Chapter 7: Biomolecules (Organic Molecules)
7.4. Enzymes
Learning Objectives
By the end of this section, you will be able to:
- Describe the structure and function of enzymes and how they catalyze reactions.
- Describe induced fit model as it relates to the formation of the active site-substrate complex.
Biological enzymes are globular proteins (proteins in tertiary structure) and act as catalysts for cellular chemical reactions.
Chemical reactions may occur spontaneously but in cells this process would happen too slowly to meet cellular needs. For example, a teaspoon of sucrose (table sugar), a disaccharide, in a glass of iced tea will take time to break down into two monosaccharides, glucose and fructose; however, if you add a small amount of the enzyme sucrase to the tea, sucrose breaks down almost immediately. Sucrase is an example of an enzyme, a type of biological catalyst.
Normal body temperatures (37 oC) are also too low to allow the cell’s chemical reactions to proceed with enough speed to sustain life. Enzymes are macromolecules—most often proteins—that speed up chemical reactions by lowering activation energy barriers.
A substance that helps a chemical reaction to occur is a catalyst, and biochemical reactions are catalyzed by the enzymes.
One of the hallmark properties of enzymes is that they remain ultimately unchanged by the reactions they catalyze. After an enzyme is done catalyzing a reaction, it releases its product(s).
Enzymes perform the critical task of lowering the activation energies of chemical reactions inside the cell. Enzymes do this by binding to the reactant molecules, and holding them in such a way as to make the chemical bond-breaking and bond-forming processes take place more readily. It is important to remember that enzymes don’t change the ∆G of a reaction. In other words, they don’t change whether a reaction is exergonic (spontaneous) or endergonic. This is because they don’t change the free energy of the reactants or products. They only reduce the activation energy required to reach the transition state (Figure 7.4.1.).
Enzyme Active Site-Substrate interaction
Most (but not all) protein enzymes can be identified by the suffix -ase; e.g., lactase acts on its substrate, sucrase acts on its substrate sucrose.
Enzymes catalyze dehydration synthesis and hydrolysis chemical reactions. The location within the enzyme where the substrate binds is called the enzyme’s active site. The active site is where the “action” happens, so to speak. Since enzymes are proteins, there is a unique combination of amino acid R groups (side chains or residues) within the active site. The unique combination of amino acid residues, their positions, sequences, structures, and properties, creates a very specific chemical environment within the active site.
This specific environment is suited to bind, albeit briefly, to a specific chemical substrate (or substrates). Due to this jigsaw puzzle-like match between an enzyme and its substrates (which adapts to find the best fit between the transition state and the active site), enzymes are known for their specificity.
Enzyme active site – substrate binding has long been described as a simple “lock-and-key” interaction which assumed there was a perfect complementary fit between the two. Research now supports a more dynamic enzyme-substrate interaction described as an induced-fit (Figure 7.4.2.).
Biological enzymes must operate at their optimal conditions. Enzymes subjected to conditions outside their optimal range undergo denaturation. Optimal temperature and pH conditions for biological enzymes:
- Temperature: Heat is thermal energy related to the kinetic energy of atoms or molecules. Temperature, a measure of kinetic energy, is directly related to the degree of molecular motion in a system. Temperature increases with increasing kinetic energy. The optimal temperature for human enzymes is body temperature (37 oC). Within an optimal range, increasing environmental temperature increases enzyme-catalyzed reaction rates because higher molecule motion increases the probability that enzyme and substrate will meet each other. Conversely, cold temperature decreases molecular motion and reaction rates. Temperature extremes can result in denaturation
- pH: Optimal pH for an enzyme varies greatly depending on the body location of the enzyme. Enzymes contained in saliva of the mouth operate at near neutral pH. In contrast, cells of the stomach secrete hydrochloric acid so the optimal pH of gastric enzymes is around pH 2.
License and attributions:
- Biology, Second edition, 2018, Clark, M.A. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-2e/pages/6-5-enzymes
