Enzyme Structure and Function

• Enzymes as Catalysts: Enzymes are proteins that speed up biochemical reactions by reducing activation energy. This allows reactions to happen faster without the enzyme being consumed.

• Active Site and Enzyme-Substrate Complex: The active site is the enzyme’s specific region where substrates bind to form the enzyme-substrate complex, initiating the reaction. The induced fit model describes how the active site changes shape slightly to better fit the substrate, enhancing efficiency.

• Allosteric Sites and Feedback Inhibition: Some enzymes have an allosteric site where molecules bind to control enzyme activity, either enhancing or inhibiting it. Feedback inhibition is a regulatory mechanism where the end product of a pathway inhibits an enzyme in that pathway to prevent waste.

Enzyme Catalysis and Metabolic Pathways

• Catabolic vs. Anabolic Pathways:

  • Catabolic pathways break down molecules, releasing energy (e.g., cellular respiration).

  • Anabolic pathways build complex molecules, requiring energy input (e.g., protein synthesis). Enzymes lower activation energy in both pathways, facilitating efficient reactions.

• Lowering Activation Energy: Enzymes reduce the amount of energy required to start a reaction, thus enabling reactions to proceed more rapidly.

Forms of Energy

• Types of Energy in Cells:

  • Kinetic energy is energy of motion (e.g., muscle contraction).

  • Thermal energy is heat energy that can be released in reactions.

  • Potential energy is stored energy (e.g., chemical energy in molecular bonds).

• Thermodynamics and Entropy:

  • The First Law of Thermodynamics states that energy can’t be created or destroyed, only transferred.

  • The Second Law of Thermodynamics states that every energy transformation increases entropy (disorder), often releasing heat.

• Spontaneous vs. Nonspontaneous Processes:

  • Spontaneous reactions occur without energy input, moving systems toward stability.

  • Nonspontaneous reactions require energy input to proceed.

Enzyme Regulation and Environmental Impacts

• Factors Influencing Enzyme Activity: Temperature, pH, and substrate concentration impact enzyme function. Each enzyme works optimally within a specific temperature and pH range, and deviations can lead to denaturation, where enzyme structure and function are disrupted.

• Inhibitors:

  • Competitive inhibitors bind to the active site, blocking substrate access.

  • Noncompetitive inhibitors bind elsewhere, altering the enzyme’s shape and reducing its effectiveness. Some inhibitors are irreversible, permanently disabling the enzyme.

Cellular Energy and ATP

• Types of Cellular Work:

  • Chemical Work: Synthesizing complex molecules.

  • Transport Work: Moving ions or molecules across membranes.

  • Mechanical Work: Physical activities like muscle contraction.

• ATP – The Cell’s Energy Currency: Adenosine Triphosphate (ATP) provides energy for cellular activities. Its high-energy phosphate bonds release energy when broken, powering various cellular functions. Cells regenerate ATP from ADP in energy-producing reactions.

Photosynthesis

• Role of Photosynthesis: Converts solar energy into glucose and oxygen, essential for life. It occurs in chloroplasts, with chlorophyll capturing light energy.

• Stages of Photosynthesis:

  • Light Reactions: Light energy splits water molecules, releasing oxygen and producing ATP and NADPH, which fuel the next stage.

  • Calvin Cycle: Occurring in the stroma, it uses CO₂, ATP, and NADPH to generate G3P, a precursor to glucose, through three phases:

    • Carbon Fixation: CO₂ attaches to a 5-carbon molecule.

    • Reduction: Molecules are converted to G3P.

    • Regeneration: The cycle’s starting molecule is regenerated for continued use.

Cellular Respiration

• Equation and Overview: Cellular respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP) breaks down glucose to release energy.

• Stages of Cellular Respiration:

  • Glycolysis: Splits glucose in the cytosol into two pyruvate molecules, generating ATP and NADH.

  • Krebs Cycle: In the mitochondrial matrix, breaks down acetyl CoA to produce CO₂, ATP, NADH, and FADH₂.

  • Electron Transport Chain and Chemiosmosis: Electrons from NADH and FADH₂ move through the ETC in the inner mitochondrial membrane, driving the pumping of protons to create a gradient. ATP Synthase uses this gradient to make ATP from ADP, in oxidative phosphorylation.

Anaerobic Respiration and Fermentation

• Anaerobic Pathways: In the absence of oxygen, cells can produce ATP through fermentation, a less efficient method.

  • Lactic Acid Fermentation: Pyruvate is converted to lactate, yielding minimal ATP and causing muscle fatigue.

  • Alcoholic Fermentation: Yeast convert pyruvate into ethanol and CO₂, used in bread and alcohol production.