Biochemistry Chapter 4: Enzymes

Chapter 4: Enzymes

• Enzymes are organic catalysts, typically proteins
• They accelerate specific chemical reactions by lowering the activation energy (Ea)
• Enzymes are globular proteins
• Enzymes are not permanently changed/consumed by the reaction
• Enzymes can be reused
• Enzymes have one or more active sites where substrates bind
• Enzymes lower activation energy (Ea) needed for a reaction to start

4.1 Catalysis and Concept of Activation Energy

• Activation energy (Ea) is the initial energy needed to start a chemical reaction
• The source of Ea is heat energy from the surroundings
• Enzymes lower Ea by:
  • Bringing substrates together with correct orientation
  • Providing a favorable cellular microenvironment
  • Stretching substrate bonds toward their transition state
  • Forming temporary covalent bonds with substrate

4.2 Mechanism of Enzyme Action: Lock and Key Model

• The substrate is complementary to the active site of the enzyme
• Substrate binds to the active site of the enzyme
• Enzyme-substrate complex formed
• Old bonds break and new ones form
• Substrates change to product
• Product has a different shape from the substrate
• Product is released from the enzyme

4.2 Mechanism of Enzyme Action: Induced-Fit Model

• Substrate not perfectly complementary to active site
• Binding of substrate induces a slight conformational change in the enzyme's active site
• Shape of substrate also changes slightly
• Catalytic groups on enzyme are correctly oriented with the substrate
• Old bonds break, and new bonds form
• Substrate is changed into a product
• Product has a different shape from the substrate
• Product is released from the enzyme
• Enzyme returns to its original shape

4.3 Factors Affecting the Rate of Enzyme Reactions

• Enzyme concentration
• Temperature
• pH
• Substrate concentration

4.4 Enzyme Kinetics

• Study of enzyme reaction rate
• Enzyme-catalyzed reactions often follow a hyperbolic curve
• Michaelis-Menten kinetics can explain how enzymes enhance reaction rates and how reaction rates depend on enzyme and substrate concentrations.
• Michaelis-Menten kinetics model is represented by the saturation plot

4.4 Lineweaver-Burk Plot

• Double-reciprocal graph of the Michaelis-Menten equation
• Plots 1/v (reaction rate) against 1/[S] (substrate concentration), where 'v' is reaction rate and '[S]' is substrate concentration
• Produces a straight line, useful for accurately determining Vmax (maximum reaction rate) and Km (Michaelis-Menten constant)

4.5 Cofactors

• Non-protein molecules or ions needed for enzyme function
  • Coenzymes: organic molecules that bind loosely and temporarily to the active site. They often carry chemical groups, atoms, or electrons between molecules
  • Metal ions: inorganic ions that can activate or stabilize the enzyme's structure and function
  • Prosthetic groups: tightly bound organic molecules essential for enzyme activity

4.6 Enzyme Inhibition

• Inhibiting agents are chemicals that selectively reduce the function of specific enzymes.
• Reversible inhibitors: bind reversibly to the enzyme, form weak bonds (like hydrogen bonds) and enzymes can return to functional structure
• Irreversible inhibitors: bind irreversibly to the enzyme, strong covalent bonds forming, which inactivates the enzyme permanently

4.6 Competitive Inhibitors

• Similar shape to the natural substrate
• Compete with the substrate for the active site
• Can be overcome by increasing substrate concentration
• Reduce the rate of reaction and product formation

4.6 Non-competitive Inhibitors

• No structural similarity to the natural substrate
• Bind to the allosteric site
• Binding causes conformational changes in the enzyme
• Substrate cannot bind to the active site
• Slower rate of reaction and reduced product formation

4.7 Regulation of Enzyme Activity

• Cells regulate their metabolic pathways by controlling when and where enzymes are active
• Mechanisms include:
  • Feedback inhibition: End product inhibits an enzyme earlier in the pathway.
  • Allosteric regulation: Regulatory molecules bind to a site (allosteric site) other than the active site, changing the enzyme's conformation and activity
  • Cooperativity (a type of allosteric regulation): Binding of a substrate to one active site affects activity at other active sites

4.8 Enzyme Classification

• Enzymes are named by adding "ase" to the name of the reaction catalyzed
• Six classes: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases, based on the type of reaction catalyzed

4.9 Enzyme Technology

• Enzymes are useful outside cellular systems for industrial and medical use
• Immobilization: Enzymes are attached or trapped in insoluble materials. This helps with product yield and reusing enzymes.
  • Immobilization methods: adsorption, entrapment, encapsulation, and covalent bonding
• Advantages of immobilization: Stability, Reusability, Prevention of product contamination, Easier separation.
• Disadvantages of immobilization: Partial loss of enzymatic activity, altered kinetics.

Explore the fascinating world of enzymes in this quiz on Chapter 4. Learn how these organic catalysts function, their role in lowering activation energy, and the mechanisms of enzyme action, including the lock and key model. Test your knowledge on key concepts from biochemistry related to enzymes.