Enzyme Activity and pH

Enzyme Characteristics and Function

• R groups of amino acids in the active site have specific charges that influence substrate binding.
• Ionization of these groups is affected by pH; optimal enzyme activity occurs at a specific pH.
• Tertiary structure stability is crucial; extreme pH levels can lead to enzyme denaturation.
• Enzyme activity is typically narrow in pH range; each enzyme has a unique optimum pH for maximum activity.

Reaction Rate and Activation Energy

• Substrate molecules must have sufficient energy to overcome an energy barrier to be converted into products.
• Reaction rate is influenced by the number of molecules that can transition to product; lower activation energy results in faster reactions.
• Enzymes do not alter the free energies of reactants and products; they do not change the reaction equilibrium.

Michaelis-Menten Model

• The Michaelis-Menten model describes enzyme-catalyzed reactions, showing the formation of the enzyme-substrate complex (ES).
• The Michaelis-Menten equation relates reaction velocity to substrate concentration.
• The Km constant reflects the enzyme's affinity for a substrate; it is the substrate concentration at which the reaction velocity is half of Vmax.

Characteristics of Enzymes

• Enzymes serve as catalysts, increasing reaction rates without being consumed.
• They are proteins, efficient, specific, and effective in small amounts.
• Enzymes require cofactors or coenzymes and can operate through multiple biochemical pathways.

Active Site and Mechanism of Action

• The active site of an enzyme binds the substrate, forming an enzyme-substrate complex that transforms into a product.
• Two binding models: lock-and-key and induced-fit, explain how enzymes interact with substrates.
• Activation energy must be absorbed to reach a transition state; enzymes lower this activation energy.

Factors Affecting Enzyme Action

• Reaction velocity increases with substrate concentration until reaching Vmax, where enzyme active sites are saturated.
• Different factors like temperature and pH significantly impact enzyme activity:
  • Optimal temperature for most human enzymes is around 37°C.
  • pH affects amino acid ionization at the active site; low Km indicates high substrate affinity, while high Km indicates low affinity.

Enzyme Kinetics and Order of Reaction

• The reaction rate is directly proportional to enzyme concentration.
• At substrate concentrations greater than Km, the reaction rate is constant (zero-order kinetics).
• The Lineweaver-Burk plot provides a double reciprocal plot allowing for easier determination of Vmax and Km.

Enzyme Inhibition

• Inhibitors reduce enzyme-catalyzed reaction velocity; they can be reversible or irreversible.
• Competitive inhibitors compete with the substrate for the active site, increasing apparent Km without affecting Vmax.
• Noncompetitive inhibitors bind to a different site, decreasing Vmax while leaving Km unchanged.

Application of Enzyme Inhibitors

• Enzyme inhibitors can be pharmaceutical agents:
  • Sarin inhibits acetylcholinesterase, causing a toxic accumulation of acetylcholine, leading to paralysis.
  • Penicillin inhibits bacterial peptidoglycan synthesis, effectively preventing bacterial cell wall formation and division.

This quiz explores the relationship between enzyme activity and pH levels, focusing on how the ionization of amino acid R groups in the active site affects substrate binding. It covers the impact of pH on enzyme structure, activity, and the conditions that lead to denaturation. Understand the optimum pH for maximum reaction rates and the consequences of extreme pH levels.