Đề 11 – Bài tập, đề thi trắc nghiệm online Hóa sinh enzyme

In feedback inhibition, the end product of a metabolic pathway inhibits an enzyme that catalyzes an early step in the pathway. This is a negative feedback mechanism that regulates the pathway`s output, preventing overproduction of the end product. By inhibiting an enzyme earlier in the pathway, the entire pathway`s flux is reduced.

Synthesizing enzymes as zymogens prevents them from being active in the cells where they are produced or stored, avoiding unwanted reactions in inappropriate locations or times. Activation usually occurs at the site where the enzyme`s activity is needed, often through proteolytic cleavage. Digestive enzymes and enzymes in blood clotting cascades are common examples.

Coenzymes are organic molecules that assist enzymes in catalysis. They often act as carriers of electrons, atoms, or functional groups during the reaction. They are not just structural components or direct substrate binders; they actively participate in the chemical transformation by being modified during the reaction and regenerated afterwards. Many coenzymes are derived from vitamins.

The catalytic efficiency (kcat/Km) is a measure of how effectively an enzyme converts substrate to product. A higher kcat/Km value indicates a more efficient enzyme. A high kcat (turnover number) means the enzyme can process substrate quickly, and a low Km (Michaelis constant) means the enzyme has high affinity for the substrate. Therefore, a higher kcat/Km reflects both high affinity and fast catalysis.

Lactase is the enzyme that hydrolyzes lactose, a disaccharide found in milk, into its component monosaccharides, glucose and galactose. Sucrase hydrolyzes sucrose, maltase hydrolyzes maltose, and cellulase hydrolyzes cellulose.

Isoenzymes, because of their different properties (like Km, Vmax, regulatory characteristics, and tissue distribution), provide a way to fine-tune metabolic processes in different tissues or cellular compartments. This allows for tissue-specific regulation and adaptation to varying physiological needs. They don`t necessarily have higher activity or simpler pathways, and inhibitor sensitivity can vary.

The Michaelis-Menten constant (Km) is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity (Vmax). Km provides an approximate measure of the substrate concentration required for significant catalysis to occur, and also reflects the affinity of the enzyme for its substrate (lower Km generally indicates higher affinity).

Proteases are commonly used in laundry detergents to break down protein-based stains like blood, grass, and food stains. Amylases are used for starch stains, lipases for fat and oil stains, and cellulases for cotton fiber cleaning and softening.

In competitive inhibition, the inhibitor resembles the substrate and competes for binding to the active site. This increases the apparent Km (more substrate is needed to reach half Vmax) because the substrate now has to compete more effectively against the inhibitor. However, at very high substrate concentrations, the substrate can outcompete the inhibitor, and Vmax remains unchanged.

The induced-fit model describes enzyme-substrate interaction where the active site of the enzyme is not perfectly complementary to the substrate initially. Upon substrate binding, the enzyme undergoes a conformational change to better fit the substrate, enhancing specificity and catalysis. While the lock-and-key model is simpler, induced-fit better explains the dynamic nature and specificity of enzyme-substrate interactions.

pH affects the ionization states of amino acid side chains, particularly those in the active site and those involved in maintaining the enzyme`s three-dimensional structure. Changes in ionization can alter substrate binding, catalytic activity, and enzyme conformation, leading to changes in enzyme activity. Extreme pH values can lead to enzyme denaturation.

Enzyme activity generally increases with temperature because higher temperatures provide more kinetic energy for molecules, leading to more frequent and energetic collisions between enzyme and substrate, up to a point. Beyond the optimal temperature, the enzyme`s protein structure starts to denature, leading to a sharp decrease in activity.

Lyases catalyze the breaking (cleavage) of chemical bonds by means other than hydrolysis or oxidation. They often form new double bonds or ring structures. For example, decarboxylases and dehydratases are lyases. Hydrolases use water to break bonds, isomerases rearrange atoms within a molecule, and transferases move functional groups.

Enzymes act as catalysts by lowering the activation energy (Ea) required for a reaction to occur. They do this by providing an alternative reaction pathway with a lower Ea. They do not increase activation energy, provide energy to reactants, or increase temperature.

The active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. It provides a specific microenvironment that facilitates substrate binding through various interactions and catalyzes the chemical transformation.

Non-competitive inhibitors bind to a site other than the active site, altering the enzyme`s conformation and reducing its catalytic efficiency. This reduces the effective concentration of active enzyme, leading to a decrease in Vmax. However, because the inhibitor does not interfere with substrate binding to the active site of the enzyme molecules that are still active, Km remains unchanged.

Enzyme immobilization involves confining enzymes to a defined space or matrix. The main advantage is the ability to recover and reuse the enzyme after a reaction, which is highly beneficial in industrial processes. It reduces enzyme consumption, simplifies product purification, and can improve process economics. While immobilization can sometimes affect stability or activity, reusability is the primary benefit.

Vmax (maximum velocity) represents the theoretical maximum rate of an enzyme-catalyzed reaction. It is achieved when the enzyme is saturated with substrate, meaning all enzyme active sites are occupied. Vmax is dependent on enzyme concentration; increasing enzyme concentration will increase Vmax.

Phenylketonuria (PKU) is caused by a deficiency in phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine. The buildup of phenylalanine and its byproducts leads to intellectual disability and other health problems if untreated. Lactase deficiency causes lactose intolerance. Glucose-6-phosphate dehydrogenase deficiency and pyruvate kinase deficiency are related to red blood cell metabolism.

Phosphorylation, the addition of a phosphate group, is a common and reversible covalent modification that regulates the activity of many enzymes. Phosphorylation is typically catalyzed by protein kinases and can either activate or inhibit an enzyme depending on the specific enzyme and site of phosphorylation. Hydrolysis is bond breaking, denaturation is protein unfolding, and competitive inhibition is non-covalent regulation.

Allosteric enzymes typically have multiple subunits and active sites. Their activity is regulated by allosteric effectors (activators or inhibitors) that bind to regulatory sites (allosteric sites), distinct from the active site. This binding induces conformational changes that alter the enzyme`s affinity for the substrate or its catalytic rate. They do not follow simple Michaelis-Menten kinetics and are regulated beyond just substrate concentration or competitive inhibition.

The Lineweaver-Burk plot is a graphical representation of the Michaelis-Menten equation. It plots the reciprocal of the reaction velocity (1/V) against the reciprocal of the substrate concentration (1/[S]). This linear transformation allows for easier determination of Km and Vmax from the intercepts and slope of the line. The x-intercept is -1/Km, and the y-intercept is 1/Vmax.

Enzymes are catalysts, meaning they speed up reactions without being consumed in the process. They do not change the equilibrium constant of a reaction; they only accelerate the rate at which equilibrium is reached. Enzymes increase the rate of both the forward and reverse reactions equally. They are known for their high specificity for substrates and the reactions they catalyze.

Oxidoreductases are enzymes that catalyze oxidation-reduction reactions, involving the transfer of electrons or hydrogen atoms between molecules. Examples include dehydrogenases and oxidases. Hydrolases catalyze hydrolysis reactions, isomerases catalyze isomerization reactions, and transferases catalyze the transfer of functional groups.

Cofactors can be inorganic ions (like metal ions) or organic molecules. Coenzymes are a specific type of cofactor that are organic molecules, often derived from vitamins. The key distinction is that cofactors is a broader term encompassing both inorganic and organic helpers, while coenzymes are specifically organic cofactors.

Ligases (also known as synthetases) catalyze the joining of two molecules together. These reactions are typically coupled with the hydrolysis of ATP or another nucleoside triphosphate to provide the energy needed for bond formation. Examples include DNA ligase and aminoacyl-tRNA synthetases.

The turnover number (kcat) represents the maximum number of substrate molecules an enzyme can convert to product per unit time when it is fully saturated with substrate. It is a measure of the intrinsic catalytic rate of an enzyme molecule. It is calculated as Vmax divided by the total enzyme concentration.

Enzyme-substrate interactions within the active site are typically mediated by non-covalent bonds, such as hydrogen bonds, ionic bonds, and van der Waals forces. These interactions are weaker and reversible, allowing for substrate binding, catalysis, and product release. Covalent bonds are usually not formed between the enzyme and substrate during the initial binding phase; they are more relevant in some catalytic mechanisms (like covalent catalysis) as transient intermediates but not for initial substrate binding itself in general enzyme-substrate interaction.

Proteases, such as papain (from papaya) and bromelain (from pineapple), are used as meat tenderizers. They break down tough muscle proteins (collagen and elastin), making the meat softer. Amylases break down starches, lipases break down fats, and cellulases break down cellulose.

Transition state analogs are stable compounds that mimic the transition state of an enzyme-catalyzed reaction. Because enzymes bind most tightly to the transition state, these analogs often bind to the enzyme`s active site with very high affinity. They are valuable for studying enzyme mechanisms and can be developed as potent enzyme inhibitors in drug design.