Enzymes PDF

## **Enzymes** ### **4. Structure of Enzymes** Enzymes are compounds that catalyze chemical reactions in living organisms, increasing the rate of substrate transformation into a product. Most enzymes are proteins (pure or heteroproteins). Ribosomes, which catalyze reactions using RNA, are an exc...

## **Enzymes** ### **4. Structure of Enzymes** Enzymes are compounds that catalyze chemical reactions in living organisms, increasing the rate of substrate transformation into a product. Most enzymes are proteins (pure or heteroproteins). Ribosomes, which catalyze reactions using RNA, are an exception. - **Pure protein enzymes**: are few, examples include chymotrypsin, lysozyme. - **Heteroprotein enzymes** consist of a protein part (**apoenzyme**) and a cofactor that can be: - **Coenzyme**: an organic non-protein molecule exhibiting weak, non-covalent bonds with the apoenzyme. Examples include FAD, FMN (derived from Vitamin B2), pyridoxal phosphate (derived from Vitamin B6). - **Prosthetic group**: an organic molecule exhibiting strong, covalent bonds with the apoenzyme. Examples include heme, oligosaccharide fragments, metal ions linked via ionic bonds. - **Monomeric enzymes:** contain a single binding site. - **Active site** is a region in the enzyme where a specific conformation of amino acids allows binding of the substrate and catalysis. - Two models explain substrate binding: - **Fischer model ("lock and key")**: A rigid model where the substrate fits perfectly into the active site. - **Koshland model ("induced fit")**: A flexible model where the active site changes its conformation to match the substrate. - **Oligomeric enzymes**: contain multiple binding sites, one per subunit. - **Allosteric enzymes** have a binding site for their substrate and an allosteric site for an effector molecule. - Effector molecules can increase (positive effector) or decrease (negative effector) the affinity of the substrate to the enzyme active site. - **Isoenzymes** are structural variants of the same enzyme, catalyzing the same reaction, but differing in structure, molecular weight, and substrate affinity. Examples include lactate dehydrogenase, creatine kinase. ### **2. Properties Common to Enzymes and Inorganic Catalysts** - Enzymes do not modify the equilibrium of the reaction. They only speed up reaching equilibrium. - Enzymes require low concentrations. - Enzymes are not consumed in the reaction. ### **2.2. Characteristics of Enzymes** **I.** Specificity **II.** Regulation of quantity and efficiency **I.2.2.1. Specificity** - **Reaction specificity:** The enzyme catalyzes only a specific type of reaction: transferase, oxidoreductase, hydrolase, lyase, isomerase, ligase. - **Substrate specificity:** - **Absolute:** The enzyme only acts on a single substrate (e.g., urease, acetylcholinesterase). - **Group:** The enzyme acts on a specific chemical class (e.g., alcohol dehydrogenase). - **Broad:** The enzyme acts on a specific chemical bond (e.g., proteases, glycosidases). - **Stereochemical:** The enzyme acts only on one enantiomer or geometric isomer (e.g., lactate dehydrogenase, fumarate hydratase). **I.2.2.2. Regulation** - **Quantity regulation:** - **Constitutive enzymes**: Their concentration remains constant since their synthesis and degradation rates are comparable. - **Inducible enzymes**: Their synthesis rate is higher than their degradation rate, often induced by substrate presence. - **Repressible enzymes**: Their degradation rate is higher than their synthesis rate, often repressed by the product. - Often, hormones can act as inducers or repressors (e.g., insulin induces glucokinase synthesis and represses phosphoenolpyruvate carboxylase synthesis). - **Activity regulation** - **Non-covalent regulation:** - **Substrate concentration:** Increasing the substrate concentration increases the enzyme activity, but only up to a certain point. - **Product inhibition:** The product can act as an inhibitor, reversing the reaction. - **Interconvertible enzymes**: Enzymes can exist in active and inactive forms, interconverted by covalent modifications (e.g., phosphorylation and dephosphorylation). - The enzyme can be activated by proteolytic cleavage of inactive proenzymes (e.g., digestive enzymes, coagulation factors). - **Allosteric regulation:** Occurs in oligomeric enzymes. - **Allosteric effectors** bind to the allosteric site, modifying the enzyme's conformation. - **Positive effectors** increase the affinity for the substrate, promoting binding. - **Negative effectors** decrease the affinity, hindering binding. ### **3. Enzyme Kinetics** - **Activity unit (UI):** The amount of enzyme that converts one micromole of substrate per minute under optimal conditions: pH, temperature. - **Enzyme kinetics:** The study of enzyme activity as a function of enzyme concentration, substrate concentration, pH, temperature, and inhibitors. **3.1. Temperature Influence** - **Optimum temperature:** The temperature at which the enzyme activity is maximal. For most enzymes, this is around 37°C. - **Q10:** The temperature coefficient quantifies the increase in reaction rate for every 10°C increase in temperature. - Extreme temperatures can denature the protein, decreasing its activity. **3.2. pH Influence** - **Optimum pH:** The pH at which the enzyme operates at maximal activity. Most enzymes have an optimal pH around 7. - **Extreme pH:** Can cause denaturation or alter the ionization state of crucial groups, affecting enzyme activity. **3.3. Enzyme Concentration Influence** - When substrate concentration is constant, the reaction rate is proportional to the enzyme concentration. **3.4. Substrate Concentration Influence** - **Michelian enzymes** (single binding site): The relationship between substrate concentration and reaction rate is described by a hyperbola. - At low substrate concentrations, the rate increases almost linearly with substrate concentration. - At high concentrations, the rate levels off, reaching the maximum rate (Vmax) due to enzyme saturation. - **Michaelis-Menten equation:** describes this relationship. - **Allosteric enzymes** (multiple binding sites): The plot of reaction rate versus substrate concentration has a sigmoidal shape. - The binding of substrate to one site facilitates subsequent binding to other sites, enhancing the enzyme's activity. - **KM:** Michaelis constant, reflecting the substrate concentration at which the reaction rate is half of Vmax. - **Low KM:** High affinity for the substrate. - **High KM:** Low affinity for the substrate. **3.4. Inhibitors** - **Inhibitors** are compounds that reduce enzyme activity. - **Types of inhibitions:** - **Irreversible inhibition:** The inhibitor binds covalently to the enzyme's active site, permanently inactivating the enzyme. - **Reversible inhibition:** The inhibitor binds non-covalently to the enzyme, allowing for its removal and the recovery of activity. - **Competitive inhibition:** The inhibitor competes with the substrate for binding to the active site. It increases the apparent KM but does not affect Vmax. - **Uncompetitive inhibition:** The inhibitor binds to the enzyme-substrate complex (ES), reducing the enzyme's activity, decreasing both KM and Vmax. - **Non-competitive inhibition:** The inhibitor binds to the enzyme at a site distinct from the active site. It does not affect KM but reduces Vmax. **.3.4.5. Serine Protease Inhibitors (Serpins):** - Serpins are proteins that inhibit serine proteases by binding to their active site, causing a conformational change that inactivates the enzyme. - Examples: antithrombin III (inhibits thrombin), a1-antitrypsin (inhibits trypsin and elastase), plasminogen activator inhibitor 1 (inhibits tissue plasminogen activator and urokinase). - Serpins have other functions besides inhibiting proteases, including: - Transporting hormones: thyroxine-binding globulin, cortisol-binding globulin. - Acting as chaperones: chaperone for collagen. ### **Enzyme Plasma** **1. Functional Plasma Enzymes** - These enzymes are synthesized by the liver and released into the blood for their function. - Examples: - LCAT (lecithin-cholesterol acyltransferase) - Pseudocholinesterase - Coagulation factors **2. Non-functional Plasma Enzymes** - These enzymes are not synthesized by the liver and are released into the plasma due to other processes. - **2.a. Exocrine Plasma** - Examples: amylase, lipase, alkaline phosphatase. - **2.b. Intracellular Plasma** - These enzymes are released in the bloodstream due to cell damage or injury. - Their levels in plasma reflect tissue injury and are used for diagnostic purposes. **Activity** is typically measured in *International Units (UI)*, which represents the amount of enzyme that converts one micromole of substrate per minute under optimal conditions (pH, temperature). **Factors influencing enzyme activity**: - **pH**: Each enzyme has an optimal pH. - **Temperature**: Optimal activity at a certain temperature. - **Substrate concentration**: Reaches a maximum at high concentrations. - **Inhibitors and activators**: Can speed up or slow down the reaction. ### **Examples of Enzymes** **1. Lactate dehydrogenase (LDH)** - Catalyzes the reversible reaction of pyruvate to lactate. - Exists as a tetramer with different subunits (H and M). - Five isoenzymes with varying electrophoretic mobility, tissue distribution, sensitivity to heat, and resistance to cold. - The five LDH isoenzymes are: - LDH1: found in heart, red blood cells, and kidneys. - LDH2: found in heart, red blood cells, and kidneys. - LDH3: found mainly in the lungs and skeletal muscle. - LDH4: found mainly in skeletal muscle and liver. - LDH5: concentrated in skeletal muscle and liver. - Clinical use: Increased LDH levels are indicators of myocardial infarction, liver disease, and anemia. **2. Creatine Kinase (CK/CPK)** - Catalyzes the reversible reaction of creatine to creatine phosphate. - Exists as a dimer with different subunits (M and B). - Three CK isoenzymes: - CK-MM: found primarily in skeletal muscle and heart. - CK-MB: found only in heart muscle. - CK-BB: found primarily in the brain and smooth muscle. - Clinical use: Increased CK-MB levels in blood are a strong indicator of myocardial infarction, especially in the first 24 hours of the event. **Zoom in on CK levels:** - Increased CK levels can also occur due to: - Skeletal muscle injury - Stroke - Myocardial infarction - Muscle dystrophy **Other enzymes:** - **Aspartate transaminase (AST)** - **Alanine transaminase (ALT)**. - These enzymes are present in the heart and liver and their elevated levels in blood indicate damage to these organs. *** Note: This is a summary created based on the image you provided and can't replace a comprehensive study guide.