Đề thi, bài tập trắc nghiệm online Hóa sinh enzyme – Đề 12

Vmax is the maximum rate of reaction when the enzyme is saturated with substrate. It is directly proportional to the enzyme concentration; doubling the enzyme concentration will double the Vmax, assuming substrate is saturating.

Allosteric regulation involves the binding of regulatory molecules (activators or inhibitors) to an allosteric site, which is a site on the enzyme distinct from the active site. This binding induces conformational changes that affect the enzyme's activity.

Ligases catalyze the formation of new bonds, often requiring energy input, typically from ATP hydrolysis. They join two molecules together, such as in DNA ligation or amino acid activation for protein synthesis.

The active site is a specific region on the enzyme where substrate molecules bind and undergo a chemical reaction. It provides a unique microenvironment that facilitates the catalytic process, including substrate binding and transition state stabilization.

Hydrolases catalyze the cleavage of bonds by the addition of water (hydrolysis). They commonly break peptide bonds in proteins, glycosidic bonds in carbohydrates, and ester bonds in lipids and nucleic acids.

Covalent modification involves the addition or removal of a chemical group to an enzyme through covalent bond formation. Phosphorylation (addition of a phosphate group) by kinases is a common example of covalent modification that can either activate or inactivate enzymes.

High temperatures can disrupt the weak bonds (hydrogen bonds, hydrophobic interactions, etc.) that maintain the enzyme's tertiary and quaternary structure, leading to denaturation. Denaturation causes the enzyme to unfold and lose its active site conformation, resulting in loss of catalytic activity.

Enzyme activity generally increases with temperature due to increased kinetic energy and molecular collisions. However, beyond an optimal temperature, the enzyme's structure starts to denature, leading to a sharp decrease in activity.

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 to regulate the pathway by preventing overproduction of the end product.

Isoenzymes often exhibit different kinetic properties (Km, Vmax), regulatory properties, or tissue distributions. This allows for fine-tuning of metabolic pathways in different tissues or at different developmental stages, adapting enzyme activity to specific physiological needs.

Zymogens or proenzymes are inactive precursor forms of enzymes. They are often synthesized and stored in an inactive state and are activated only when needed, usually by proteolytic cleavage. This mechanism prevents enzymes from being active at inappropriate times or locations.

pH changes can alter the ionization of amino acid side chains, particularly those in the active site. This can disrupt the enzyme's tertiary structure, substrate binding, and catalytic mechanism, leading to changes in activity. Each enzyme has an optimal pH range for maximal activity.

Enzyme activity is commonly regulated by allosteric regulation, covalent modification, and controlling enzyme concentration through transcriptional regulation. Increasing atmospheric pressure is not a physiologically relevant or common mechanism for regulating enzyme activity in biological systems.

Both models describe enzyme specificity. The Lock-and-Key model proposes a rigid active site complementary to the substrate. The Induced-Fit model suggests the active site is flexible and changes shape upon substrate binding to achieve optimal fit and interaction.

The transition state is a fleeting, high-energy, unstable intermediate species formed during a reaction. It represents the point of highest energy in the reaction pathway between reactants and products. Enzymes lower the activation energy by stabilizing the transition state.

The Enzyme Commission (EC) number is a numerical classification system for enzymes, based on the type of reaction they catalyze. It provides a systematic and unambiguous way to identify and categorize enzymes, regardless of their common names.

Metal ions can function as essential cofactors for many enzymes. They can participate in various aspects of catalysis, including binding to substrates, mediating redox reactions (as in metalloenzymes), or stabilizing the enzyme's active conformation.

Coenzymes are organic cofactors that are loosely bound to enzymes and can dissociate, often participating in the reaction and being regenerated in a separate reaction. Prosthetic groups are also organic cofactors but are tightly or covalently bound to the enzyme and remain associated with it throughout the reaction.

Transferases catalyze the transfer of functional groups from one molecule to another. Kinases, a type of transferase, transfer phosphate groups, while transaminases transfer amino groups.

Initially, as substrate concentration increases, the reaction rate increases because more enzyme active sites become occupied. However, as substrate concentration further increases, the enzyme becomes saturated, and all active sites are occupied. Beyond this point, increasing substrate concentration no longer significantly increases the reaction rate, and the rate plateaus at Vmax.

Lyases catalyze the cleavage of bonds by means other than hydrolysis or oxidation, often resulting in the formation of new double bonds or ring structures. Examples include decarboxylases and dehydratases.

Oxidoreductases are enzymes that catalyze oxidation-reduction reactions, involving the transfer of electrons or hydrogen atoms between molecules. Examples include dehydrogenases and oxidases.

A competitive inhibitor competes with the substrate for binding to the active site. It increases the apparent Km (because more substrate is needed to reach half Vmax) but does not change Vmax (because at sufficiently high substrate concentrations, the substrate can outcompete the inhibitor).

Enzymes are highly specific (acting on specific substrates and catalyzing specific reactions) and very efficient catalysts (accelerating reactions by many orders of magnitude). These properties make them highly valuable in industrial applications requiring precise and efficient chemical transformations, such as in food processing, pharmaceuticals, and biofuel production.

Enzymes control the rate of individual steps in metabolic pathways. By regulating enzyme activity (e.g., through allosteric regulation, covalent modification, or changes in enzyme concentration), cells can control the flux of metabolites through these pathways.

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

Isomerases catalyze intramolecular rearrangements, converting one isomer to another. They facilitate reactions like cis-trans isomerizations and epimerizations, essentially changing the form of a molecule without altering its molecular formula.

Enzymes accelerate reactions by lowering the activation energy, the energy barrier that must be overcome for a reaction to occur. They do not increase temperature or reactant concentration, nor are they consumed in the reaction; they are regenerated.

Cofactors (including coenzymes and metal ions) assist enzymes by providing functional groups, participating in catalysis, or stabilizing enzyme structure. Substrate specificity is primarily determined by the shape and chemical properties of the enzyme's active site, not by cofactors.

Non-competitive inhibitors bind to a site other than the active site, causing a conformational change in the enzyme that reduces its catalytic activity. This reduces Vmax (because even at high substrate concentrations, some enzyme molecules are inactive) but does not affect Km (because the inhibitor does not directly interfere with substrate binding to the active site of the enzyme molecules that are still active).