzymatic reaction:

V = V_max * [S] / (K_m + [S])

Factors Governing Enzymatic Activity

• Substrate concentration
• pH
• Temperature

Mechanisms for Increasing Enzyme Efficiency

• Cell Compartmentalization
• Reaction Cascades
• Multienzyme Complexes
• Isozymes

Regulation of Enzyme Activity

• Enzyme Activation

• Enzyme Inhibition

  • Irreversible Inhibition
  • Reversible Inhibition
    • Reversible Competitive Inhibition
    • Reversible Non-Competitive Inhibition

Allosteric Regulation

Allosteric enzymes exhibit the following characteristics:

• They consist of several subunits.
• They have several regulatory sites (positive/negative).
• They have two different conformations:
  • State A (Relaxed): The affinity for the substrate is high and is stabilized when activators are bound to the regulatory centers.
  • T State (Tense): The affinity for the substrate is low, stabilized by the binding of allosteric inhibitors.