Đề thi, bài tập trắc nghiệm online Hóa sinh enzyme – Đề 2

Catalysts, including enzymes, only affect the rate at which a reaction reaches equilibrium. They do not change the position of the equilibrium itself. The equilibrium constant is determined by the difference in Gibbs free energy between reactants and products, which is unaffected by the enzyme.

Coenzymes are organic non-protein molecules that are essential for the catalytic activity of many enzymes. They often participate in the reaction by transporting substrates or functional groups.

The Lineweaver-Burk plot is a double reciprocal plot of the Michaelis-Menten equation (1/V vs 1/[S]). The y-intercept of this plot is equal to 1/Vmax, and the x-intercept is equal to -1/Km. The slope is Km/Vmax.

Enzymes speed up reactions by lowering the activation energy (ΔG‡), which is the energy barrier that must be overcome for a reaction to occur. They do not change the overall Gibbs free energy change (ΔG) of the reaction or provide energy themselves; they simply make it easier for the reaction to proceed.

Hydrolases catalyze hydrolysis reactions, which involve the breaking of chemical bonds with the addition of water. Examples include peptidases (break peptide bonds), lipases (break ester bonds in lipids), and glycosidases (break glycosidic bonds in carbohydrates).

Amylase is widely used in the food industry, particularly in baking and brewing, to break down starch into simpler sugars. DNA polymerase is used in molecular biology, hemoglobin is an oxygen-carrying protein, and insulin is a hormone.

Km is defined as the substrate concentration at which the reaction velocity 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.

Competitive inhibitors resemble the substrate and compete for binding to the enzyme's active site. They do not prevent the substrate from binding altogether, but their presence reduces the enzyme's activity at a given substrate concentration.

While some enzymes function optimally at acidic or alkaline pHs (e.g., pepsin in the stomach, alkaline phosphatase), the majority of enzymes in the human body operate most efficiently around a neutral pH of 7, reflecting the physiological pH of most cells and body fluids.

Oxidoreductases catalyze oxidation-reduction reactions, which involve the transfer of electrons or hydrogen atoms from one molecule (the reductant or electron donor) to another (the oxidant or electron acceptor). Common examples include dehydrogenases and oxidases.

Competitive inhibition can be overcome by increasing substrate concentration because higher substrate concentration can outcompete the inhibitor for binding to the active site. In contrast, non-competitive and uncompetitive inhibition cannot be overcome by increasing substrate concentration because the inhibitor binds at a different site.

The 'induced fit' model proposes that the active site of the enzyme is not a rigid, pre-shaped pocket. Instead, it is more flexible and undergoes a conformational change upon substrate binding. This conformational change optimizes the interaction with the substrate, leading to a better fit and facilitating catalysis.

Allosteric regulation occurs when a regulator molecule binds to an enzyme at a site distinct from the active site (the allosteric site). This binding induces a conformational change in the enzyme, which can either increase or decrease its catalytic activity.

Enzymes act by providing an alternative reaction pathway with a lower activation energy. This lower energy barrier allows the reaction to proceed at a faster rate.

The active site of an enzyme has a unique three-dimensional shape that is complementary to the shape of its specific substrate. This 'lock and key' or 'induced fit' mechanism ensures specificity.

Non-competitive inhibitors bind to an enzyme at a site other than the active site, thereby reducing the concentration of active enzyme molecules. This primarily affects Vmax, decreasing the maximum rate the reaction can achieve. Km remains unchanged because the inhibitor does not interfere with substrate binding to the active site of the enzyme molecules that are still active.

The transition state is a transient, high-energy state in a reaction pathway where bonds are in the process of being broken and formed. Enzymes stabilize the transition state, reducing the activation energy required to reach it and thereby accelerating the reaction.

Feedback inhibition is a crucial regulatory mechanism. The end product of a metabolic pathway acts as an allosteric inhibitor of an enzyme, typically the first committed step enzyme, in the same pathway, thus preventing overproduction of the end product.

Enzymes are catalysts that accelerate the attainment of equilibrium but do not alter the equilibrium position. They lower the activation energy barrier for both the forward and reverse reactions to the same extent, thus speeding up both equally. This means the ratio of forward to reverse rate constants remains unchanged, and thus the equilibrium constant also remains unchanged.

Ligases (also known as synthetases) catalyze the joining of two large molecules by forming a new chemical bond, often coupled with the hydrolysis of ATP or a similar energy-rich molecule. A common example is DNA ligase, which joins DNA fragments.

Uncompetitive inhibitors bind specifically to the enzyme-substrate complex, but not to the free enzyme. This binding distorts the active site and prevents the reaction from proceeding effectively. Uncompetitive inhibition cannot be overcome by increasing substrate concentration.

Polymers are macromolecules, not a class of enzymes. The six major classes of enzymes are Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases.

Many enzymes, particularly digestive enzymes and enzymes involved in blood clotting, are synthesized as inactive precursors called zymogens or proenzymes. Proteolytic activation involves the cleavage of specific peptide bonds in the zymogen to convert it into its active enzymatic form. This is an irreversible activation mechanism.

The Enzyme Commission (EC) number classifies enzymes into six main classes (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases) based on the type of chemical reaction they catalyze. Each class is further divided into subclasses, sub-subclasses, and individual enzymes.

Isomerases catalyze intramolecular rearrangements, meaning they convert one isomer into another. They facilitate changes within a single molecule, such as converting glucose-6-phosphate to fructose-6-phosphate.

Enzyme activity typically increases with temperature as molecules have more kinetic energy and collisions are more frequent. However, beyond an optimal temperature, the enzyme starts to denature, leading to a sharp decrease in activity.

Transferases catalyze the transfer of functional groups (e.g., methyl, acyl, amino, phosphate groups) from one molecule (donor) to another (acceptor). Examples include kinases (transfer phosphate groups) and transaminases (transfer amino groups).

The color of the reaction mixture is generally irrelevant to enzyme activity. Factors that significantly affect enzyme reaction rates include enzyme concentration, substrate concentration, temperature, pH, and the presence of inhibitors or activators.

Lyases catalyze the breaking of chemical bonds by elimination, leaving double bonds, or conversely, adding groups to double bonds. They differ from hydrolases and oxidoreductases in their mechanism of action.

According to Michaelis-Menten kinetics, as substrate concentration increases, the reaction rate increases initially, eventually reaching a maximum velocity (Vmax). At Vmax, all enzyme active sites are saturated with substrate, and further increases in substrate concentration do not significantly increase the reaction rate.