Enzymes and Metabolic Reactions

Enzymes

• Enzymes are biocatalysts, speeding up reactions by lowering activation energy.
• They are not consumed in the reaction.
• Enzymes are highly specific for their substrate and reaction.
• Most enzymes are proteins, but ribozymes are catalytic RNA.
• Enzymes change reaction rate (efficiency), not the reaction equilibrium.

Learning Objectives

• Describe enzymes and their common properties
• List the enzyme classifications correctly
• Describe factors changing enzyme activity
• Explain the relationship between Km and activity
• Explain enzyme inhibition and types of inhibition

Metabolic Reactions

• Enzymes catalyze metabolic reaction, speeding up anabolic and catabolic pathways.
• Enzymes modify molecules, facilitate reactions, and aid in molecule transport.
• Some reactions involve multiple steps with multiple enzymes.

Glycolysis

• Glycolysis is a metabolic pathway where glucose is broken down to pyruvate.
• Enzymes such as HK, PGI, PFK, ALDO, and others catalyze specific steps in this pathway

Activation Energy

• Activation energy is the energy needed for a chemical reaction to start.
• Enzymes lower the activation energy required for reactions to occur.
• Enzymes speed up reactions by lowering the energy barriers and making them more favorable.

Activation Energy and Catalysts

• Catalysts speed up reactions.
• Catalysts lower the activation energy of reactions without being consumed in the process.
• Enzymes are biological catalysts.

Enzymes (Biocatalysts)

• Enzymes increase reaction rates.
• Enzymes are biocatalysts that increase the reaction rate
• Enzymes bind to substrates and make reactions faster.

Properties of Enzymes

• Enzymes are proteins (except ribozymes).
• They exhibit reaction specificity, meaning they only work on particular reactions.
• They demonstrate substrate specificity (bind to a specific substrate).
• Enzymes are highly efficient, increasing reaction rate up to millions of times faster.

Efficiency

• Enzyme-catalyzed reactions occur much faster than non-catalyzed ones.
• Carbonic anhydrase is exceptionally fast, hydrating CO2 molecules.

Specificity

• Reaction specificity: A given enzyme catalyzes a specific reaction type.
• Substrate specificity: A given enzyme uses a specific molecule or chemically related molecules.

Enzyme Nomenclature (Classification)

• Enzymes are classified by EC numbers, a hierarchical system.
• A classification system exists to differentiate enzymes by the specific reactions they catalyze.

Enzyme Kinetics and Km

• Km represents enzyme affinity or how well an enzyme binds to its substrate.
• A smaller Km signifies a higher affinity.
• Vmax is the maximum reaction rate of an enzyme when saturated with substrate.

Km, A Clinical Example

• Km values for alcohol dehydrogenase and aldehyde dehydrogenase vary depending on factors like the enzymes involved and the genes.
• Lowering the Km of aldehyde dehydrogenase decreases the toxicity of acetaldehyde, relating to alcohol metabolism.

Regulation of Enzyme Activity

• Enzyme activity is controlled by numerous processes.
• Enzyme amount, modification, and location regulate enzyme activity.
• Non-covalent and covalent modifications, and compartments regulate activity.

Regulation of Enzyme Activity (continued)

• Phosphorylation/Dephosphorylation:
  • Kinase activates the enzyme by adding a phosphate.
  • Phosphatase deactivates the enzyme by removing the phosphate.
• Feed-forward activation:
  • A reaction activates a cascade of enzymes

Regulation of Enzyme Activity (Continued)

• Feed-back inhibition:
  • A reaction product inhibits an earlier enzyme step.
• Compartmentalization:
  • Enzymes are localized to specific cellular compartments.
• Isozymes:
  • Different forms of an enzyme with varying activities

Enzyme Structure (Active Site)

• Apoenzyme: Protein portion of an enzyme.
• Cofactor: Non-protein part of an enzyme (metal ion or coenzyme).
• Holoenzyme: Complete enzyme with cofactor and apoenzyme.

Enzyme Activity

• Activity (V) is the speed or rate at which an enzyme works.
• Unit activity: enzyme activity that converts 1 µmol substrate in 1 minute under optimal conditions.

Factors Affecting Enzyme Activity

• Enzyme concentration: increasing concentration means more reactions happen faster.
• Substrate concentration: higher substrate leads to faster reaction initially, but only to a certain point.
• Temperature: Enzymes operate best at particular temperatures.
• pH: Optimal pH range varies based on enzymes and the environment.
• Inhibitors: Molecules that reduce enzyme activity.

This quiz covers the properties and classifications of enzymes, their role in metabolic reactions, and specific pathways like glycolysis. Explore how enzymes function as biocatalysts and the factors influencing their activity. Test your understanding of enzyme inhibition and the relationship between Km and activity.