Lecture (5) PDF - Energy-Yielding Metabolism

1 Shady ElShehawy ,2023 - [email protected] 10/30/2023  Dietetic Program Nutrition & Metabolism (1) (Fdt309) Lecture (5) Energy-yielding metabolism Professor...

1 Shady ElShehawy ,2023 - [email protected] 10/30/2023  Dietetic Program Nutrition & Metabolism (1) (Fdt309) Lecture (5) Energy-yielding metabolism Professor Shady M. ElShehawy Professor of Food Science & Technology Food Industries Department, Faculty of Agriculture Mansoura University, Egypt 10/30/2023 Shady ElShehawy ,2023 - [email protected] 2  Topics  Glycolysis: Oxidation of Glucose:  Energy-Investing Reactions 1 to 5.  Energy-Generating Reactions 6 to 10.  Pathways for Pyruvate:  Aerobic Conditions.  Anaerobic Conditions. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 3  Glycolysis: Oxidation of Glucose The major source of energy for the body is the glucose produced when we digest the carbohydrates in our food, or from glycogen, a polysaccharide stored in the liver and skeletal muscle. Glucose in the bloodstream enters our cells, where it undergoes degradation in a pathway called glycolysis. Early organisms used glycolysis to release energy from simple nutrients long before there was any oxygen in Earth’s atmosphere. Glycolysis is an anaerobic process; no oxygen is required. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 4  Glycolysis: Oxidation of Glucose In glycolysis, a six-carbon glucose molecule is broken down to two molecules of three-carbon pyruvate. All the reactions in glycolysis take place in the cytoplasm of the cell. In the first five reactions, called the energy-investing phase, energy from the hydrolysis of two ATP is used to form two three-carbon, energy-rich phosphate compounds. In the last five reactions, called the energy-generating phase, energy from the hydrolysis of the energy-rich phosphate compounds is used to synthesize four ATP. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 5  Energy-Investing Reactions 1 to 5 Reaction 1 Phosphorylation: In the initial reaction, a phosphate group from ATP is added to glucose to form glucose- 6-phosphate and ADP. Reaction 2 Isomerization: The glucose-6-phosphate, the aldose from reaction 1, undergoes isomerization to fructose- 6- phosphate, which is a ketose. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 6  Energy-Investing Reactions 1 to 5 Reaction 3 Phosphorylation: The hydrolysis of another ATP provides a second phosphate group, which converts fructose- 6- phosphate to fructose-1,6-bisphosphate. The word bisphosphate is used to show that the two phosphate groups are on different carbons in fructose and not connected to each other. Reaction 4 Cleavage: Fructose-1,6-bisphosphate is split into two three- carbon phosphate isomers: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 7  Energy-Investing Reactions 1 to 5 Reaction 5 Isomerization: Because dihydroxyacetone phosphate is a ketone, it cannot undergo further oxidation. However, it undergoes isomerization to provide a second molecule of glyceraldehyde- 3-phosphate, which can be oxidized. Now all six carbon atoms from glucose are contained in two identical triose phosphates. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 8  Energy-Generating Reactions 6 to 10 In our discussion of glycolysis from this point, the two molecules of glyceraldehyde-3- phosphate produced in step 5 are undergoing the same reactions. For simplicity, we show the structures and reactions for only one three-carbon molecule for reactions 6 to 10. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 9  Energy-Generating Reactions 6 to 10 Reaction 6 Oxidation and Phosphorylation: The aldehyde group of each glyceraldehyde-3-phosphate is oxidized to a carboxyl group, while the coenzyme NAD+ is reduced to NADH and H+. A phosphate group (Pi) adds to each of the new carboxyl groups to form two molecules of the high- energy compound 1,3-biphosphoglycerate. Reaction 7 Phosphate Transfer A phosphate group from each 1,3-bisphosphoglycerate is transferred to two ADP molecules, yielding two molecules of the high-energy compound ATP. At this point in glycolysis, two ATP are produced, which balance the two ATP consumed in reactions 1 and 3. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 10  Energy-Generating Reactions 6 to 10 Reaction 8 Isomerization: Two 3-phosphoglycerate molecules undergo isomerization, which moves the phosphate group from carbon 3 to carbon 2, yielding two molecules of 2-phosphoglycerate. Reaction 9 Dehydration: Each of the phosphoglycerate molecules undergoes dehydration (loss of water), producing two molecules of phosphoenolpyruvate, a high-energy compound. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 11  Energy-Generating Reactions 6 to 10 Reaction 10 Phosphate Transfer In a second direct phosphorylation, phosphate groups from two phosphoenolpyruvate molecules are transferred to two ADP to form two pyruvate and two ATP. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 12 In the glycolysis pathway, a six-carbon glucose molecule is converted to two three-carbon pyruvate molecules. Initially, two ATP are required to form fructose-1,6-bisphosphate. In later reactions, phosphate transfers produce a total of four ATP. Overall, glycolysis yields two ATP and two NADH when a glucose molecule is converted to two pyruvate. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 13 10/30/2023 Shady ElShehawy ,2023 - [email protected] 14 10/30/2023 Shady ElShehawy ,2023 - [email protected] 15 10/30/2023 Shady ElShehawy ,2023 - [email protected] 16  Pathways for Pyruvate The pyruvate produced from glucose can now enter pathways that continue to extract energy. The available pathway depends on whether there is sufficient oxygen in the cell. Under aerobic conditions, oxygen is available to convert pyruvate to acetyl coenzyme A (acetyl CoA). When oxygen levels are low, pyruvate is reduced to lactate. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 17 10/30/2023 Shady ElShehawy ,2023 - [email protected] 18  Aerobic Conditions In glycolysis, two ATP were generated when one glucose molecule was converted to two pyruvate. However, much more energy is obtained from glucose when oxygen levels are high in the cells. Under aerobic conditions, pyruvate moves from the cytoplasm into the mitochondria to be oxidized further. In a complex reaction, pyruvate is oxidized, and a carbon atom is removed from pyruvate as CO2. The coenzyme NAD+ is reduced during the oxidation. The resulting two-carbon acetyl compound is attached to CoA, producing acetyl CoA, an important intermediate in many metabolic pathways. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 19 10/30/2023 Shady ElShehawy ,2023 - [email protected] 20 10/30/2023 Shady ElShehawy ,2023 - [email protected] 21 10/30/2023 Shady ElShehawy ,2023 - [email protected] 22 10/30/2023 Shady ElShehawy ,2023 - [email protected] 23  Anaerobic Conditions When we engage in strenuous exercise, the oxygen stored in our muscle cells is quickly depleted. Under anaerobic conditions, pyruvate remains in the cytoplasm where it is reduced to lactate. NAD+ is produced and is used to oxidize more glyceraldehyde-3-phosphate in the glycolysis pathway, which produces a small but needed amount of ATP. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 24 10/30/2023 Shady ElShehawy ,2023 - [email protected] 25 The accumulation of lactate causes the muscles to tire and become sore. After exercise, a person continues to breathe rapidly to repay the oxygen debt incurred during exercise. Most of the lactate is transported to the liver, where it is converted back into pyruvate. 10/30/2023 Shady ElShehawy ,2023 - [email protected] 26 10/30/2023 Shady ElShehawy ,2023 - [email protected] 27 10/30/2023 Shady ElShehawy ,2023 - [email protected] 28 29 10/30/2023 Shady ElShehawy ,2023 - [email protected]