Enzymes and Coenzymes in Photosynthesis and Cellular Respiration

1. The General Role of Enzymes

• Enzymes are biological catalysts, primarily proteins, that speed up biochemical reactions within cells.
• They are highly specific, with each enzyme catalyzing a particular reaction or a set of closely related reactions.
• Enzymes lower the activation energy required for a reaction to occur, thereby increasing the reaction rate.
• Enzymes are not consumed in the reactions they catalyze, allowing them to be reused repeatedly.

KEY TAKEAWAY: Enzymes are crucial for life because they enable biochemical reactions to occur at rates necessary to sustain cellular processes.

1.1 Enzyme Action

1. Substrate Binding: The enzyme binds to its specific substrate(s) at the active site.
2. Enzyme-Substrate Complex Formation: The enzyme and substrate form a temporary complex.
3. Catalysis: The enzyme catalyzes the reaction, converting the substrate(s) into product(s).
4. Product Release: The product(s) are released, and the enzyme returns to its original state, ready to catalyze another reaction.

1.2 Enzymes and Activation Energy

• All chemical reactions require an initial input of energy called activation energy.
• Enzymes reduce the activation energy by:
  • Providing an alternative reaction pathway with a lower energy barrier.
  • Stabilizing the transition state.
  • Bringing reactants into close proximity and optimal orientation.

EXAM TIP: Be able to draw and label a reaction coordinate diagram illustrating the effect of an enzyme on activation energy.

2. The General Role of Coenzymes

• Coenzymes are non-protein organic molecules that assist enzymes in catalyzing reactions.
• They bind to the enzyme and participate directly in the reaction, often by carrying electrons or chemical groups.
• Coenzymes are recycled within the cell but may be chemically altered during the reaction.
• Many coenzymes are derived from vitamins.

REMEMBER: Think of coenzymes as “helpers” that enable enzymes to function effectively.

2.1 Examples of Important Coenzymes

COMMON MISTAKE: Students often confuse NAD+ and NADP+. Remember that NADP+ is primarily used in anabolic reactions like photosynthesis.

3. Enzymes and Coenzymes in Photosynthesis

3.1 Light-Dependent Reactions

• Location: Thylakoid membranes of chloroplasts
• Overall: Light energy is converted into chemical energy in the form of ATP and NADPH.
• Key Enzymes & Coenzymes:
  • Photosystems I & II: Protein complexes containing chlorophyll and other pigments that absorb light energy.
  • ATP synthase: Enzyme that catalyzes the synthesis of ATP using a proton gradient (chemiosmosis).
  • NADP+ reductase: Enzyme that catalyzes the reduction of NADP+ to NADPH.
  • Plastoquinone (PQ): A mobile electron carrier within the thylakoid membrane.
  • Plastocyanin (PC): A mobile electron carrier that transfers electrons from cytochrome b6f complex to Photosystem I.
• Role of Enzymes & Coenzymes:
  • Photosystems I & II: Convert light energy to chemical energy by exciting electrons.
  • ATP synthase: Uses the proton gradient to drive ATP synthesis.
  • NADP+ reductase: Transfers electrons to NADP+, forming NADPH, a reducing agent for the Calvin cycle.

3.2 Light-Independent Reactions (Calvin Cycle)

• Location: Stroma of chloroplasts
• Overall: Carbon dioxide is fixed and converted into glucose using ATP and NADPH.
• Key Enzyme & Coenzyme:
  • RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase): Enzyme that catalyzes the initial fixation of carbon dioxide to ribulose-1,5-bisphosphate (RuBP).
  • ATP: Provides energy for the reduction and regeneration phases.
  • NADPH: Provides reducing power (electrons) for the reduction of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P).
• Role of Enzyme & Coenzymes:
  • RuBisCO: Catalyzes the crucial first step of carbon fixation.
  • ATP and NADPH: Provide the necessary energy and reducing power to convert fixed carbon into sugars.

VCAA FOCUS: VCAA often asks about the specific roles of RuBisCO, ATP synthase, and NADPH in photosynthesis.

4. Enzymes and Coenzymes in Cellular Respiration

4.1 Glycolysis

• Location: Cytosol
• Overall: Glucose is broken down into pyruvate, producing ATP and NADH.
• Key Enzymes & Coenzymes:
  • Hexokinase: Phosphorylates glucose.
  • Phosphofructokinase (PFK): A key regulatory enzyme that phosphorylates fructose-6-phosphate.
  • Glyceraldehyde-3-phosphate dehydrogenase: Oxidizes glyceraldehyde-3-phosphate and reduces NAD+ to NADH.
  • Pyruvate kinase: Catalyzes the final step, producing pyruvate and ATP.
  • NAD+: Acts as an electron acceptor, forming NADH.
  • ATP: Provides initial energy and is produced during the process.
• Role of Enzymes & Coenzymes:
  • Enzymes catalyze the series of reactions that break down glucose and generate ATP and NADH.
  • NAD+ accepts electrons, forming NADH, which carries electrons to the electron transport chain.
  • ATP provides energy for the initial steps and is generated as a product.

4.2 Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial matrix
• Overall: Pyruvate is oxidized to carbon dioxide, generating ATP, NADH, and FADH2.
• Key Enzymes & Coenzymes:
  • Pyruvate dehydrogenase complex: Converts pyruvate to acetyl CoA.
  • Citrate synthase: Catalyzes the first step, combining acetyl CoA with oxaloacetate.
  • Isocitrate dehydrogenase: Key regulatory enzyme that catalyzes the oxidation of isocitrate.
  • α-ketoglutarate dehydrogenase complex: Catalyzes the oxidation of α-ketoglutarate.
  • Succinate dehydrogenase: Oxidizes succinate to fumarate, producing FADH2.
  • NAD+: Accepts electrons, forming NADH.
  • FAD: Accepts electrons, forming FADH2.
  • CoA: Carries acetyl groups.
  • GDP: Is phosphorylated to GTP which then donates the phosphate to ADP to make ATP.
• Role of Enzymes & Coenzymes:
  • Enzymes catalyze the cyclic series of reactions that oxidize acetyl CoA, releasing carbon dioxide and generating ATP, NADH, and FADH2.
  • NAD+ and FAD accept electrons, forming NADH and FADH2, which carry electrons to the electron transport chain.
  • CoA carries acetyl groups into the cycle.

4.3 Electron Transport Chain (ETC) and Oxidative Phosphorylation

• Location: Inner mitochondrial membrane
• Overall: Electrons from NADH and FADH2 are passed along a series of electron carriers, generating a proton gradient that drives ATP synthesis.
• Key Enzymes & Coenzymes:
  • NADH dehydrogenase (Complex I): Accepts electrons from NADH.
  • Succinate dehydrogenase (Complex II): Accepts electrons from FADH2.
  • Cytochrome bc1 complex (Complex III): Transfers electrons from ubiquinone to cytochrome c.
  • Cytochrome c oxidase (Complex IV): Transfers electrons to oxygen, forming water.
  • Ubiquinone (Coenzyme Q): A mobile electron carrier.
  • Cytochrome c: A mobile electron carrier.
  • ATP synthase: Enzyme that catalyzes the synthesis of ATP using the proton gradient (chemiosmosis).
• Role of Enzymes & Coenzymes:
  • Electron carriers (Complexes I-IV, ubiquinone, and cytochrome c) facilitate the transfer of electrons, releasing energy to pump protons across the inner mitochondrial membrane.
  • ATP synthase uses the proton gradient to drive ATP synthesis (oxidative phosphorylation).
  • Oxygen acts as the final electron acceptor, forming water.

STUDY HINT: Create a flowchart illustrating the flow of electrons and the roles of key enzymes and coenzymes in both photosynthesis and cellular respiration.

5. Summary Table

APPLICATION: Understanding enzyme and coenzyme function is crucial in fields like medicine (drug design) and biotechnology (industrial enzyme production).