What is an enzyme?

Enzymes are globular proteins that act as biological catalysts, meaning they speed up chemical reactions in living organisms without being used up or permanently changed in the process.

• Composition and Structure

  • Enzymes are proteins.

  • They are typically roughly spherical in shape due to the tight folding of polypeptide chains, making them soluble.

  • A critical feature of an enzyme is its active site, a specific region where substrate molecules bind. The active site's shape is determined by the enzyme's tertiary structure.

  • Specificity: Enzymes are highly specific; they usually catalyze only one type of reaction because only one complementary substrate will fit into their active site. This specificity is vital for controlling the thousands of metabolic reactions occurring in cells.

• Mechanism of Action

  • Enzymes speed up reactions by lowering the activation energy. They achieve this by forming a temporary enzyme-substrate complex.

  • Models of Enzyme Action:

    • Lock-and-Key Hypothesis: An early model suggesting the substrate fits into the enzyme's active site like a key into a lock, implying rigid structures.

    • Induced-Fit Hypothesis: The current understanding, where the active site (and sometimes the substrate) changes shape slightly to allow a tighter, more precise fit as the substrate binds. This change in shape helps to momentarily raise the substrate to a transition state, making it easier to react.

• Location of Action

  • Intracellular enzymes catalyze reactions inside cells, such as respiration.

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

• Key Properties

  • Enzymes are effective in small amounts and remain unchanged at the end of the reaction, allowing them to be reused.

  • Their activity is influenced by various factors, including temperature, pH, enzyme concentration, substrate concentration, and inhibitors.

  • Enzymes have an optimum temperature and optimum pH at which they work fastest. Temperatures or pH values outside this optimum range can lead to denaturation, where the enzyme loses its specific tertiary structure and the active site changes shape, preventing proper substrate binding and catalytic activity. High temperatures cause increased vibration that breaks bonds holding the enzyme's shape, while extreme pH disrupts ionic and hydrogen bonds.

In essence, enzymes are vital protein catalysts that enable the rapid and controlled chemical reactions necessary for life by specifically interacting with substrates at their active sites and lowering the activation energy required for these reactions.