Đề 9 – Bài tập, đề thi trắc nghiệm online Hóa sinh enzyme

Transferases catalyze the transfer of functional groups from one molecule (donor) to another (acceptor). Examples include kinases (transfer of phosphate groups) and transaminases (transfer of amino groups). Isomerases catalyze isomerization reactions, ligases join two molecules together, and hydrolases catalyze hydrolysis.

Enzyme activity generally increases with temperature because higher temperatures provide more kinetic energy, increasing the frequency of enzyme-substrate collisions. However, beyond an optimal temperature, the enzyme`s structure begins to denature due to disruption of non-covalent bonds, leading to a decrease in activity.

Oxidoreductases are a major class of enzymes that catalyze oxidation-reduction reactions, involving the transfer of electrons (or hydride ions) between molecules. Examples include dehydrogenases, oxidases, and reductases. Hydrolases catalyze hydrolysis reactions, transferases catalyze transfer of functional groups, and lyases catalyze bond cleavage by means other than hydrolysis or oxidation.

In uncompetitive inhibition, the inhibitor binds to the enzyme-substrate complex, preventing the complex from proceeding to form product. This effectively reduces both Km and Vmax. Km decreases because the inhibitor binding stabilizes the enzyme-substrate complex, increasing the apparent affinity. Vmax decreases because the inhibitor reduces the concentration of functional enzyme-substrate complex.

Salivary amylase, present in saliva, begins the digestion of carbohydrates in the mouth by breaking down starch into smaller sugars (maltose and dextrins). Pepsin and trypsin are proteases active in the stomach and small intestine, respectively, and lipase digests fats.

Ligases catalyze the formation of new bonds, usually requiring energy input in the form of ATP or another nucleoside triphosphate. These enzymes are involved in joining two large molecules by forming new chemical bonds. DNA ligase, for example, joins DNA fragments.

Some enzymes, particularly digestive enzymes and enzymes involved in blood clotting, are synthesized as inactive zymogens to prevent them from acting prematurely or in the wrong location. They are activated only when and where their activity is required, often by proteolytic cleavage. This allows for rapid and localized activation of enzyme function.

Enzyme denaturation is the process where an enzyme loses its native three-dimensional structure due to disruption of non-covalent bonds (hydrogen bonds, hydrophobic interactions, ionic bonds). This structural change usually leads to a loss of enzyme activity because the active site is disrupted and can no longer bind substrate effectively. Denaturation is caused by factors like heat, extreme pH, or certain chemicals.

In feedback inhibition, the end product of a metabolic pathway typically acts as an allosteric inhibitor of an enzyme early in the pathway, often the enzyme catalyzing the first committed step. This mechanism helps to regulate the pathway by preventing overproduction of the end product; when enough product is present, it signals to slow down its own production.

Proteases, such as papain (from papaya), bromelain (from pineapple), and ficin (from figs), are used as meat tenderizers. They break down tough muscle proteins (collagen and elastin), making the meat more tender. Amylase breaks down starch, lipase breaks down fats, and cellulase breaks down cellulose.

pH affects the ionization state of amino acid side chains, particularly those in the active site, which are crucial for substrate binding and catalysis. Changes in pH can alter the enzyme`s 3D structure, disrupt ionic and hydrogen bonds, and affect enzyme activity. Each enzyme has an optimal pH range where it functions most efficiently.

In competitive inhibition, the inhibitor competes with the substrate for binding to the active site. This increases the apparent Km (lower affinity), because a higher substrate concentration is needed to reach half-Vmax. However, at sufficiently high substrate concentrations, the inhibitor can be overcome, and Vmax remains unchanged.

Non-competitive inhibitors bind to a site on the enzyme other than the active site, altering the enzyme`s conformation and reducing its catalytic activity. This reduces Vmax because even at high substrate concentrations, the enzyme`s maximum rate is lowered. However, Km remains unchanged because the inhibitor does not affect substrate binding to the active site of the enzyme molecules that are not bound by the inhibitor.

Phosphorylation, the addition of a phosphate group, is a reversible covalent modification that can regulate enzyme activity. It can either activate or inhibit an enzyme, depending on the specific enzyme and the site of phosphorylation. For some enzymes, phosphorylation increases activity, while for others it decreases activity. This is a key mechanism for signal transduction and metabolic control.

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. Binding at the allosteric site induces conformational changes in the enzyme, affecting its active site and thus its activity. This is a key difference from competitive inhibition where the inhibitor binds at the active site.

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, meaning it has a higher catalytic rate (kcat) and/or a higher affinity for its substrate (lower Km). It reflects the overall ability of the enzyme to catalyze a reaction under physiological conditions.

The Michaelis-Menten constant (Km) is defined as the substrate concentration at which the reaction rate is half of the maximum velocity (Vmax). Km is an indicator of the affinity of an enzyme for its substrate; a lower Km indicates a higher affinity.

Metal ions, when required for enzyme activity, typically act as cofactors. They can participate in catalysis by serving as Lewis acids, stabilizing negative charges, or mediating redox reactions within the enzyme active site. They are essential non-protein components that assist enzymes in their catalytic function.

While the Lock-and-Key model was an early concept, the Induced-Fit model more accurately describes enzyme-substrate interaction. In this model, the enzyme`s active site and the substrate are not perfectly complementary initially, but the enzyme changes shape upon substrate binding to create a better fit, enhancing specificity and catalysis. Koshland model is another name for the induced-fit model.

Vmax, or maximum velocity, represents the highest rate at which an enzyme can catalyze a reaction. It is achieved when the enzyme is saturated with substrate, meaning all enzyme active sites are occupied by substrate molecules. Vmax is directly proportional to the enzyme concentration.

Enzymes are essential for life because they dramatically accelerate the rates of biochemical reactions that would otherwise occur too slowly to support life. They do not increase temperature or produce energy directly; their primary role is catalytic, making biological processes happen at physiologically relevant speeds.

When substrate concentration is below saturation levels and enzyme concentration is constant, increasing substrate concentration will generally increase the reaction rate. This is because there are more substrate molecules available to bind to enzyme active sites, leading to more enzyme-substrate complex formation and product generation.

Hydrolases catalyze the cleavage of bonds by the addition of water (hydrolysis). They commonly break various bonds such as peptide bonds (in proteins by proteases), ester bonds (in lipids by lipases), and glycosidic bonds (in carbohydrates by glycosidases). Peptide bonds are specifically broken in proteins during digestion or protein turnover.

Creatine kinase (CK), specifically the CK-MB isoenzyme, is released into the bloodstream from damaged heart muscle cells following a myocardial infarction. Elevated CK-MB levels are a key diagnostic indicator of heart attack. ALT is more indicative of liver damage, amylase of pancreatic issues, and alkaline phosphatase of liver or bone disorders.

The active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. It is specifically shaped to fit the substrate(s) and contains amino acid residues that facilitate catalysis.

Lyases catalyze bond cleavage through mechanisms other than hydrolysis or oxidation, often forming new double bonds or rings, or conversely, cleaving double bonds or rings. Examples include decarboxylases and synthases (in some classifications, synthases are lyases). Isomerases catalyze rearrangements, ligases join molecules, and oxidoreductases involve electron transfer.

The transition state is a transient, high-energy, unstable intermediate state in a chemical reaction. Enzymes stabilize the transition state, thereby lowering the activation energy and accelerating the reaction. It`s not the substrate, product, or a stable complex, but a fleeting, energetically unfavorable configuration that enzymes help to achieve more readily.

Isomerases catalyze the intramolecular rearrangement of atoms, converting one isomer to another. This involves changing the spatial arrangement of atoms within a single molecule without changing its molecular formula. For example, phosphohexose isomerase converts glucose-6-phosphate to fructose-6-phosphate.

Cofactors can be inorganic ions or organic molecules (coenzymes). Magnesium ion (Mg²⁺) is an example of an inorganic cofactor that is essential for the activity of many enzymes. Glucose is a substrate, ribonuclease is an enzyme, and DNA is a nucleic acid.

Enzymes work by lowering the activation energy (Ea) of a reaction, which is the energy barrier that must be overcome for a reaction to occur. By lowering Ea, enzymes make it easier for reactions to proceed at a faster rate.