Chapter 22 Metabolic Pathways for Carbohydrates PDF

Chapter Twenty Two Metabolic Pathways for Carbohydrates HW(no credit): 1-7, 15, 16, 20-31, 33, 35, 37-44, 47, 49, 51-78, 85, 89, 125, 126, 127, 129, 131, 133, 135  Metabolism Metabolism refers to all chemical reactions that provide ener...

Chapter Twenty Two Metabolic Pathways for Carbohydrates HW(no credit): 1-7, 15, 16, 20-31, 33, 35, 37-44, 47, 49, 51-78, 85, 89, 125, 126, 127, 129, 131, 133, 135  Metabolism Metabolism refers to all chemical reactions that provide energy and substances required for continued cell growth. Two Types of Metabolic Reactions: 1. catabolic reactions – complex molecules are broken down to smaller molecules with release of energy. 2. anabolic reactions - use of energy to build larger molecules.  Stages of Catabolism Catabolic reactions are organized in stages as shown below. Stage 1: Digestion and hydrolysis break down large polymers to monomers that enter the bloodstream. Stage 2: Cellular degradation breaks down each set of molecules differently into two- and three-carbon compounds. Stage 3: Oxidation of ALL small molecules in the citric acid cycle and electron transport provides ATP energy. Stages of Catabolism: From Digestion to Cell ATP, Adenosine Triphosphate: Each Cell Makes The ATP molecule, composed of the base adenine, a ribose sugar, and three phosphate groups, hydrolyzes to form ADP and AMP along with a release of energy. The more phosphates, the more stored energy ATP Drives Reactions  Metabolic Reactions A coenzyme that gains hydrogen ions and electrons is reduced (and has energy stored in H or H-), whereas a coenzyme that loses hydrogen ions and electrons to a substrate is oxidized.  Structure of Coenzyme NAD+ NAD+, nicotinamide adenine dinucleotide, ► Has ADP attached to a derivative of coenzyme niacin (vitamin B3) ► Collects an H+ and 2 electrons- Coenzyme NAD+ or NADP+  Coenzyme FAD FAD, flavin adenine dinucleotide, ► contains ADP and riboflavin (vitamin B2). ► Collects 2H+ atoms (+ 2 e−), reducing it to FADH2.  Structure of Coenzyme A Coenzyme A (CoA) is made up of several components: pantothenic acid (vitamin B5), phosphorylated ADP, and aminoethanethiol.  Function, Coenzyme A Important functions of coenzyme A include ►preparation of small acyl groups such as acetyl for reactions with enzymes. ►production of the energy-rich thioester acetyl CoA.  Digestion of Carbohydrates ► Stage 1 of catabolism is digestion, the breakdown of food into small molecules. ► In mouth: physical grinding & mixing of food ► In mouth: chemical enzyme-catalyzed hydrolysis of carbohydrates ► α-amylase in saliva catalyzes hydrolysis of the glycosidic bonds in carbohydrates. ► Salivary α-amylase continues to act on polysaccharides in the stomach until it is denatured by stomach acid. ► Digestion: In Small Intestine, a second α-amylase is secreted by the pancreas and finishes conversion of polysaccharides to glucose. Sucrase, Maltase, and lactase (if present) breaks disaccharides ► Absorption: monosaccharides transported across the intestinal wall into the bloodstream  Digestion of Carbohydrates  Lactose Intolerance ► Lactase can stop being produced after childhood ► Lactose can’t be digested in the small intestine. ► Lactose in the large intestine, can be used by bacteria to form CO2, H2, & CH4 causing cramps and diarrhea. ► Lactose free milk products predigested by lactase ► Eat lactase enzyme with lactose meals  In The Cell Stage 2: Glycolysis: Splitting Glucose (& Oxidation) ► Glucose from bloodstream enters our cells, where it undergoes degradation (Stage 2 of catabolism) in a pathway called glycolysis. ► 1 glucose (6C) is converted to 2 pyruvate (3C). ► ATP put into the system will get more back when finished ► NADH electron energy recieved ► Glycolysis takes place in the cytosol of the cell.  Glycolysis: Reaction 1 In reaction 1, phosphorylation, ►a phosphate group is transferred from ATP to glucose. ►glucose-6-phosphate and ADP are produced. ►the enzyme hexokinase catalyzes the reaction.  Glycolysis: Reaction 2 In reaction 2, isomerization, ►glucose-6-phosphate, the aldose from reaction 1, is converted to fructose-6- phosphate. ►the isomerization is catalyzed by the enzyme phosphoglucose isomerase.  Glycolysis: Reaction 3 In reaction 3, phosphorylation, ►hydrolysis of another ATP provides a second phosphate group. ►the phosphate group is transferred to fructose-6- phosphate, producing fructose-1,6-bisphosphate. ►a second kinase enzyme called phosphofructokinase catalyzes the reaction.  Glycolysis: Reaction 4 In reaction 4, cleavage, ►fructose-1,6-bisphosphate is split into two three-carbon phosphate isomers. ►the enzyme aldolase produces dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3- phosphate (G3P)  Glycolysis: Reaction 5 In reaction 5, isomerization, ►dihydroxyacetone phosphate undergoes isomerization catalyzed by triose phosphate isomerase. ►a 2nd molecule of glyceraldehyde-3- phosphate is produced ►all six carbon atoms from glucose are contained in 2 molecules of Glyceraldehyde-3-phosphate (G3P)  Glycolysis: Reaction 6 In reaction 6, oxidation and phosphorylation, ►the aldehyde group of each glyceraldehyde-3-phosphate is oxidized to a carboxyl group by glyceraldehyde 3-phosphate dehydrogenase. ►NAD+ is reduced to NADH and H+ (x2) ►a phosphate group is added forming 2 molecules of 1,3- bisphosphoglycerate.  Glycolysis: Reaction 7 In reaction 7, phosphate transfer, ►a phosphate group from each 1,3- bisphosphoglycerate is transferred to ADP by phosphoglycerate kinase. ►ATP is produced (x 2)!  Glycolysis: Reaction 8 In reaction 8, isomerization, ►3-phosphoglycerate undergoes isomerization by phosphoglycerate mutase. ►the phosphate group is moved from carbon 3 to carbon 2, yielding 2-phosphoglycerate  Glycolysis: Reaction 9 In reaction 9, dehydration, each phosphoglycerate molecule undergoes dehydration by the enzyme enolase. high-energy phosphoenolpyruvate produced  Glycolysis: Reaction 10 In reaction 10, phosphate transfer, ►phosphate group from phosphoenolpyruvate transferred by pyruvate kinase to ADP ►ATP produced (x 2)! ►a fourth kinase enzyme transfers a phosphate with ATP production  Glycolysis: Overall Reaction In glycolysis, ► two ATP add phosphate to glucose and fructose-6-phosphate. ► four ATP are formed in energy generation by direct transfers of phosphate groups to four ADP. ► there is a net gain of 2 ATP and 2 NADH  Fructose and Galactose Galactose and fructose form intermediates that enter the glycolysis pathway to be metabolized.  Regulation of Glycolysis ► In reaction 1, hexokinase ►inhibited by high levels of glucose-6-phosphate (which means pathway not needed) ► In reaction 3, phosphofructokinase ►inhibited by high levels of ATP ►activated by high levels of AMP ► In reaction 10, pyruvate kinase ►inhibited by high levels of ATP ►Inhibited by acetyl CoA (Citric Acid Cycle backing up)  Fates of Pyruvate: Fermentation or Oxidation  Pyruvate: Anaerobic Conditions Under anaerobic conditions (without oxygen): ►NADH concentration increases, NAD+ is in short supply, and glycolysis cannot continue. ►Pyruvate is reduced to lactate and NAD+ by lactate dehydrogenase solves this problem ►NAD+ is used to oxidize glyceraldehyde-3-phosphate in reaction 6 of glycolysis, producing a small amount of ATP. ►Using NADH energy, we can continue Glycolysis and make ATP ►Lactic acid will accumulate and cannot continue indefinitely  Pyruvate: Anaerobic Conditions ► Under anaerobic conditions, glycolysis has a net gain of only two ATP ► Same for other forms of fermentation  Pyruvate: Aerobic Conditions Under aerobic conditions (oxygen present), pyruvate ► moves from the cytosol into the mitochondria to be oxidized. ► is oxidized by pyruvate dehydrogenase to acetyl-CoA and CO2 as the coenzyme NAD+ is reduced to NADH energy Acetyl-CoA can enter the citric acid cycle. NADH will go to the electron transport chain to produce ATP  Gluconeogenesis ► Glucose is the primary energy source for the brain, nervous system, testes, embryonic tissues, and red blood cells. ► Critical during fasting and the early stages of starvation. Glycogen storage in liver is empty ► liver cells synthesize glucose by gluconeogenesis and release into blood stream for all cells ► Gluconeogenesis – is the pathway for making glucose from other metabolites—pyruvate, amino acids, and glycerol.  Glycolysis and Gluconeogenesis Bypassed Regulated Reactions of Glycolysis: 1.Reaction 1 2.Reaction 3 3.Reaction 10 Has 4 new enzymes to Regulate Gluconeogenesis (backwards, but also only in the liver)  Energy Cost of Gluconeogenesis ► The pathway consists of seven reversible reactions of glycolysis and four new reactions that replace the three irreversible reactions. ► Overall, glucose synthesis requires 4 ATP, 2 GTP, and 2 NADH ► This is a heavy cost to the liver, but it will be worth it for cells to have glucose for energy ► Not using fermentation, but the Citric Acid Cycle makes much more energy per glucose molecule  Regulation of Blood Glucose ► 2 hormones from the pancreas regulate blood glucose ► insulin, is released after meals when blood glucose concentrations rise to bring into cells ► glucagon, is released after fasting when blood glucose concentrations drop to signal to liver to release glucose  The Biochemistry of Extreme Activity ► In intense activity/weights, the energy comes from available ATP (depleted in seconds), creatine phosphate (depleted in seconds), and then glycolysis ► ADP + creatine phosphate ↔ATP + creatine ► During extreme muscle exertion, oxygen is used at max capacity ► Anaerobic glycolysis suffices until the lactate buildup causes muscle fatigue (cramps) The Biochemistry of Longer Exercise ► Long distance/time activity, oxygen supply to the muscle tissues must be maintained. ► As breathing and heart rate speed up, oxygen- carrying blood flows more quickly to the muscles. ATP is produced by aerobic glycolysis. ► Anaerobic threshold – oxygen is in short supply, ATP is supplied only by glycolysis, and lactate is recycled to pyruvate.  Lactate and the Cori Cycle ► is the flow of lactate from muscles to the liver ► Liver converts pyruvate to glucose, which is carried back to the muscles.