Factors Impacting Enzyme Function in Photosynthesis and Cellular Respiration

Introduction to Enzymes

• Enzymes: Biological catalysts, typically proteins, that speed up biochemical reactions by lowering the activation energy.
• Active Site: The specific region of an enzyme where the substrate binds and catalysis occurs.
• Substrate: The molecule upon which an enzyme acts.
• Enzyme-Substrate Complex: The temporary association formed when a substrate binds to the active site of an enzyme.
• Biochemical Pathways: Series of enzyme-catalyzed reactions where the product of one reaction is the substrate for the next. Photosynthesis and cellular respiration are both biochemical pathways.

KEY TAKEAWAY: Enzymes are essential for life; they accelerate the rates of reactions in metabolic pathways, including photosynthesis and cellular respiration.

General Factors Affecting Enzyme Function

1. Temperature

• Effect: Temperature affects the rate of enzyme-catalyzed reactions.
• Increasing Temperature: Generally increases reaction rate up to a certain point, as molecules have more kinetic energy, leading to more frequent collisions between enzyme and substrate.
• Optimum Temperature: The temperature at which an enzyme exhibits maximum activity.
• High Temperatures: Beyond the optimum, the enzyme’s structure begins to break down (denaturation), leading to a loss of function. This is because the heat disrupts the bonds (hydrogen bonds, ionic bonds) that maintain the enzyme’s three-dimensional shape.
• Low Temperatures: Enzyme activity is reduced at low temperatures because molecules move slower, leading to fewer collisions. However, the enzyme does not denature.

COMMON MISTAKE: Students often think that enzymes are “killed” by high temperatures. Enzymes are not alive; they are denatured, meaning their structure is changed.

2. pH

• Effect: pH affects the ionization of amino acid residues in the enzyme, which can alter its shape and the charge properties of the active site.
• Optimum pH: Each enzyme has an optimum pH at which it functions most effectively.
• Deviations from Optimum: Changes in pH can disrupt the hydrogen bonds and ionic interactions that maintain the enzyme’s three-dimensional structure, leading to denaturation or altered active site.
• Photosynthesis: Enzymes in the chloroplasts have optimal pH ranges for their specific reactions.
• Cellular Respiration: Enzymes in the cytoplasm and mitochondria have their respective optimal pH ranges.

VCAA FOCUS: Be prepared to interpret graphs showing enzyme activity vs. temperature or pH. Pay attention to the optimum and the range of activity.

3. Enzyme and Substrate Concentration

• Enzyme Concentration:
  • Increasing enzyme concentration generally increases the reaction rate, provided there is sufficient substrate available.
  • If the substrate is limiting, increasing the enzyme concentration will have no significant effect on the reaction rate once all substrate molecules are bound.
• Substrate Concentration:
  • Increasing substrate concentration generally increases the reaction rate, up to a point.
  • Saturation: At high substrate concentrations, the enzyme becomes saturated, meaning all active sites are occupied. Adding more substrate will not increase the reaction rate significantly. This is because the enzyme is working at its maximum velocity (\(V_{max}\)).

EXAM TIP: Understand the concept of saturation. A graph of reaction rate vs. substrate concentration will plateau at high substrate concentrations.

4. Enzyme Inhibitors

• Enzyme Inhibitors: Substances that reduce enzyme activity.
• Competitive Inhibitors:
  • Mechanism: Bind to the active site of the enzyme, preventing the substrate from binding.
  • Structure: Similar in structure to the substrate.
  • Effect: Decreases the rate of reaction. The effect can be overcome by increasing the substrate concentration.
• Non-Competitive Inhibitors:
  • Mechanism: Bind to a site on the enzyme other than the active site (allosteric site), causing a conformational change in the enzyme, which reduces its ability to bind to the substrate.
  • Effect: Decreases the maximum rate of reaction (\(V_{max}\)). Increasing substrate concentration cannot overcome the effect.

REMEMBER: Competitive inhibitors “compete” for the active site, while non-competitive inhibitors change the enzyme’s shape.

Enzyme Function in Photosynthesis and Cellular Respiration

• Photosynthesis: Enzymes play crucial roles in both the light-dependent and light-independent (Calvin cycle) reactions. Temperature, pH and presence of inhibitors can affect the rate of photosynthesis.
• Cellular Respiration: Enzymes are essential for glycolysis, the Krebs cycle, and the electron transport chain. Temperature, pH and presence of inhibitors can affect the rate of cellular respiration.
• Regulation: Enzyme activity is tightly regulated in both photosynthesis and cellular respiration to maintain cellular homeostasis and respond to changing environmental conditions.

STUDY HINT: Create a table listing the key enzymes in photosynthesis and cellular respiration and how their activity is affected by temperature, pH, and inhibitors.

Examples in Photosynthesis and Cellular Respiration

• Photosynthesis:
  • RuBisCO: The enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle. Its activity is affected by temperature and pH.
  • ATP Synthase: Uses a proton gradient to generate ATP. Its function can be inhibited by certain chemicals.
• Cellular Respiration:
  • Hexokinase: The enzyme that catalyzes the first step of glycolysis. Its activity is inhibited by glucose-6-phosphate (a product of the reaction).
  • Cytochrome c oxidase: The final enzyme in the electron transport chain. Cyanide is a non-competitive inhibitor of this enzyme, blocking cellular respiration.

APPLICATION: Understanding enzyme inhibition is crucial in pharmacology. Many drugs work by inhibiting specific enzymes involved in disease pathways.