Enzyme Structure, Classification, and Regulation

Posted on Mar 13, 2025 in Biotechnology

Enzyme Structure and Function

Enzyme activity depends on the integrity of its structure. Maintaining primary, secondary, tertiary, and quaternary structures ensures that the enzyme retains its functionality.

Cofactors and Coenzymes

Some enzymes require the participation of other chemicals:

  • When one or more metal ions, such as iron, magnesium, or zinc, are involved, they are designated as cofactors.
  • If the chemical components are more complex organic molecules, they are called coenzymes.
  • When the coenzyme or cofactor binds to the enzyme through a covalent bond, it is designated as a prosthetic group.

Holoenzymes and Apoenzymes

The active enzyme (cofactor + coenzyme) is called a holoenzyme. The protein portion is called the apoenzyme or apoprotein.

Classification of Enzymes

Enzymes are classified into six main categories based on the type of reaction they catalyze:

  1. Oxidoreductases: Catalyze electron transfer reactions.
  2. Transferases: Catalyze the transfer of functional groups.
  3. Hydrolases: Catalyze hydrolysis reactions.
  4. Lyases: Catalyze addition to double bonds.
  5. Isomerases: Catalyze isomerization reactions.
  6. Ligases: Catalyze the formation of bonds with ATP cleavage.

Mechanism of Enzymatic Action

The catalytic ability of enzymes is related to a specific structure called the active site, where the substrate binds, forming the enzyme-substrate complex.

Models of Enzyme-Substrate Binding

  • Lock-and-Key Model: The substrate binds to the active site of the enzyme in a manner analogous to a key fitting into a lock.
  • Induced Fit Model: The substrate induces changes in the structure or conformation of the active site, allowing for proper orientation of chemical groups involved in the catalytic reaction.

Enzyme Kinetics

Enzyme kinetics is the study of the rate of enzymatic reactions and how they change in response to changes in experimental conditions. These studies begin when the enzyme and substrate meet to form one or more products. This can be represented in a graph called a progress curve, which shows the reaction rate and concentration.

  • At very high concentrations, the reaction rate reaches a maximum and remains constant.

Regulatory Enzymes

Regulatory enzymes increase or decrease their activity in response to certain chemical signals, acting as control points in a metabolic pathway. They are typically located at the start of pathways to ensure cellular requirements are met. When the cell requires a product, regulatory enzymes increase their activity; when needs are met, enzyme activity decreases.

Allosteric Enzymes

Allosteric enzymes are crucial in metabolic reactions. Modulators affect their function, either stimulating (+) or inhibiting (-) enzyme activity. The binding of the modulator causes a conformational change in the enzyme’s active site, altering substrate affinity and affecting enzyme activity.

Enzyme Inhibition

Chemicals can affect enzyme function, partially or totally diminishing their activity.

Types of Enzyme Inhibition

  • Competitive Inhibition: A decrease in enzyme activity results from the binding of a chemical compound similar to the substrate. The inhibitor competes with the substrate for binding to the active site.
  • Noncompetitive Inhibition: The inhibitor binds directly to the enzyme-substrate complex or the free enzyme at a site different from the active site.
  • Uncompetitive Inhibition: The inhibitor binds directly to the enzyme-substrate complex, but not to the free enzyme, and affects the active site structure.