Lec3_CHO Metabolism (1) PDF

Module: Biochemistry Introduction to metabolism & Carbohydrates metabolism Dr Mohammed Mansour Senior Lecturer in Biomedical Science, London South Bank University, UK E: [email protected] INTRODUCTION TO METABOLISM Metabolism is the entire spectrum of chemical...

Module: Biochemistry Introduction to metabolism & Carbohydrates metabolism Dr Mohammed Mansour Senior Lecturer in Biomedical Science, London South Bank University, UK E: [email protected] INTRODUCTION TO METABOLISM Metabolism is the entire spectrum of chemical reactions that occur in the living system. A metabolic pathway consists of a series of enzymatic reactions to produce desired products. The term metabolite is applied to a substrate or an intermediate or a product in metabolic reactions. Metabolism is broadly divided into two categories: catabolism and anabolism. Outline of catabolism and anabolism Catabolism: Catabolism is defined as the degradative process concerned with the breakdown of complex molecules to simpler ones, with a release of energy. The very purpose of catabolism is to trap the energy of the biomolecules in the form of ATP and generate the substances (precursors) required for the synthesis of complex molecules. Catabolism occurs in three stages as shown. Stage I – Conversion of complex into their building blocks: Polysaccharides are broken down to monosaccharide, lipids to free fatty acids and glycerol, and proteins to amino acids.  Stage II – Formation of simple intermediates: The building blocks produced in stage I are degraded to simple intermediates such as pyruvate and acetyl-CoA. These intermediates are not readily identifiable as carbohydrates, lipids, or proteins. A small quantity of energy (as ATP) is captured in stage II. Stage III – final oxidation of acetyl-CoA: Acetyl-CoA is completely oxidised to CO2 , liberating NADH and FADH2 that finally get oxidised to release large quantity of energy (as ATP). Krebs’s cycle or citric acid cycle is the common metabolic pathway involved in the final oxidation of all energy-rich molecules. This pathway accepts the carbon compounds (pyruvate, succinate, and so on) derived from carbohydrates, lipids, or proteins. Anabolism It is the biosynthetic reaction involving the formation of complex molecules from simpler precursors. For the synthesis of a large variety of complex molecules, the starting materials are relatively few. These include pyruvate, acetyl-CoA, and the intermediates of citric acid cycle. Besides the availability of precursors, the anabolic reactions depend on the supply of energy (as ATP or GTP) and reducing equivalents as (NADPH+ H+).  Carbohydrate metabolism is concerned with the fate of glucose Glucose is metabolised to pyruvate and lactate in all mammalian cells by the pathway of glycolysis. Glycolysis can occur in the absence of oxygen (anaerobic) when the end product is lactate only. Tissues that can utilise oxygen (aerobic) are able to metabolise pyruvate to acetyl-CoA, which can enter the citric acid cycle for complete oxidation to CO2 and H2O, with the liberation of much free energy as ATP in the process of oxidative phosphorylation. Thus, glucose is a major fuel of many tissues. But it also takes part in other processes as follows: 1. Conversion to its storage polymer, glycogen, particularly in skeletal muscle and liver. 2. The pentose phosphate pathway, which arises from intermediates of glycolysis. It is a source of reducing equivalents (2H) for biosynthesis. Example includes fatty acids, and it is also the source of ribose, which is important for nucleotide and nucleic acid formation. 3. Triose phosphate gives rise to the glycerol moiety of acylglycerol (fat). 4. Pyruvate and intermediates of the citric acid cycle provide the carbon skeletons for the synthesis of amino acid and acetyl-CoA is building block for the long chain fatty acids and cholesterol, the precursor of all steroids synthesised in the body. Lipid metabolism The source for the fatty acid synthesis is acetyl-CoA. Fatty acid is produced either by de novo synthesis from acetyl-CoA derived from carbohydrate or from dietary lipid. In the tissues, fatty acids are oxidised to acetyl-CoA (β-oxidation) or esterified to acylglycerol, whereas in triacylglycerol (fat), they constitute the body’s main caloric reserve. Acetyl-CoA formed by β-oxidation has several important facts: 1. As in the case of acetyl-CoA derived from carbohydrate, it is oxidised completely to CO2 and H2O via the citric acid cycle. Fatty acids yield considerable energy both in β-oxidation and in the citric acid cycle and are, therefore, very effective of tissue fuels. 2. It is a source of the carbon atoms in cholesterol and other steroids. 3. In the liver, it forms acetoacetate, the parent ketone body. Ketone bodies are alternative water-soluble tissue fuels, which become important sources of energy under certain conditions (Example: starvation). Amino acid metabolism Amino acids are necessary for protein synthesis. Some must be supplied specifically in the diet (the essential amino acids). The non-essential amino acids are also supplied in the diet, but they can be formed from intermediates by transamination using the amino nitrogen from other surplus amino acids. After deamination, excess amino nitrogen is removed as urea, and the carbon skeletons that remain after transamination:  1. are oxidised to CO2 via citric acid cycle, 2. form glucose (gluconeogenesis) and 3. form ketone bodies. -In addition to their requirement for protein synthesis, the amino acids are also the precursors of many other important compound. Examples: Purine, pyrimidine, and hormones such as epinephrine and thyroxine. Metabolic pathways – levels of organisation The location and integration of metabolic pathways are released by studies at lower levels of organisation, namely the following: 1. At the subcellular level, each cell organelle (mitochondria) or compartment (cytosol) carries out specific biochemical roles that from part of a subcellular pattern of metabolic pathway. 2. At the tissue and organ level, the nature of the substrate entering and metabolites are leaving tissues and organs. At the tissue and organ level, the blood circulation integrates metabolism. Amino acids resulting from digestion of dietary protein and glucose resulting from the digestion of carbohydrate share a common route of absorption via the hepatic portal vein. This ensures that both of these metabolites and other water- soluble products of digestion are initially directed to liver. Liver has the primary metabolic function of regulating the blood concentration of most metabolites, particularly glucose and amino acids. In the case of glucose, this is achieved by taking up excess glucose and converting it to glycogen (glycogenesis) or fat (lipogenesis). Between meals, it can draw upon its glycogen. Glycolysis Biomedical importance: 1. Glycolysis is an almost universal central pathway of glucose catabolism, the pathway with the largest flux of carbon in most cells. 2. The glycolytic breakdown of glucose is the sole source of metabolic energy in some mammalian tissues and cell types (erythrocytes, renal medulla, brain, and sperm, for example). 3. Many anaerobic microorganisms are entirely dependent on glycolysis.  Thank you Dr. Mohammed Mansour E: [email protected] Location: -Glycolysis occurs in all cells, and in some cells, it is the sole source of energy like brain cells, erythrocytes. -Organelle: It occurs in cytoplasm of the cells. Steps: In glycolysis, the breakdown of the six-carbon glucose into two molecules of the three-carbon pyruvate occurs in ten steps, Glycolysis is divided in to two phases: 1. Preparatory phase 2. Pay-off phase  1. Preparatory phase Step 1: Glucose to glucose 6-phosphate (Phosphorylation of glucose). In the first step of glycolysis, glucose is activated for subsequent reactions by its phosphorylation at C-6 to yield glucose 6-phosphate with ATP as the phosphoryl donor. Step 2: Conversion of glucose 6- phosphate to fructose 6-phosphate The enzyme phosphohexose isomerase (phosphoglucose isomerase) catalyses the reversible isomerisation of glucose 6-phosphate, an aldose, to fructose 6-phosphate, a ketose. Step 3: Phosphorylation of fructose 6- phosphate to fructose 1,6-bisphosphate In the second of the two priming reactions of glycolysis, phosphofructokinase-1 (PFK-1) catalyses the transfer of a phosphoryl group from ATP to fructose 6-phosphate to yield fructose 1,6- bisphosphate. Step 4: Cleavage of fructose 1, 6- bisphosphate The enzyme fructose 1,6-bisphosphate aldolase, often called simply aldolase, catalyses a reversible aldol condensation. Fructose 1,6-bisphosphate is cleaved to yield two different triose phosphates, glyceraldehyde 3-phosphate, an aldose, and dihydroxyacetone phosphate, a ketose. Step 5: Inter-conversion of the triose phosphates Only one of the two triose phosphates formed by aldolase, glyceraldehydes 3-phosphate, can be directly degraded in the subsequent steps of glycolysis. 2. Pay-off phase The pay-off phase of glycolysis includes the energy- conserving phosphorylation steps in which some of the free energy of the glucose molecule is conserved in the form of ATP. Remember that one molecule of glucose yields two molecules of glyceraldehyde 3- phosphate; both halves of the glucose molecule follow the same pathway in the second phase of glycolysis. Step 6: Oxidation of glyceraldehyde 3- phosphate to 1,3-bisphosphoglycerate The first step in the pay-off phase is the oxidation of glyceraldehyde 3-phosphate to 1,3 bisphosphoglycerate, catalysed by glyceraldehyde 3 phosphate dehydrogenase. Step 7: Phosphoryl transfer from 1,3- bisphosphoglycerate to ADP The enzyme phosphoglycerate kinase transfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. Step 8: Conversion of 3-phosphoglycerate to 2- phosphoglycerate The enzyme phosphoglycerate mutase catalyses a reversible shift of the phosphoryl group between C-2 and C-3 of glycerate; Mg2+ is essential for this reaction. Step 9: Dehydration of 2-phosphoglycerate to phosphoenolpyruvate In the second glycolytic reaction that generates a compound with high phosphoryl group transfer potential, enolase promotes reversible removal of a molecule of water from 2-phosphoglycerate to yield phosphoenolpyruvate (PEP). Step 10: Transfer of the phosphoryl group from phosphoenolpyruvate to ADP The last step in glycolysis is the transfer of the phosphoryl group from phosphoenolpyruvate to ADP, catalysed by pyruvate kinase, which requires K+ and either Mg2+ or Mn2+. Energy production in Glycolysis During glycolysis, ATP is formed and used in the following reactions:  The net gain of ATP molecules during glycolysis is eight (10 – 2 = 8). During anaerobic condition, NAD+ is regenerated from NADH by the reduction of pyruvate to lactate catalysed by lactate dehydrogenase.  Thank you Dr. Mohammed Mansour E: [email protected]