Chapter 4: Enzymes

General Characteristics and Properties of Enzymes

• Definition: Enzymes are proteins that function as biological catalysts.
• Catalytic Function: Enzymes catalyze and speed up the rate of chemical reactions without being consumed or altered by the process.
• Chemical Stability: Enzymes remain chemically unchanged at the end of every chemical reaction.
• Required Quantities: Enzymes are required in minute amounts to perform their functions effectively.
• Reusability:
  • Because enzymes remain unchanged at the end of reactions, they can be used over and over again.
  • A small amount of enzyme is capable of catalyzing a reaction involving a large amount of substrate.
• Examples of Digestive Enzymes:
  • Amylase: Functions in carbohydrate breakdown.
  • Maltase: Catalyzes the digestion of maltose into simpler sugars.
  • Protease: Catalyzes the degradation of proteins into amino acids.
  • Lipase: Facilitates the breakdown of lipids into fatty acids and glycerol.

Activation Energy and the Catalytic Process

• Definition of Activation Energy: Activation energy is the exact amount of energy needed to start a chemical reaction.
• Mechanism of Lowering Activation Energy:
  • Enzymes speed up chemical reactions by lowering the activation energy needed to start a chemical reaction.
  • Enzymes provide an alternative reaction pathway that requires a lower activation energy to start the chemical reaction.
• Energy Profile Dynamics:
  • Uncatalyzed Pathway: In the absence of an enzyme, a reaction requires a higher activation energy barrier to reach the transition state.
  • Catalyzed Pathway: In the presence of an enzyme, the activation energy barrier is significantly lower.
  • Overall Energy Change: The overall energy change between reactants and products remains identical regardless of whether an enzyme is present or absent.

Enzyme Specificity and Active Site Dynamics

• Enzyme Specificity: Enzymes are highly specific in their action, functioning only on substrates that complement their structure.
• Active Site Characteristics:
  • Every enzyme possesses a specific active site.
  • The substrate is defined as the substance upon which an enzyme reacts.
  • Only a substrate with a three-dimensional ($3\text{-D}$) shape complementary to the active site can fit into the enzyme.
• Formation of Enzyme-Substrate Complex:
  • Only substrates complementary to the enzyme's active site can fit and bind to form an enzyme-substrate complex.
• Sequential Steps of Enzyme Action:
  • Binding: The substrate molecule collides with and binds to the complementary active site of the enzyme, forming the enzyme-substrate complex.
  • Chemical Reaction: While the substrate is attached to the active site, a chemical reaction occurs.
  • Product Conversion: The substrate is converted into products.
  • Product Release: The resulting products leave the active site.
  • Enzyme Recycling: The enzyme remains completely unchanged and can catalyze another reaction.

Factors Affecting Enzyme Activity: Temperature

• Temperature Sensitivity: The rate of enzyme activity is directly affected by temperature.
• Low Temperature Environment:
  • At low temperatures, enzymes are less active because the kinetic energy ($KE$) of molecules is low.
  • Enzyme and substrate molecules move slowly.
  • The rate at which enzyme and substrate molecules collide is very low, leading to a low rate of reaction.
• Increasing Temperature:
  • At higher temperatures, kinetic energy ($KE$) is increased.
  • This results in a higher rate of effective collisions between enzyme and substrate molecules.
  • Consequently, the rate of enzyme-substrate complex formation increases, elevating the reaction rate.
• Optimum Temperature:
  • The optimum temperature is the specific temperature at which an enzyme is most active.
• Temperatures Above Optimum:
  • An increase in temperature above the optimum temperature causes the enzyme to denature.
  • Upon denaturation, the active site is no longer complementary to the substrate.
  • This prevents substrate binding and decreases the rate of enzyme action.

Factors Affecting Enzyme Activity: pH and Denaturation

• Definition of Denaturation:
  • Denaturation is defined as the change in the three-dimensional ($3\text{-D}$) structure of an enzyme or any other soluble protein.
  • It is caused by heat, acids, or alkalis.
• Influence of pH:
  • Enzymes require specific pH conditions to maintain their active site shape and function at maximum capacity.
• Optimum pH Values of Specific Digestive Enzymes:
  • Stomach Protease: Demonstrates an optimum pH at $1$ (acidic environment).
  • Amylase: Demonstrates an optimum pH at $6$ (slightly acidic environment).
  • Small Intestine Protease: Demonstrates an optimum pH at $7.5$ (slightly alkaline environment).