Activation energy Knowledge-Based Study

Study notes

1. What is Activation Energy?

   Every chemical reaction involves breaking existing chemical bonds in reactant molecules and forming new bonds to produce products. Even in spontaneous or exergonic reactions that release energy overall, bonds do not break on their own. Reactant molecules must first absorb an initial amount of energy to destabilize their bonds.

   This initial investment of energy required to start a chemical reaction is called the activation energy ($E_a$). It acts as a kinetic barrier that prevents complex, energy-rich molecules such as proteins, carbohydrates, and nucleic acids from spontaneously decomposing at normal cellular temperatures.

   Reaching the Transition State

   When reactant molecules absorb activation energy, usually in the form of thermal energy from their surroundings, their internal bonds bend and stretch to a critical breaking point. At this high-energy peak, the molecules enter an unstable, short-lived condition called the transition state.

   • In the transition state, existing chemical bonds are maximally strained and ready to break.
   • Once reactants cross this energy peak, new bonds form, releasing energy as the molecules stabilize into products.
   • Without sufficient activation energy, reactant collisions do not reach the transition state, and the reaction cannot proceed.

   Enzymatic Catalysis

   In living systems, increasing temperature to overcome high activation energy barriers is harmful because high heat denatures cellular proteins and non-specifically accelerates all reactions. Cells solve this problem using biological catalysts called enzymes.

   Enzymes speed up metabolic reactions specifically by lowering the activation energy barrier ($E_a$). They do this through several mechanisms within their active site:

   • Orienting substrates: Bringing reactants together in the exact physical orientation needed for a reaction to occur.
   • Straining substrate bonds: Distorting substrate molecules toward their transition-state conformation, reducing the amount of thermal energy needed to break bonds.
   • Providing a favorable microenvironment: Creating localized conditions (such as acidic or basic amino acid side chains) that facilitate chemical transitions.
   • Direct participation: Forming brief covalent bonds with the substrate that are resolved before the reaction concludes.

   Free Energy Change ($\Delta G$) Remains Constant

   A crucial principle in enzyme kinetics is that while enzymes lower the activation energy, they do not change the overall free energy change ($\Delta G$) of a reaction.

   • $\Delta G$ represents the difference in potential energy between the reactants and the products. Because an enzyme does not alter the energy levels of the initial reactants or final products, $\Delta G$ remains identical in both catalyzed and uncatalyzed reactions.
   • Enzymes cannot make an endergonic (non-spontaneous) reaction occur without an external energy source like ATP.
   • Enzymes speed up the rate at which equilibrium is reached, but they do not alter the final equilibrium concentrations of reactants and products.