Enzymes - Chemistry LibreTexts

Introduction

In chemistry, a catalyst is a chemical that drives a reaction forward. Catalysts lower the activation energy, which is the amount of energy required for reactants to form products (Figure 1). Catalysts also lower the kinetic barrier, which is needed to drive a reaction forward and backward. A certain amount of energy contained in the molecules is required when the two molecules react together to form a product. If the two molecules do not have enough energy to react, then no product is produced. By lowering the activation energy, a catalyst allows the molecules to gain sufficient energy to overcome the barrier and form products.

Figure 1: Compare the red curve with the blue curve. Which hill would you want to climb over? This figure shows the decrease in the activation energy and the kintetic barrier in a reaction in which there is a catalyst or enzyme (red curve). Extracted from Dr. Delmar Larsen's Lecture 22 on 5/24/10.

Catalysts increase the rates of the forward and backward reaction (Kf to denote the rate of the forward reaction and Kb to denote the rate of the backward reaction). Exergonic forward reactions convert reactants to products, whereas endergonic backward reactions convert products to reactants. Typically, the conversion of products to reactants requires more energy, measured in Gibbs energy. Recall that the difference in energy between the products and the reactants is measured as ΔG (Gibbs energy). It is very important to note that catalysts do not change the free energy, G, they simply affect the speed of the reaction. Catalysts are very beneficial in biological systems because they drive individual reactions forward. Our bodies are a vast combination of redox reactions. Heat may drive a reaction forward. For example, when you catch a fever, your body raises its temperature to drive reactions forward in your body, dissipating energy in the form of heat. Note that the body raises its temperature to drive reactions forward. This extra energy drives your immune system forward to get rid of the germs faster. Often, life does not want whole systems to be driven forward; instead, the biological system merely wants to produce a little extra product or a small amount of excess reactant of one reaction. It would be a waste of energy to constantly have our body hotter than needed, and we would probably die much faster. Therefore, our body uses reaction specific catalysts. These reaction specific catalysts are required to keep our body alive. In this section, we will talk about the chemistry of inorganic and organic biological catalysts, also called enzymes, and how their composition is evaluated in medicine.

Figure 2