Metabolic Pathways: Enzyme Inhibition (PDF)

1. Metabolism: Metabolism is the sum of all enzyme-catalysed reactions within an organism, essential for maintaining life. 2. Enzyme-Catalysed Reactions: Most chemical changes in a cell occur through pathways of enzyme-catalysed reactions, with each step controlled by a specific...

1. Metabolism: Metabolism is the sum of all enzyme-catalysed reactions within an organism, essential for maintaining life. 2. Enzyme-Catalysed Reactions: Most chemical changes in a cell occur through pathways of enzyme-catalysed reactions, with each step controlled by a specific enzyme. These reactions can happen both intracellularly (e.g., glycolysis, Krebs cycle) and extracellularly (e.g., digestion in the gut). 3. Intracellular vs. Extracellular Enzyme Reactions: Intracellular reactions: Occur within the cell, like glycolysis and the Krebs cycle, key steps in cellular respiration. Extracellular reactions: Occur outside the cell, such as the breakdown of nutrients during chemical digestion in the gut. 4. Metabolic Pathways: Linear pathways: Involve a series of reactions where the product of one step becomes the reactant for the next. Examples: glycolysis (intracellular) and blood clotting (extracellular). Cyclic pathways: Involve cycles where the end product regenerates the starting molecule. Examples: Krebs cycle(aerobic respiration) and Calvin cycle (photosynthesis). 5. Regulation of Metabolic Pathways: Metabolic pathways are regulated through intermediates, allowing for more control over the chemical changes within the cell. Summary: Enzyme-catalysed reactions drive metabolism and occur in both intracellular and extracellular environments. Linear pathways (e.g., glycolysis, blood clotting) and cyclic pathways (e.g., Krebs cycle, Calvin cycle) are key to cellular functions. Metabolic pathways are tightly regulated to ensure efficient cellular processes. 1. Competitive Inhibition: A competitive inhibitor binds reversibly to the active site of an enzyme, blocking the substrate from binding. The inhibitor is chemically and structurally similar to the substrate, allowing it to compete for the active site. Effect: The inhibitor reduces the enzyme's activity, but this effect can be reduced by increasing the substrate concentration, which "outcompetes" the inhibitor. Example: Statins (cholesterol-lowering drugs) are competitive inhibitors that block the enzyme HMG-CoA reductase, reducing cholesterol production. 2. Non-Competitive Inhibition: A non-competitive inhibitor binds to an allosteric site (a site other than the active site), causing a conformational change in the enzyme's structure. The conformational change altered the active site, preventing the substrate from binding effectively. Effect: Since the inhibitor does not compete with the substrate, increasing substrate concentration does not mitigate the inhibition. Example: Cyanide is a non-competitive inhibitor that binds to cytochrome oxidase (part of the electron transport chain), preventing it from passing electrons to oxygen, halting ATP production and causing death. Key Takeaways: Competitive inhibition blocks the active site but can be overcome by increasing substrate concentration. Non-competitive inhibition changes the enzyme's structure and cannot be overcome by substrate concentration. Both types of inhibition can have significant impacts on metabolic pathways and cellular functions, as seen in statins and cyanide. 1. End-product (Feedback) Inhibition: End-product inhibition is a form of negative feedback used to regulate metabolic pathways. The final product of a metabolic pathway inhibits an enzyme from an earlier step in the pathway to prevent excess production.  This inhibition occurs when the product binds to an allosteric site on the enzyme, leading to a non-competitive inhibition that temporarily inactivates the enzyme. 2. Example: Isoleucine Synthesis in Bacteria and Plants: In bacteria and plants, isoleucine is synthesized from threonine in a five-step pathway. The enzyme threonine deaminase catalyzes the first step, and isoleucine can act as a non-competitive inhibitor of this enzyme. When excess isoleucine is produced, it inhibits further synthesis, ensuring that threonine is not depleted and the pathway is regulated. 3. Role of Feedback Inhibition: Feedback inhibition ensures that the production of key products, like isoleucine, is tightly regulated. When product levels are high, the product inhibits further synthesis, decreasing production. When product levels drop, the inhibition is removed, and the pathway proceeds to increase production. Key Takeaways: End-product inhibition ensures that metabolic pathways are efficient and not wasteful by stopping excess production when enough product is made. Isoleucine synthesis in bacteria and plants is an example of feedback inhibition, where the product regulates its own production through non-competitive inhibition. 1. Mechanism-based Inhibition (Suicide Inhibition): Mechanism-based inhibition occurs when an enzyme binds to a competitive inhibitor that forms an irreversible complex with the enzyme. This process involves covalent bonds being formed during the normal catalytic reaction, preventing the inhibitor from being released. As a result, the enzyme is permanently inactivated and can no longer catalyze reactions. The enzyme’s activity can only be restored by synthesizing new enzymes. 2. Penicillin as an Example:  Penicillin is a classic example of a mechanism-based inhibitor. It irreversibly inhibits specific transpeptidases, enzymes that are involved in synthesizing the bacterial cell wall. This inhibition disrupts the bacterial cell wall, leading to the destruction of the bacterial cell as it can no longer regulate its internal pressure and ruptures (lysis). Penicillin specifically targets prokaryotic cells, making it a highly effective antibiotic. 3. Bacterial Resistance: Some bacteria have evolved modified versions of the transpeptidase enzyme, which have a low affinity for penicillin. These resistant strains can survive penicillin treatment and may transfer this resistance through bacterial conjugation, which poses a challenge for treatment. Key Takeaways: Mechanism-based inhibition is a permanent inactivation of an enzyme via the irreversible binding of an inhibitor, often involving covalent bonds. Penicillin is a prime example, targeting bacterial cell wall synthesis to cause cell lysis, but bacterial resistance through modified enzymes and conjugation is a growing concern.