Chapter 3: Enzymes Student Notes - PDF

# Chapter 3: Energy, Chemical Reactions, and Cellular Respiration ## Enzymes ### Learning Objectives - Explain the structure and function of enzymes and why they are important to the human body. - Describe the effects of substrate concentration, temperature, and pH on reaction rates. - Compare a...

# Chapter 3: Energy, Chemical Reactions, and Cellular Respiration ## Enzymes ### Learning Objectives - Explain the structure and function of enzymes and why they are important to the human body. - Describe the effects of substrate concentration, temperature, and pH on reaction rates. - Compare and contrast competitive and noncompetitive inhibition. ## Lactose Intolerance - Caused by a deficiency in the enzyme lactase (or abnormal lactase) - Lactase is required to break bond in lactose into glucose and galactose monosaccharides. - Common symptoms: abdominal upset, nausea, diarrhea, bloating, gas. A diagram shows the breakdown of lactose by lactase into glucose and galactose. Under the diagram, a graphic shows the prevalence of lactase persistence and intolerance across the lifespan. - Treat with lactase enzymes, avoidance of milk, or drinking lactose-free milk. A box of Lactaid lactase enzyme supplement is shown. ## Case Study - 5 days old baby presents to pediatric clinic with excessive crying and failure to thrive. - Her parents say she has frequent watery stools and appears uncomfortable after feeding. - Her mother has been exclusively breastfeeding her. An image depicts a crying baby being held by a parent. - On physical exam baby is irritable. - Vital signs (temp, HR, RR) are normal. - Abdomen has mild distension. - Stool is watery with a sour smell and frequent (5-6 times a day). An image depicts a yellow baby diaper. ### Poop Color Guide for Breastfed Babies - Day 1-2: Meconium: dark green, black; sticky and tar-like; Within 24 hours of birth - Days 2-4: Transitional poop: dark green or brown; less sticky - Days 3-5: Breastfeeding poop: can vary from baby to baby and can be green or brown then yellow - Day 5: Breastfeeding poop: mustard yellow, loose and seedy A diagram shows the color of breastfed baby poop across the first 5 days of life. ## Case Study - Stool analysis reveals presence of undigested lactose. - Blood glucose levels did not rise after lactose ingestion. - Genetic testing positive for congenital lactase deficiency gene mutations. A diagram shows a 3D model of lactase enzyme. - Baby responded well to lactose free formula. - Abdominal discomfort and diarrhea resolved within a week. - Baby was thriving with normal growth patterns by end of the month. An image of a smiling baby is shown. ## Reaction Rates and Activation Energy - **Reaction rate**: Measure of how quickly a chemical reaction takes place. - Activation energy is a primary factor determining reaction rate. A diagram showing uncatalyzed and catalyzed reactions is shown. It shows the change in activation energy with the presence of a catalyst. ### Activation energy (Ea) - Energy required to break existing chemical bonds - Overcoming the activation energy... - In a lab, increasing temperature provides energy to break bonds - Significant temperature increase in a cell would denature proteins The diagram is used here again to point out the aspects of activation energy. ## Function of Enzymes - Biologically active catalysts that accelerate chemical reactions by decreasing activation energy. - **Uncatalyzed reaction** - no enzyme present. - **Catalyzed reaction** - enzyme present. - Increase rate of product formation. The previous diagram is used here again. - Sucrose has higher potential energy than total potential energy of products, glucose and fructose. - Activation energy barrier must be overcome to initiate reaction. - Presence of enzyme **lowers** Ea, speeds up reaction. ## Enzyme Structure - Range in size from small (60 amino acids) to large (2500 amino acids). - Unique 3-dimensional structure in protein chain called active site. - Temporarily forms enzyme-substrate complex. A diagram shows the structure of an enzyme with an active site and a substrate, and the formation of an enzyme-substrate complex. - The active site has **specificity**. - It permits only a single substrate to bind. - Helps to catalyze only one specific reaction. The diagram of an enzyme with an active site and substrate is used here again. ## Enzyme Location - Some remain within cells. - For example, DNA polymerase, which helps form new DNA. - Some become embedded in plasma membrane. - For example, lactase in walls of small intestine cells helps digest lactose. - Some are secreted from the cell. - For example, pancreatic amylase released from pancreas to participate in starch digestion. A diagram shows a cell with lactase enzyme, lactose substrate, and glucose and galactose products. ## Mechanisms of Enzyme Action A diagram shows three steps in enzyme action: - 1: Enzyme+substrate - 2: Enzyme-substrate complex - 3: Enzyme+product - 1. Substrate enters **active site**, forming enzyme-substrate complex. - 2. Enzyme changes shape slightly, resulting in even closer fit (induced fit model). - 3. Change in enzyme shape stresses chemical bonds, permitting new bonds to be formed. - 4. Products are released; enzyme may repeat process. A diagram shows lactose breakdown into glucose and galactose and glycogen synthesis from glucose. It describes the four steps of enzyme action for each process. ### Cofactors - Molecules or “helper” ions required to ensure that a reaction occurs. A diagram shows an enzyme, a cofactor, and a substrate. - **Nonprotein organic (coenzymes) or inorganic substance:** - Zinc ion (inorganic substances) required for carbonic anhydrase to function. - Vitamins or modified nucleotides serving as coenzymes. The diagram is used here again. ## Enzyme Classification and Naming Enzymes are organized into six major functional classes: 1. Oxidoreductases - redox reactions 2. Transferases - transfer atoms or molecules between chemical structures 3. Hydrolases - split chemical bonds using water 4. Isomerases - convert one isomer to another 5. Ligases - bond two molecules together 6. Lyases - split bonds without using water ### Enzyme names based on: - Name of substrate or product - Subclass - Suffix -ase ### Examples: - Pyruvate dehydrogenase transfers hydrogen from pyruvate. - DNA polymerase helps form DNA. - Lactase digests lactose ## Effect of Concentration - Increase in enzyme concentration. - Increase in substrate concentration. - Increases reaction rate only until point of saturation. - **Saturation** occurs when so much substance is present that all enzymes are engaged in reaction. A diagram showing an increase in reaction rate with an increase in substrate concentration until saturation is reached. ## Effect of Temperature - Human enzymes function best at **optimal temperature** (usually 40°C; 104°F). - Moderate fever - Results in more efficient enzyme activity. - Severe increases in temperature - Cause protein denaturation with loss of function. A diagram shows an increase in reaction rate as temperature increases, reaching an optimal temperature, followed by a decrease in reaction rate as temperature increases further. ## Effect of pH - Enzymes function best at **optimal pH** (pH of 6 and 8 for most enzymes). - Enzyme loss of shape, denaturation. - Optimal pH may differ. - For example, enzymes working in the lower pH of the stomach. A diagram shows an increase in reaction rate as pH increases, reaching an optimal pH, followed by a decrease in reaction rate as pH increases further. ## Controlling Enzymes - **Inhibitors** bind enzymes and turn them off. - Prevents overproduction of product. - Later release of inhibitor allows enzyme to function again. - Inhibitors can be **competitive** or **noncompetitive** A diagram shows the effects of an uninhibited reaction, a competitive inhibitor, and a noncompetitive inhibitor on reaction rates. ## Competitive Inhibition - Resembles substrate and **binds to active site** of enzyme. - Compete for occupation of active site. - With **more substrate**: Less likely competitive inhibitor will occupy active site. - With **less substrate**: More likely inhibitor will occupy active site. A diagram shows a competitive inhibitor interfering with the active site of an enzyme, blocking the binding of the substrate. ## Noncompetitive Inhibition - Do not resemble substrate. - Bind a site other than active site (**allosteric site**). - Induce conformational change to enzyme and active site. - Also called **allosteric inhibitors**. - Not influenced by concentration of substrate. A diagram shows an allosteric inhibitor binding at a site other than the active site of an enzyme, changing the shape of the active site and blocking the substrate binding. ## Metabolic Pathways - **Metabolism**: All the chemical process occurring inside the body. - Series of enzymes. - Product of one enzyme becomes substrate of the next. - Ex: glucose breakdown. - Regulated through negative feedback. - Product acts as an allosteric inhibitor. A diagram shows a simplified metabolic pathway. A more detailed diagram showing the steps of glycolysis and gluconeogenesis is shown. ## Metabolic Pathways - Substrate - Enzyme - Enzyme cannot bind substrate - Metabolic pathway - Original form - End product - Acts as inhibitor A diagram shows an overview of a metabolic pathway, including its regulation.