3.6. Thermodynamics and Mechanisms of Enzyme Catalysis

Enzymes can act in several ways, all of which lower activation free energy of the reaction (ΔG‡):

• The activation energy is lowered by creating an environment in which the transition state is stabilized. The shape of a substrate is strained in shape upon binding the transition-state conformation of the substrate/product molecules → the enzyme distorts the bound substrate into their transition state form → thus, the amount of energy required to complete the transition is reduced.
• Enzyme may Lower the energy of the transition state without distorting the substrate by creating an opposite charge distribution to that of the transition state.
• Enzyme provides an alternative pathway. For example, temporarily reacting with the substrate to form an intermediate ES complex, which would be impossible in the absence of the enzyme.
• Bring the substrates together in the correct orientation to react → thus reducing the reaction entropy change by bringing.
• Increases in temperatures speed up reactions. Thus, temperature increases help the enzyme function and develop the end product even faster. However, if heated too much, the enzyme becomes denatured.

Figure 3.4: Energetic of reaction showing the role of an enzyme in lowering the activation energy.

• As all catalysts, enzymes also do not change the position of the chemical equilibrium, but only the speed at which it is reached.
• Under irreversible reaction conditions the enzyme, in fact, only catalyzes the reaction in the thermodynamically allowed direction.

3.7. Kinetics of Enezyme Catalysis

• Definition: The study of the reaction rates and how the rates are affected by changes in experimental conditions of enzyme catalyzed reactions is called enzyme kinetics.
• Study of enzyme kinetics is an approach to understanding how changes in the physical and chemical environments change function
• The rate of a reaction is influenced by several factors:
  • concentrations of enzymes, substrates and products
  • modifiers like inhibitors, activators or cofactors
  • environmental conditions (like temperature and pH)
• Saturation (Vmax): It is an essential feature of enzyme-catalyzed reactions. It is the stage where anincrease in concentrations of substrates the rate increases and approaches a limit where there is no dependence of rate on concentration.
• The first scientists to experiment with enzyme kinetics in a more detailed way are Leonor Michaelis and Maud Menten.
• Enzyme-Substrate (E-S) Complex: Enzyme binds the transition state structure about 1012 times more tightly than it binds the substrate or products. Then, how does an enzyme release product so efficiently?
  • As bonds are broken/made at transition state, electronic redistribution can occur → interactions for transition-state stabilization are no longer present → generating a repulsive interaction → Therefore products are poorly bound → lead expulsion of products.
• Enzymatic reaction:

Where, E = Enzyme; S = Substrate; [E-S] = Enzyme-Substrate Complex; P = Product; k₁ = Rate of forward reaction; k_-1 = Rate of reverse reaction; k₂ = Rate of product formation = also known as k_cat.