Mode of action of enzymes

Enzymes are globular proteins that serve as biological catalysts, meaning they accelerate the rate of chemical reactions in living organisms without being consumed or permanently altered in the process. Their efficiency allows metabolic reactions to occur at speeds necessary for life, even at relatively low temperatures.

• Structure and Active Site

  • Enzymes are proteins. Specifically, they are globular proteins, characterized by their spherical shape due to the precise folding of their polypeptide chains, making them soluble in water.

  • A critical feature of an enzyme is its active site, a specific region where substrate molecules (the substances the enzyme acts upon) bind. The shape of the active site is highly specific and is determined by the enzyme's tertiary structure. This specificity ensures that generally only one type of substrate or a very small group of similar substrates will fit and be acted upon. The R groups of amino acids at the active site form temporary bonds (like ionic and hydrogen bonds) with the substrate, holding it in place.

• Enzyme-Substrate Complex and Activation Energy

  • When a substrate binds to the enzyme's active site, a temporary enzyme-substrate complex is formed.

  • Enzymes speed up reactions by lowering the activation energy. Activation energy is the minimum energy required to raise reacting molecules to a "transition state" before a reaction can occur. Enzymes achieve this by holding the substrate molecules in a way that makes them react more easily, for instance, by bringing two substrate molecules closer together or by straining bonds in a substrate for breakdown reactions.

• Models of Enzyme Action

  • Lock-and-Key Hypothesis: An earlier model proposed that the enzyme's active site and the substrate have complementary, rigid shapes that fit together precisely, like a key in a lock.

  • Induced-Fit Hypothesis: The more current and refined model, which suggests that the active site is not entirely rigid but undergoes a slight conformational change (or "induced fit") as the substrate binds. This allows for a tighter, more precise fit, optimizing the enzyme's ability to catalyze the reaction by momentarily raising the substrate to a transition state. After the products are released, the active site typically returns to its original shape and can bind to new substrate molecules.

• Location of Enzyme Action

  • Intracellular enzymes function inside cells, where they catalyze metabolic reactions such as respiration.

  • Extracellular enzymes are produced by cells but are secreted to catalyze reactions outside cells, for example, digestive enzymes in the gut of mammals.

• Reusability

  • Enzymes are not used up or permanently changed in the reaction. This allows them to be reused for many cycles of reaction, and they are effective even in small quantities.