Chapter 20 Enzymes and Vitamins Describe how enzymes function as catalysts; name and classify them.

Enzymes are Biological Catalysts Enzymes are proteins that catalyze nearly all the chemical reactions taking place in the cells of the body increase the rate of reaction by lowering the energy of activation

Names of Enzymes The name of an enzyme usually ends in ase 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)

Classification of Enzymes Enzymes are classified by the types of reaction they catalyze. Class Type of Reactions catalyzed Oxidoreductases Oxidation–reduction Transferases Transfer groups of atoms Hydrolases Hydrolysis Lyases Add atoms/remove atoms to or from a double bond Isomerases Rearrange atoms Ligases Use ATP to combine small molecules

Oxidoreductases and Transferases

Hydrolases and Lyases

Isomerases and Ligases

Learning Check Match the type of reaction with an enzyme 1) aminase 2) dehydrogenase 3) isomerase 4) synthetase A. converts a cis-fatty acid to a trans-fatty acid B. removes 2H atoms to form a double bond C. combines two molecules to make a new compound D. adds NH3

Solution Match the type of reaction with an enzyme 1) aminase 2) dehydrogenase 3) isomerase 4) synthetase 3 A. converts a cis-fatty acid to a trans-fatty acid 2 B. removes 2H atoms to form a double bond 4 C. combines two molecules to make a new compound 1 D. adds NH3

Chapter 20 Enzymes and Vitamins 20.2 Enzyme Action Describe the role of an enzyme in an enzyme–catalyzed reaction.

Active Site The active site How enzyme works p 706 The active site is a region within an enzyme that fits the shape of the reacting molecule, called a substrate contains amino acid R groups that bind the substrate releases products when the reaction is complete

Enzyme-Catalyzed Reaction The proper fit of a substrate (S) in an active site on an enzyme (E) forms an enzyme–substrate (ES) complex. E + S ES Within the ES complex, the reaction occurs to convert substrate to product (P). ES E + P The products, which are no longer attracted to the active site, are released. Overall, substrate is converted to product. E + S ES E + P

Enzyme-Catalyzed Reaction (continued) In an enzyme-catalyzed reaction, a substrate attaches to the active site an enzyme–substrate (ES) complex forms reaction occurs and products are released an enzyme (E) is used over and over E + S ES E + P

Enzyme Specificity Enzymes may recognize and catalyze a single substrate a group of similar substrates a particular type of bond

Lock-and-Key Model In the lock-and-key model of enzyme action, the active site has a rigid shape only substrates with the matching shape can fit the substrate is the key that fits that lock

Induced-Fit Model In the induced-fit model of enzyme action, enzyme structure is flexible, not rigid shapes of enzyme and substrate adjust for best fit at the active site to improve catalysis of reaction substrate specificity increases

Example of an Enzyme-Catalyzed Reaction

Learning Check A. The active site is 1) the entire enzyme 2) a section of the enzyme 3) the substrate B. In the induced-fit model, the shape of the enzyme when substrate binds 1) stays the same 2) adapts to the shape of the substrate

Solution A. The active site is 2) a section of the enzyme B. In the induced-fit model, the shape of the enzyme when substrate binds 2) adapts to the shape of the substrate

Isoenzymes as Diagnostic Tools H.N. P 709 Isoenzymes as Diagnostic Tools Isoenzymes catalyze the same reaction in different tissues in the body such as lactate dehydrogenase (LDH), which converts lactate to pyruvate, consist of five isoenzymes can be used to identify the organ or tissue involved in damage or disease such as LDH have one form more prevalent in heart muscle and another form in skeletal muscle and liver tissue

Isoenzymes (continued)

Diagnostic Enzymes Diagnostic enzymes determine the amount of damage in tissues that are elevated may indicate damage or disease in a particular organ

Diagnostic Enzymes (continued) Levels of the enzyme creatine kinase (CK), lactic dehydrogenase (LDH), and aspartate transaminase (AST) are elevated following a heart attack are used to determine the severity of the attack

Chapter 20 Enzymes and Vitamins 20.3 Factors Affecting Enzyme Activity Describe the effect of temperature, pH, concentration of enzyme, and concentration of substrate on enzyme activity.

Temperature and Enzyme Action Enzymes are most active at an optimum temperature (usually 37 °C in humans) show little activity at low temperatures lose activity at high temperatures as denaturation occurs

pH and Enzyme Action Enzymes are most active at optimum pH contain R groups of amino acids with proper charges at optimum pH lose activity in low or high pH as tertiary structure is disrupted

Optimum pH Values Enzymes in the body have an optimum pH of about 7.4 certain organs have enzymes that operate at lower and higher optimum pH values

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

Learning Check Sucrase has an optimum temperature of 37 °C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction. 1) no change 2) increase 3) decrease A. increasing the concentration of sucrase B. changing the pH to 4 C. running the reaction at 70 °C

Solution Sucrase has an optimum temperature of 37 °C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction 1) no change 2) increase 3) decrease 2 A. increasing the concentration of sucrase 3 B. changing the pH to 4 3 C. running the reaction at 70 °C

Chapter 20 Enzymes and Vitamins 20.4 Enzyme Inhibition Describe competitive and noncompetitive inhibition, and reversible and irreversible inhibition.

Enzyme Inhibitors Inhibitors are molecules that cause a loss of catalytic activity prevent substrates from fitting into the active sites E + S ES E + P E + I EI no P

Competitive Inhibitor A competitive inhibitor has a structure that is similar to that of the substrate competes with the substrate for the active site has its effect reversed by increasing substrate concentration

Noncompetitive Inhibitor A noncompetitive inhibitor has a structure that is much different than the substrate distorts the shape of the enzyme, which alters the shape of the active site prevents the binding of the substrate cannot have its effect reversed by adding more substrate

Malonate and Succinate Dehydrogenase is a competitive inhibitor of succinate dehydrogenase has a structure that is similar to succinate inhibition is reversed by adding succinate

Irreversible Inhibition In irreversible inhibition, an inhibitor bonds with R groups at the active site destroys enzyme activity Enzyme Inhibition p 714

Irreversible Enzyme Inhibitors

Learning Check Identify each description as an inhibitor that is 1) competitive or 2) noncompetitive. A. increasing substrate reverses inhibition B. binds to enzyme surface but not to the active site C. structure is similar to substrate D. inhibition is not reversed by adding more substrate

Solution Identify each description as an inhibitor that is 1) competitive or 2) noncompetitive. 1 A. increasing substrate reverses inhibition 2 B. binds to enzyme surface but not to the active site 1 C. structure is similar to substrate 2 D. inhibition is not reversed by adding more substrate

Chapter 20 Enzymes and Vitamins 20.5 Regulation of Enzyme Activity Describe the role of zymogens, feedback control, and allosteric enzymes in regulating enzyme activity.

Zymogens Zymogens (proenzymes) are inactive forms of enzymes are activated when one or more peptides are removed Example: The zymogen proinsulin is converted to its active form, insulin, by removing a small peptide chain.

Digestive Enzymes Digestive enzymes are produced as zymogens in one organ and transported to another, such as the pancreas, when needed activated by removing small peptide sections

Digestive Enzymes (continued) Zymogen Active Enzyme (from pancreas) (in small intestine) Enteropeptidase Trypsinogen trypsin + peptide Trypsin Chymotrypsinogen chymotrypsin + 2 dipeptides Procarboxypeptidase carboxypeptidase + peptide

Summary of Digestive Enzymes

Allosteric Enzymes An allosteric enzyme is an enzyme in a reaction sequence that binds a regulator substance a positive regulator when it enhances the binding of substrate and accelerates the rate of reaction a negative regulator when it prevents the binding of the substrate to the active site and slows down the rate of reaction

Feedback Control In feedback control, a product acts as a regulator an end product binds with the first enzyme (E1) in a sequence when sufficient product is present so shuts down the reaction

Feedback Control by Threonine Deaminase When isoleucine accumulates in a cell, it binds to threonine deaminase, the first enzyme in the pathway from threonine to isoleucine a change in the shape of threonine deaminase blocks the binding of threonine feedback control decreases the synthesis of isoleucine Enzyme Inhibition p 714

Learning Check Identify each statement as: zymogen (Z) allosteric enzyme (A) positive regulator (PR) feedback control (FC) A. an enzyme in a pathway that controls the rate of the reaction B. speeds up a reaction by combining with an enzyme in the pathway C. removal of a peptide activates the enzyme D. some product binds to the first enzyme to limit the synthesis of product

Solution Identify each statement as: zymogen (Z) allosteric enzyme (A) positive regulator (PR) feedback control (FC) A A. an enzyme in a pathway that controls the rate of the reaction PR B. speeds up a reaction by combining with an enzyme in the pathway Z C. removal of a peptide activates the enzyme FC D. some product binds to the first enzyme to limit the synthesis of product

Chapter 20 Enzymes and Vitamins 20.6 Enzyme Cofactors and Vitamins Describe the types of cofactors found in enzymes.

Enzyme Cofactors A simple enzyme is an active enzyme that consists only of protein. Many enzymes are active only when they combine with cofactors such as metal ions or small molecules. A coenzyme is a cofactor that is a small organic molecule such as a vitamin.

Enzyme Cofactors (continued)

Function of Coenzymes A coenzyme prepares the active site for catalytic activity.

Metal Ions as Cofactors Many active enzymes require a metal ion. For example, Zn2+, a cofactor for carboxypeptidase, stabilizes the carbonyl oxygen during the hydrolysis of a peptide bond.

Some Enzymes and Their Cofactors

Learning Check Identify each enzyme as 1) a simple enzyme 2) an enzyme that required a cofactor A. requires Mg2+ for hydrolysis of phosphate esters B. requires vitamin B3 to transfer an acetyl group C. is active with four polypeptide subunits

Solution Identify each enzyme as 1) a simple enzyme 2) an enzyme that required a cofactor 2 A. requires Mg2+ for hydrolysis of phosphate esters 2 B. requires vitamin B3 to transfer an acetyl group 1 C. is active with four polypeptide subunits

Water-Soluble Vitamins Water-soluble vitamins are soluble in aqueous solutions cofactors for many enzymes not stored in the body

Descriptions of Water-Soluble Enzymes

Fat-Soluble Vitamins Fat-soluble vitamins are vitamins A, D, E, and K soluble in lipids but not in aqueous solutions stored in the body important in vision, bone formation, antioxidants, and blood clotting

Learning Check Identify each of the following as a water-soluble vitamin (WS) or fat-soluble vitamin (FS). A. folic acid B. retinol (vitamin A) C. vitamin C D. vitamin E E. niacin

Solution Identify each of the following as a water-soluble vitamin (WS) or fat-soluble vitamin (FS) WS A. folic acid FS B. retinol (vitamin A) WS C. vitamin C FS D. vitamin E WS E. niacin

Learning Check Identify the vitamin associated with each 1) riboflavin (B2) 2) vitamin A 3) vitamin K 4) vitamin D 5) ascorbic acid A. collagen formation B. part of the coenzymes FAD and FMN C. absorption of phosphorus and calcium in bone D. vision E. blood clotting

Solution Identify the vitamin associated with each 1) riboflavin (B2) 2) vitamin A 3) vitamin K 4) vitamin D 5) ascorbic acid 5 A. collagen formation 1 B. part of the coenzymes FAD and FMN 4 C. absorption of phosphorus and calcium in bone 2 D. vision 3 E. blood clotting