Enzymes Notes

Enzymes & Biological Catalysts

• Enzymes are biological catalysts that speed up reactions in the body without being used up. They lower the activation energy required for a reaction to start.
• Most enzymes are proteins with a specific shape due to their tertiary structure.

Enzyme Function

• Enzymes work by having a specific active site that is complementary in shape to the substrate.
• The substrate binds to the active site, forming an enzyme-substrate complex.
• In the complex, the substrate bonds are broken or new bonds are formed.
• The enzyme remains unchanged and can catalyze further reactions.

Models of Enzyme Action

• Lock and Key Model: Originally, it was thought that the enzyme's active site was perfectly complementary to the substrate.
• Induced Fit Model: New evidence suggests the enzyme can change the shape of its active site as the substrate binds for a better fit.

Activation Energy

• Activation energy is the energy needed to start a chemical reaction.
• Enzymes lower the activation energy, making it easier for reactions to occur.

Visual Representation of Activation Energy

• The slide presents a diagram showing the free energy of reactants and products, with a curve illustrating the transition state and the activation energy required with and without an enzyme.
• Enzymes reduce the activation energy needed for the reaction.

Graphical Representation

• A graph illustrates the energy levels of reactants and products, showing the activation energy required for a reaction with and without an enzyme.
• Important labels include:
  • Reactants (e.g., C6H12O6 + O2)
  • Products (CO2 + H2O)
  • Activation energy with and without enzyme
  • Overall energy released during the reaction

Environmental Impacts on Enzyme Function

pH

• pH measures how acidic or basic a substance is.
• Different enzymes have different optimal pH levels at which they function best, depending on their environment.

pH Effect on Enzymes

• Pepsin: Found in stomach acid (approximately pH 2), with an optimal pH in the acidic range.
• Salivary Amylase: Found in the mouth (approximately pH 7), with an optimal pH around neutral.

pH Scale

• pH = -log_{10}[H^+]
• A change of 1 pH unit represents a 10x change in H+ concentration.
• The pH scale ranges from 0 to 14, with:
  • 0-6 being acidic
  • 7 being neutral
  • 8-14 being alkaline

Denaturation

• If the pH is too far outside the enzyme's optimal range, it disrupts the bonds (e.g., hydrogen bonds) holding the enzyme together.
• This can change the shape of the active site, preventing substrates from binding and stopping the reaction.
• This process is called denaturation.
• In some cases, denaturation is reversible if conditions return to the optimal range, but sometimes it is permanent.

Temperature

• Different organisms have different body temperatures, so their enzymes have different optimal temperatures.
• Increasing the temperature raises the kinetic energy, making molecules move faster and increasing the likelihood of successful collisions between enzymes and substrates.
• However, increasing the temperature too far above the optimum can break bonds and denature the enzyme.
• Human lipase, for example, has an optimal temperature around 37°C (average body temperature).

Effects of increasing temperature

• Increasing temperature up to the optimal temperature raises kinetic energy, leading to more collisions and successful reactions.
• Increasing temperature above the optimal temperature can break bonds, causing the enzyme to denature.

Inhibitors

Non-Competitive Inhibitors

• Bind to the allosteric site of an enzyme, causing the active site to change shape and preventing it from catalyzing reactions.
• Increasing substrate concentration has little effect.

Competitive Inhibitors

• Bind to the active site of an enzyme, preventing the enzyme from forming enzyme-substrate complexes.
• Increasing substrate concentration decreases the effect of a competitive inhibitor, as the substrate is more likely to collide with the enzyme.

Concentration

• Concentration refers to how much of a substance is in a specified area.

Enzyme Concentration

• As enzyme concentration increases, the rate of reaction increases until all substrate molecules have been bound to active sites.
• After this point, adding more enzymes makes no difference.

Substrate Concentration

• Increasing substrate concentration increases the rate of reaction until all enzyme active sites are fully occupied.
• After this point, further increasing substrate concentration makes no difference.

Cofactors & Coenzymes

• Some enzymes require cofactors to function optimally.
• Cofactors can attach temporarily or permanently to enzymes.
• Many cofactors are inorganic ions, such as calcium or magnesium.
• Coenzymes are organic cofactors, often obtained from vitamins in our diet.
• The cofactor helps the substrate bind to the active site.