Đề thi, bài tập trắc nghiệm online Hóa sinh enzyme – Đề 14

Proteolytic activation involves activating an inactive enzyme precursor (zymogen or proenzyme) by cleaving one or more peptide bonds. This cleavage often removes a blocking peptide sequence, allowing the enzyme to fold into its active conformation. It's not about activator binding, temperature, or pH changes, but about structural modification by proteolysis.

While the lock-and-key model is a simplified early concept, the induced-fit model more accurately describes enzyme-substrate interaction. In induced-fit, the enzyme's active site and the substrate both undergo conformational changes upon binding to achieve optimal interaction and catalysis.

The turnover number (kcat) is a measure of the catalytic activity of an enzyme. It represents the maximum number of substrate molecules an enzyme molecule can convert to product per unit time when the enzyme is fully saturated with substrate. Km is substrate concentration at half Vmax, and Vmax is the maximum rate but not per enzyme molecule, and Km relates to affinity, not catalytic rate directly.

Ligases catalyze the formation of new chemical bonds, typically requiring energy input, often in the form of ATP hydrolysis. They join two molecules together, for example, DNA ligase joins DNA fragments. Water is used by hydrolases, oxygen by oxidases (a type of oxidoreductase), and reducing agents are involved in reduction reactions.

Vmax represents the maximum velocity of an enzyme-catalyzed reaction. It is achieved when the enzyme is saturated with substrate, meaning all enzyme active sites are occupied. Vmax reflects the enzyme's catalytic efficiency under optimal conditions.

Hydrolases catalyze hydrolysis reactions, where a chemical bond is cleaved by the addition of water. For example, peptidases hydrolyze peptide bonds, and lipases hydrolyze ester bonds in lipids.

The active site is a specific region on the enzyme where substrate molecules bind and undergo a chemical reaction. It is responsible for both substrate binding and catalysis, due to the specific arrangement of amino acid residues.

Cofactors can be inorganic ions (like metal ions such as Mg²⁺, Zn²⁺, Fe²⁺) or organic molecules (coenzymes). They assist enzymes in catalysis. Glucose, RNA, and DNA are not cofactors; glucose is a substrate/energy source, and RNA/DNA are nucleic acids involved in genetic information.

Covalent modification involves the chemical addition or removal of groups (like phosphate, methyl, or acetyl groups) to or from an enzyme. This modification can alter the enzyme's conformation and activity. Phosphorylation is a common example. Allosteric regulation involves non-covalent binding, feedback inhibition is a type of allosteric regulation, and competitive inhibition is a type of reversible inhibition.

Metabolic pathways are series of enzyme-catalyzed reactions. Enzymes play a vital role in catalyzing each specific step, allowing metabolic pathways to proceed efficiently and be regulated to meet the cell's needs. Enzymes do not directly provide energy (that's the role of energy-carrying molecules like ATP), nor do they slow down reactions or store genetic information.

Enzyme activity is highly sensitive to temperature and pH. For enzyme assays to be accurate and reproducible, these factors must be carefully controlled and kept constant. Changes in temperature and pH can significantly alter enzyme activity and affect the assay results. The goal is not to slow down the reaction or denature the enzyme, but to measure its true activity under defined conditions.

Non-competitive inhibitors bind to a site on the enzyme other than the active site (allosteric site). This binding causes a conformational change in the enzyme, reducing its catalytic activity. Non-competitive inhibitors decrease Vmax (because the enzyme's inherent catalytic rate is reduced), but they do not affect Km (substrate binding affinity is unchanged, although the enzyme's efficiency is reduced).

In feedback inhibition, the end product of a metabolic pathway acts as an allosteric inhibitor of an enzyme earlier in the pathway. This mechanism helps regulate the pathway's output by slowing down production when the end product accumulates, maintaining homeostasis.

Competitive inhibitors compete with the substrate for binding to the active site. They increase the apparent Km (because more substrate is needed to reach half Vmax in the presence of the inhibitor), but they do not affect Vmax because, at sufficiently high substrate concentrations, the substrate can outcompete the inhibitor.

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

Enzymes like proteases (for meat tenderizing) and pectinases (for fruit juice clarification) are widely used in the food industry to improve texture, taste, and clarity of food products. Sterilization and antibiotic production are more related to antimicrobial agents, while genetic disorder diagnosis involves molecular biology techniques.

Enzyme immobilization involves attaching enzymes to a solid support or confining them within a membrane or matrix. This technique offers advantages in industrial applications, such as enzyme reuse, easier product separation, and improved enzyme stability. It is not about destroying enzyme activity, isolating enzymes (though it can be part of purification), or just purifying enzymes.

Allosteric enzymes have regulatory sites, called allosteric sites, separate from their active sites. Regulatory molecules (activators or inhibitors) bind to these allosteric sites, inducing conformational changes that alter the enzyme's activity.

Km is defined as the substrate concentration at which the reaction rate is half of the maximum rate (Vmax). It is an indicator of the enzyme's affinity for its substrate; a lower Km indicates a higher affinity.

Uncompetitive inhibitors bind only to the enzyme-substrate complex, not to the free enzyme. This binding distorts the active site and reduces the enzyme's catalytic activity. Uncompetitive inhibitors decrease both Km and Vmax. Km decreases because the inhibitor effectively increases the apparent affinity of the enzyme for the substrate (by trapping the ES complex), and Vmax decreases because the inhibitor reduces the concentration of functional enzyme-substrate complex.

Metal ions act as inorganic cofactors for many enzymes. They can play various roles, including: stabilizing the enzyme's structure, facilitating substrate binding and orientation in the active site, and participating directly in catalytic mechanisms, especially in redox reactions. They are not just for structural support or redox reactions, and gene expression regulation is not their direct role in enzyme activity.

Generally, increasing temperature increases the rate of enzyme-catalyzed reactions because it provides more kinetic energy for molecules to react. However, beyond an optimal temperature, the enzyme's structure (especially the active site) can denature, leading to a decrease in reaction rate.

Enzymes function by lowering the activation energy (Ea) of a reaction, which is the energy barrier that must be overcome for a reaction to occur. By reducing Ea, enzymes accelerate the reaction rate without being consumed in the process.

Isomerases catalyze isomerization reactions, which involve the rearrangement of atoms within a molecule to form isomers. They convert one isomer to another without changing the molecular formula. Examples include racemases and epimerases.

Coenzymes are organic cofactors that participate in the catalytic reaction, often acting as carriers of electrons, atoms, or functional groups. They are regenerated after the reaction and can participate in multiple catalytic cycles. They don't just provide structural support or block substrate binding; and genetic regulation is a different mechanism for enzyme control.

pH affects the ionization state of amino acid side chains in the enzyme. Changes in ionization can alter the enzyme's folding, active site shape, and the charges involved in substrate binding and catalysis, thus significantly impacting enzyme activity. Each enzyme typically has an optimal pH range for maximal activity.

Lyases catalyze bond cleavage through mechanisms other than hydrolysis or oxidation, often resulting in the formation of double bonds or new rings. Examples include decarboxylases (removing carboxyl groups) and dehydratases (removing water to form double bonds).

Enzymes as biomarkers mean their levels in body fluids (like blood or urine) can be measured to diagnose or monitor diseases. For example, elevated levels of certain liver enzymes in blood can indicate liver damage. Enzymes are not directly used for treatment as biomarkers, nor are they used for drug synthesis or instrument cleaning in this context.

Enzymes are highly specific, meaning they typically catalyze only one or a few specific reactions and usually act on specific substrates. This specificity arises from the unique three-dimensional structure of the enzyme's active site, which is complementary to the shape and chemical properties of the substrate.

Transferases catalyze the transfer of functional groups (e.g., methyl, acyl, phosphate groups) from one molecule (donor) to another (acceptor). Examples include kinases (transferring phosphate groups) and transaminases (transferring amino groups). Lyases catalyze bond cleavage by means other than hydrolysis or oxidation, ligases join molecules together, and hydrolases use water to break bonds.