Chapter 20 Enzymes and Vitamins PDF

General, Organic, and Biological Chemistry: Structures of Life Chapter 20 Enzymes and Vitamins Enzymes and Enzyme Action Enzymes are biological catalysts that increase the rate of a reaction are not chemically changed in the reaction allow fast conversion of r...

General, Organic, and Biological Chemistry: Structures of Life Chapter 20 Enzymes and Vitamins Enzymes and Enzyme Action Enzymes are biological catalysts that increase the rate of a reaction are not chemically changed in the reaction allow fast conversion of reactants to products  Enzymes and Active Sites Active site is where the substrate fits and reactions occur The substrate, lactose (gray), is held in place in by hydrogen bonds with amino acid R groups of the enzyme lactase Enzyme–Substrate Complex The active site in lactase is flexible to fit the substrate and complete the reaction Once the disaccharide is hydrolyzed, the monosaccharide products are released from the enzyme, which is ready to bind another lactose. Models of Enzyme Action A lock-and-key model is great for the specificity of one enzyme working with one substrate The induced-fit model is a more dynamic model of with a flexible active site to adapt to the shape of the substrate and help make the products. The induced-fit model has the substrate and enzyme working together to acquire a geometrical arrangement that lowers the activation energy of the reaction Learning Check 1. Which is the active site? A. the entire enzyme B. a section of the enzyme C. the substrate 2. In the induced-fit model, what happens to the shape of the enzyme when the substrate binds? A. stays the same B. adapts to the shape of the substrate Names of Enzymes By convention end in ase (more historic ones do not) identifies the reacting substance; for example, sucrase catalyzes the reaction of sucrose. describes the function of the enzyme; for example, oxidases catalyze oxidation. can be a common name, particularly for the digestive enzymes, such as pepsin and trypsin. Factors Affecting Enzyme Activity Enzyme activity describes how fast it catalyzes the reaction -strongly affected by reaction conditions, such as temperature. pH. concentration of the enzyme and substrate. Enzyme Activity and Temperature Enzymes are most active at an optimum temperature show little activity at low temperatures. lose activity at high temperatures as denaturation occurs. Enzyme Activity and pH Enzymes Have an optimum p H, where tertiary structure of the protein is maintained pH will vary amino acid R groups and charges-less effective low or high p H loss of function as tertiary structure is disrupted Optimum pH Values Enzymes in certain organs operate at lower or higher optimum pH values. Table 20.3 Optimum pH for Selected Enzymes Enzyme Location Substrate Optimum pH Pepsin Stomach Peptide bonds 1.5–2.0 Lactase GI tract Lactose 6.0 Sucrase Small intestine Sucrose 6.2 Amylase Pancreas Amylose 6.7–7.0 Urease Liver Urea 7.0 Trypsin Small intestine Peptide bonds 7.7–8.0 Lipase Pancreas Lipid (ester bonds) 8.0 Arginase Liver Arginine 9.7 Enzyme Concentration An increase in enzyme concentration increases the rate of reaction (at constant substrate concentration). binds more substrate with enzyme. Substrate Concentration An increase in substrate concentration increases the rate of reaction (at constant enzyme concentration) eventually saturates an enzyme with substrate to give maximum activity Feedback Control: Metabolism when Needed the end product of a series of reactions acts as a negative regulator on 1st enzyme to stop activity the subsequent reaction sequence stops until end product is used up and no longer binds to 1st enzyme so pathway goes again Zymogens: Proenzymes Digestive enzymes; protein hormones, such as insulin; and blood clotting enzymes. Larger, inactive proteins are produced now, and cleaved to smaller active proteins when needed Table 20.4 Examples of Zymogens and Their Active Forms Zymogen (Inactive Enzyme) Produced in Activated in Enzyme (Active) Chymotrypsinogen Pancreas Small intestine Chymotrypsin Pepsinogen Gastric chief cells Stomach Pepsin Trypsinogen Pancreas Small intestine Trypsin Fibrinogen Liver Damaged tissues Fibrin Prothrombin Liver Damaged tissues Thrombin Proinsulin Pancreas Pancreas Insulin Enzyme Activity Regulation Enzymes can be turned on or off using a phosphate group a) A kinase activates an inactive b) A phosphatase activates an inactive enzyme by phosphorylation. enzyme by removal of a phosphate. Enzyme Cofactors A simple enzyme is Table 20.9 Enzymes and the Metal Ions Required as Cofactors active with only a protein Metal Ion Enzymes Requiring Metal Ion Many need cofactors C u super 2 plus, C u super plus Cofactors Cytochrome oxidase such as metal ions or F e super 2 plus, F e super 3 plus Catalase Cytochrome oxidase small molecules present Z n super 2 plus Alcohol dehydrogenase Carbonic anhydrase Carboxypeptidase A A coenzyme is a M g super 2 plus Glucose-6-phosphatase Hexokinase cofactor that is a small M n super 2 plus Arginase organic molecule such N i super 2 plus Urease as a vitamin Function of Cofactors A cofactor is required so that many enzymes can become active. Vitamins and Coenzymes Vitamins are organic molecules that are essential for normal health and growth. are required in trace amounts. need to be obtained from the diet. grouped into water-soluble vitamins and fat-soluble vitamins. Water-Soluble Vitamins (Bs, C, H) Must be replenished frequently Are not stored; excess excreted in urine are cofactors for many enzymes. Can be destroyed by boiling, less heat better for preservation Fat-Soluble Vitamins include A, D, E, and K and are not involved as coenzymes in catalytic reactions. are stored in the body are important in vision, bone formation, antioxidants, and blood clotting.