Đề 7 – Bài tập, đề thi trắc nghiệm online Hóa sinh enzyme

Isoenzymes allow for fine-tuning of metabolic pathways in different tissues or cellular locations. Their varying properties (e.g., Km, Vmax, regulatory sensitivity) enable specific metabolic adaptations in different contexts.

The active site is a specific three-dimensional pocket or cleft on the enzyme where substrates bind and catalysis occurs. It is formed by specific amino acid residues that create a unique microenvironment conducive to substrate binding and reaction.

The three-dimensional structure of an enzyme, determined by its amino acid sequence and folding, is critical for its function. The active site`s shape and chemical environment are precisely arranged to bind specific substrates and catalyze reactions efficiently. Changes in this structure can lead to loss of function.

Allosteric enzymes often display sigmoidal kinetics due to cooperativity between subunits. Their activity is modulated by allosteric effectors binding at regulatory sites, leading to conformational changes that alter enzyme activity.

Competitive inhibitors compete with the substrate for the active site. They increase the apparent Km (lower affinity) because more substrate is needed to reach half-maximal velocity, but Vmax remains unchanged as inhibition can be overcome at high substrate concentrations.

In feedback inhibition, the end product of a metabolic pathway inhibits an enzyme earlier in the pathway. This is a form of negative regulation, preventing overproduction of the end product.

Coenzymes are organic, non-protein molecules (often vitamin-derived) that assist enzymes, usually by carrying chemical groups. Cofactors can be either inorganic ions or organic molecules (including coenzymes) and are required for enzyme activity.

Uncompetitive inhibitors bind specifically to the enzyme-substrate complex, after the substrate has already bound to the enzyme. This type of inhibition reduces both Vmax and Km in parallel.

Reversible covalent modification is a regulatory mechanism where enzyme activity is altered by the covalent attachment and removal of chemical groups, such as phosphate groups. Phosphorylation (addition of phosphate) and dephosphorylation (removal of phosphate) are common examples.

The plateau at Vmax occurs because, at high substrate concentrations, all available enzyme active sites are occupied by substrate molecules. Increasing substrate concentration further cannot increase the reaction rate beyond this point.

Km is the Michaelis constant, representing 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.

Vmax (maximum velocity) is the theoretical maximum rate of an enzyme-catalyzed reaction. It is achieved when the enzyme is fully saturated with substrate, and all active sites are occupied, representing the enzyme`s maximal turnover capacity.

The six major classes of enzymes are Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases. Polymerases, while enzymes, are a subclass within Transferases (nucleotidyltransferases) and not a major class on their own in the primary classification.

kcat (turnover number) represents the maximum number of substrate molecules converted to product per enzyme molecule per unit time. Km reflects substrate affinity. A high kcat/Km ratio indicates both high catalytic rate and high substrate affinity, thus high catalytic efficiency.

Drastic changes in pH, especially moving away from the optimal pH, can disrupt the ionic bonds and hydrogen bonds that maintain the enzyme`s tertiary structure. This denaturation leads to a loss of the enzyme`s functional shape and thus, activity.

Competitive inhibitors bind to the active site, competing with the substrate. By increasing substrate concentration, the substrate can outcompete the inhibitor for binding to the active site, effectively overcoming the inhibition and reaching Vmax.

In a Lineweaver-Burk plot (double reciprocal plot), the y-intercept corresponds to 1/Vmax. The x-intercept corresponds to -1/Km, and the slope is Km/Vmax. This plot is used to linearize Michaelis-Menten kinetics for easier determination of Km and Vmax.

Enzyme concentration, substrate concentration, and the presence of inhibitors all directly affect the rate of enzyme-catalyzed reactions. The volume of the reaction mixture, assuming concentrations are kept constant, does not directly affect the rate per unit volume, although it might affect the total amount of product formed over time.

At high temperatures, the increased kinetic energy can disrupt the weak bonds (hydrogen bonds, hydrophobic interactions) that maintain the enzyme`s tertiary and quaternary structure, leading to denaturation and loss of activity.

A prosthetic group is a non-protein, organic molecule that is tightly or even covalently bound to an enzyme and is crucial for its function. It is a type of cofactor. Examples include heme in hemoglobin and cytochromes.

An apoenzyme is an enzyme that is inactive because it is missing its required cofactor or coenzyme. When the cofactor is bound, the enzyme becomes a holoenzyme and is catalytically active.

Transferases are enzymes that catalyze the transfer of functional groups, such as phosphate groups, from one molecule to another. Kinases, which transfer phosphate groups, are a subclass of transferases.

Synthesizing enzymes as zymogens prevents them from being active in the cells where they are produced, which could be harmful. Activation usually occurs at the site of action or in response to a specific signal, ensuring controlled enzyme activity.

Enzymes primarily function by lowering the activation energy of reactions. This reduction in energy barrier accelerates the rate at which reactions proceed, without altering the equilibrium.

The induced-fit model describes the dynamic interaction between enzyme and substrate, where the active site of the enzyme changes shape to optimally bind the substrate, enhancing catalysis.

Enzymes are catalysts, which means they are not consumed in the reaction and are regenerated. They speed up reactions by lowering activation energy but do not change the reaction equilibrium or Gibbs free energy change.

The transition state is a fleeting, high-energy state during a reaction where bonds are being broken and formed. Enzymes stabilize the transition state, thereby lowering the activation energy required to reach it and accelerating the reaction.

Metal ions can play various roles in enzyme catalysis, including structural stabilization, redox reactions, and substrate binding. However, they do not provide energy for reactions; enzymes lower activation energy, but the energy source is inherent in the reactants.

Enzymes are catalysts; they speed up both the forward and reverse reactions equally. They do not change the position of the equilibrium or the equilibrium constant, but only accelerate the rate at which equilibrium is attained.

According to the Michaelis-Menten equation, v = (Vmax * [S]) / (Km + [S]). When [S] = Km, the equation becomes v = (Vmax * Km) / (Km + Km) = (Vmax * Km) / (2Km) = Vmax / 2. Thus, the reaction rate is half of Vmax.