Enzymes: Structure and Function

• Enzymes are biological catalysts that speed up chemical reactions in living organisms.
• They are typically proteins, although some catalytic RNA molecules (ribozymes) exist.
• Enzymes are highly specific, catalyzing only certain reactions or a set of closely related reactions.
• They are essential for various biological processes, including metabolism, digestion, and DNA replication.

Enzyme Structure

• Enzymes have a complex three-dimensional structure.
• The active site is a specific region on the enzyme where the substrate binds and catalysis occurs.
• The active site's shape and chemical properties are complementary to the substrate.
• Some enzymes require cofactors (inorganic ions) or coenzymes (organic molecules) to function.
• Apoenzyme refers to the protein part of an enzyme without its cofactor.
• Holoenzyme refers to the complete, catalytically active enzyme with its cofactor.

Enzyme Function

• Enzymes accelerate reactions by lowering the activation energy.
• They do not change the equilibrium of a reaction; they only affect the rate.
• Enzymes form temporary complexes with their substrates.
• The enzyme-substrate complex facilitates the reaction.
• After the reaction, the products are released, and the enzyme is free to catalyze another reaction.

Enzyme Specificity

• Enzyme specificity refers to the ability of an enzyme to catalyze a single reaction or a set of closely related reactions.
• Lock-and-key model: The substrate fits perfectly into the active site.
• Induced-fit model: The enzyme's active site changes shape upon substrate binding to achieve a better fit.
• Enzymes exhibit different types of specificity:
  • Absolute specificity: Catalyzes only one specific reaction.
  • Relative specificity: Catalyzes reactions involving similar molecules.
  • Stereospecificity: Distinguishes between stereoisomers.

Factors Affecting Enzyme Activity

• Temperature:
  • Enzyme activity increases with temperature up to an optimal point.
  • High temperatures can denature the enzyme, causing loss of activity.
• pH:
  • Enzymes have an optimal pH range.
  • pH changes can affect the enzyme's structure and active site.
• Substrate concentration:
  • Enzyme activity increases with substrate concentration up to a maximum rate (Vmax).
  • Vmax is the maximum rate of reaction when the enzyme is saturated with substrate.
  • The Michaelis constant (Km) is the substrate concentration at which the reaction rate is half of Vmax; it indicates the affinity of the enzyme for the substrate.
• Enzyme concentration:
  • Enzyme activity is directly proportional to enzyme concentration (if substrate is in excess).
• Inhibitors:
  • Substances that reduce enzyme activity.

Enzyme Inhibition

• Competitive inhibition:
  • The inhibitor binds to the active site, preventing substrate binding.
  • It increases Km but does not affect Vmax.
  • The effect can be overcome by increasing substrate concentration.
• Non-competitive inhibition:
  • The inhibitor binds to a site other than the active site, altering the enzyme's shape and reducing its activity.
  • It decreases Vmax but does not affect Km.
  • Increasing substrate concentration does not overcome the effect.
• Uncompetitive inhibition:
  • The inhibitor binds only to the enzyme-substrate complex.
  • It decreases both Km and Vmax.
• Irreversible inhibition:
  • The inhibitor binds covalently to the enzyme, permanently inactivating it.
  • These inhibitors are often toxic.
• Allosteric regulation:
  • The binding of a molecule to a site other than the active site affects enzyme activity.
  • Allosteric activators increase enzyme activity.
  • Allosteric inhibitors decrease enzyme activity.
• Feedback inhibition:
  • The end product of a metabolic pathway inhibits an enzyme earlier in the pathway.
  • This helps regulate the production of the end product.

Enzyme Classification

• Enzymes are classified into six main classes based on the type of reaction they catalyze:
  • Oxidoreductases: Catalyze oxidation-reduction reactions.
  • Transferases: Catalyze the transfer of functional groups.
  • Hydrolases: Catalyze hydrolysis reactions (addition of water).
  • Lyases: Catalyze the breaking of chemical bonds by means other than hydrolysis or oxidation.
  • Isomerases: Catalyze the rearrangement of atoms within a molecule.
  • Ligases: Catalyze the joining of two molecules, often coupled with ATP hydrolysis.
• Each enzyme is assigned a unique Enzyme Commission (EC) number.
• The EC number consists of four numbers separated by periods, representing the class, subclass, sub-subclass, and serial number.

Enzyme Regulation

• Enzyme activity can be regulated by several mechanisms:
  • Genetic control: Regulation of enzyme synthesis (transcription and translation).
  • Covalent modification: Addition or removal of chemical groups (e.g., phosphorylation).
  • Proteolytic cleavage: Activation of an enzyme by cleaving a proenzyme or zymogen.
  • Allosteric regulation: Binding of molecules to sites other than the active site.
  • Compartmentalization: Localization of enzymes in specific cellular compartments.

Enzymes in Medicine and Industry

• Enzymes are used in various medical applications:
  • Diagnostic enzymes: Used to detect diseases by measuring enzyme levels in body fluids.
  • Therapeutic enzymes: Used to treat diseases by replacing deficient enzymes or degrading harmful substances.
• Enzymes find wide applications in industry:
  • Food industry: Used in baking, brewing, and cheese production.
  • Textile industry: Used for bleaching and desizing fabrics.
  • Detergent industry: Used to remove stains.
  • Pharmaceutical industry: Used in drug synthesis and analysis.
  • Paper industry: Used for pulp bleaching.

Summary of Key Concepts

• Enzymes are biological catalysts that speed up chemical reactions.
• They have specific active sites that bind substrates.
• Factors like temperature, pH, and substrate concentration affect enzyme activity.
• Enzyme inhibitors can reduce enzyme activity through various mechanisms.
• Enzymes are classified into six main classes based on their function.
• Enzyme regulation involves genetic control, covalent modification, and allosteric regulation.
• Enzymes have diverse applications in medicine and industry.