VCE bio Unit 3 AOS 2

Key Question: How are biochemical pathways regulated?

  1. Enzymes: Biological Catalysts

Key Concepts:

  • Enzymes are specialized proteins that act as biological catalysts, significantly increasing the rate of chemical reactions within cells without being consumed in the process.

  • They lower activation energy, which is the minimum energy required to initiate a chemical reaction, thereby enabling reactions to occur at lower temperatures.

  • Enzymes have a unique active site where specific substrates bind, forming an enzyme-substrate complex that facilitates the chemical reaction and yields products.

Models of Action:

  • Lock-and-Key Model: Proposes that the substrate fits precisely into the enzyme's active site, akin to a key fitting into a lock.

  • Induced Fit Model: Suggests that the enzyme undergoes a conformational change upon substrate binding, enhancing its fit and increasing catalytic efficiency.

Factors Affecting Enzyme Activity:

  • Temperature: The activity of enzymes typically increases with temperature due to increased kinetic energy, up to an optimum temperature (usually around 37°C for human enzymes). Past this point, high temperatures can lead to denaturation, where the enzyme loses its functional shape.

  • pH: Each enzyme has an optimal pH range in which it functions best. Extreme pH levels can disrupt ionic and hydrogen bonds, leading to changes in the enzyme's structure and a loss of function.

  • Substrate Concentration: Increasing substrate concentration raises reaction rates, as more substrate molecules are available to bind to the available enzyme active sites; however, this effect plateaus and reaches a maximum velocity (Vmax) when all active sites are saturated.

  • Enzyme Concentration: Adding more enzymes typically increases the rate of reactions, given that the substrate is in excess; this allows more enzyme-substrate complexes to form efficiently.

  1. Enzyme Inhibition

Types and Mechanisms:

  • Competitive Inhibition: This occurs when an inhibitor, which resembles the substrate, competes for binding at the active site. This type of inhibition can often be reversed by increasing substrate concentration.

  • Non-competitive Inhibition: The inhibitor binds to an allosteric site (a site distinct from the active site), altering the enzyme's conformation and reducing its activity without competing with substrate binding. This inhibition is usually not reversible by merely increasing substrate concentration.

  • Irreversible Inhibition: This type of inhibitor permanently binds to and modifies the active site of the enzyme (e.g., via covalent bonding), resulting in a lasting loss of catalytic activity, as seen with certain poisons or drugs.

  1. Biochemical Pathways

Definition:

  • Biochemical pathways consist of a series of enzyme-controlled reactions in which the product of one reaction becomes the substrate for the next, leading to a complex network of interactions. These pathways are tightly regulated and ensure metabolic efficiency and homeostasis.

Examples:

  • Photosynthesis:

    • Light-dependent reactions: Occur in the thylakoid membranes of chloroplasts; these reactions utilize sunlight to split water (H₂O), producing oxygen (O₂) and capturing energy in the form of ATP and NADPH.

    • Light-independent reactions (Calvin Cycle): Occur in the stroma of chloroplasts; in this phase, ATP and NADPH produced in the light-dependent reactions are used to convert carbon dioxide (CO₂) into glucose through a series of enzymatic reactions.

  • Cellular Respiration:

    • The pathway includes Glycolysis, which splits glucose into pyruvate and generates small amounts of ATP; the Krebs Cycle (also known as the Citric Acid Cycle) further breaks down pyruvate to release high-energy electrons for the Electron Transport Chain (ETC), resulting in the production of substantial amounts of ATP along with carbon dioxide (CO₂) and water (H₂O) as byproducts.

  1. Feedback Mechanisms and Regulation

Feedback Inhibition (End-product Inhibition):

  • This regulatory mechanism occurs when an end-product of a metabolic pathway binds to an allosteric site on an early enzyme in the pathway, changing its shape, which ultimately reduces further production of the end-product. An example includes ATP acting as an inhibitor of phosphofructokinase in glycolysis.

Importance of Feedback Mechanisms:

  • Feedback mechanisms are crucial for maintaining metabolic balance and homeostasis within the cell. They prevent the waste of resources by stopping the overproduction of products when their concentrations are sufficiently high, allowing cells to adjust to changing environmental conditions and avoid energy expenditure on unnecessary synthesis.

  1. Coenzymes

Definition:

  • Coenzymes are organic non-protein molecules that play essential roles in assisting enzymes, often acting as carriers for electrons, atoms, or functional groups that are necessary for the enzymatic reaction to occur. They are typically recycled during metabolic processes.

Major Coenzymes and Their Roles:

  • ATP: Adenosine triphosphate - transfers phosphate groups, serving as the primary energy currency of the cell.

  • NAD⁺ / NADH: Nicotinamide adenine dinucleotide, which accepts electrons during cellular respiration, playing a critical role in redox reactions.

  • FAD / FADH₂: Flavin adenine dinucleotide, functioning as an electron carrier in the Krebs cycle, facilitating the transfer of electrons.

  • Coenzyme A: Transfers acetyl groups (as acetyl-CoA) during the metabolism of fatty acids and synthesis of cholesterol, playing a pivotal role in various metabolic pathways.

  • NADP⁺ / NADPH: Nicotinamide adenine dinucleotide phosphate, utilized mainly in photosynthesis; it assists in the synthesis of glucose by providing reducing power through its reduced form, NADPH.