Metabolism PDF

METABOLISM Metabolism - is the sum total of all the biochemical reactions that take place in a living organism. - is the central theme in biochemistry. - It keeps the cells and organisms alive. - It gives them the energy to carry on and the building blocks requi...

METABOLISM Metabolism - is the sum total of all the biochemical reactions that take place in a living organism. - is the central theme in biochemistry. - It keeps the cells and organisms alive. - It gives them the energy to carry on and the building blocks required for growth and propagation. - The metabolism of animals and humans, which depend entirely in foodstuffs, can be conceptually divided into catabolic and anabolic reactions. Types: 1. Catabolism - is all metabolic reactions in which large biochemical molecules are broken down to smaller ones. - Catabolic reactions usually release energy. The reactions involved in the oxidation of glucose are catabolic. What are the key roles of catabolic reactions? Production of energy (ATP) Production of the reducing power (NADPH), and Regeneration of the building blocks for anabolic metabolism. ATP – energy currency of the cell or “electric energy” of the cell. 2. Anabolism - is all metabolic reactions in which small biomolecules are joined together to form larger ones. - Anabolic reactions usually require energy in order to proceed. The synthesis of proteins from amino acids is an anabolic process. - The processes of catabolism and anabolism are opposite in nature. The first usually produces energy and the second usually consumes energy. - The metabolic reactions that occur in a cell are usually organized into sequences called metabolic pathways.  Metabolic pathway - is a series of consecutive biochemical reactions used to convert a starting material into an end product. - There are several “mainstream” metabolic pathways that occur in a wide variety of living organisms. A good example is “glycolysis”, which is the main pathway of glucose degradation. - Glycolysis, also known as ‘Embden – Meyerhof Pathway’ – is a pathway of carbohydrate catabolism that begins with the substrate D-glucose. - The pathway evolved at a time when the earth’s atmosphere was anaerobic, no free oxygen was available. As a result, glycolysis requires no oxygen; it is an anaerobic process. Glycolysis can be divided into two major segments: 1. Investment of ATP energy - Without this investment, glucose would not have enough energy for glycolysis to continue; and there would be no ATP produced. - It includes the first 5 reactions of the pathway. 2. Involves the remaining reactions of the pathway, those that result in a net energy yield. There are 10 steps involved in the ‘Glycolysis Pathway’: hexokinase Reaction 1. Glucose Glucose-6-phosphate phosphoglucose isomerase 2. Glucose-6-phosphate Fructose-6-phosphate Phosphofructokinase 3. Fructose-6-phosphate Fructose-1,6-biphosphate aldolase 4. Fructose-1,6-biphosphate DHA phosphate + Glyceraldehyde-3-phosphate triose phosphate 5. DHA phosphate isomerase Glyceraldehyde-3-phosphate  glyceraldehyde-3-phosphate dehydrogenase 6. Glyceraldehyde-3-phosphate + NAD+ + P1 1,3 biphosphoglycerate + NADH + H+ phosphoglycerate kinase 7. 1,3-biphosphoglycerate + ADP H+ 3-phosphoglycerate + ATP phosphoglycerate mutase 8. 3-phosphoglycerate 2-phosphoglycerate enolase 9. 2-phosphoglycerate Phosphoenolpyruvate pyruvate kinase 10. Phosphoenolpyruvate + ADP + H+ Pyruvate + ATP Note: It should be noted that reactions 6 thru 10 occur twice per glucose molecule, because the starting six-carbon sugar is split into two 3-carbon molecules. Thus, in reaction 6, two NADH molecules are generated, and a total of four ATP molecules are made (steps 7 and 10). The net ATP gain from this pathway is, however, only two ATP molecules because there was an investment of two ATP molecules in the early steps of the pathway. This investment was paid back by the two ATP molecules produced by substrate-level phosphorylation in step 7. The actual energy yield is produced by substrate-level phosphorylation in reaction 10. Gluconeogenesis - the synthesis of glucose. Glycogenolysis - the degradation of glycogen. Glycogenesis - the synthesis of glycogen CONVERSION OF PYRUVATE ACETYL CoA Under aerobic conditions, the cells can use oxygen and completely glucose to CO 2 in a metabolic pathway called the Citric Acid Cycle, TCA, Krebs cycle. The pathway is often referred to as the Krebs cycle in honor of Sir Han Krebs who worked out the steps in the cyclic pathway. Also known as TCA cycle because several of the early intermediates in the pathway have 3 carboxyl groups. Once pyruvate enters the mitochondria, it must be converted to a two-carbon acetyl group. The acetyl group must be ‘activated’ to enter the reaction of the TCA cycle; where activation occurs when the acetyl group (CH3CO) is bonded to the thiol group of CoA. CoA - is a large thiol derived from ATP and the vitamin pantothenic acid. Acetyl CoA - is the ‘activated ‘form of the acetyl group. OVERVIEW OF THE BIOCHEMICAL ENERGY PRODUCTION Energy production - summarizes the four general stages in the process of production of biochemical energy from ingested food. Stage 1: The first stage, digestion, begins in the mouth (saliva contains starch-digesting enzymes), continues in the stomach (gastric juices), and is completed in the small intestine (majority of digestive enzymes and bile salts). The end products of digestion- glucose and other monosaccharides from carbohydrates, amino acids from proteins, and fatty acids and glycerol from fats and oils are small enough to pass across intestinal membranes and into the blood, with the aid of membrane transport systems. Once in the blood, they are then distributed to the cells in various parts of the body. Stage 2: The second stage, acetyl group formation, involves numerous reactions, some of which occur in the cytosol of cells and some in cellular mitochondria. The small molecules from digestion are further oxidized during this stage. Primary products include two-carbon acetyl units (which become attached to coenzyme A to give acetyl CoA) and the reduced coenzyme NADH. Stage 3: The third stage, the citric acid cycle, occurs inside the mitochondria. Here acetyl groups are oxidized to produce CO2, and energy. Some of the energy released by these reactions is lost as heat, and some is carried by the reduced coenzymes NADH and FADH2 to the fourth stage. The CO2 that is exhaled as part of the breathing process comes primarily from this stage. Stage 4: The fourth stage, the electron transport chain and oxidative phosphorylation, also occurs inside mitochondria. NADH and FADH2 supply the “fuel” (hydrogen ions and electrons) needed for the production of ATP molecules, the primary energy carriers in metabolic pathways. Molecular O 2, inhaled via breathing, is converted to H2O in this stage.