BIOCHEMISTRY_OF_CARBOHYDRATES DAVID.pptx

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1. BIOCHEMISTRY OF CARBOHYDRATES GATLUAK JAMES KEDOK JIEK JANY (BBLT, MUK)

2. Lecture at a Glance Introduction Definition Classification Identification Importance Carbohydrate Metabolism and B iosynthesis of Glycogen Questions-Answers

3. 1. Introduction Carbohydrate is one of the major component of food. The percentage of carbohydrate in diet = 60-70% Examples of foods containing carbohydrates Bread Beans Potatoes Cookies Popcorn Rice

4. 2. Definition Carbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones. Or carbohydrates are defined as the aldehydic or ketonic derivatives of higher polyhydroxy alcohols or anhydrides of such derivatives. Carbohydrate means ………. Hydrated (H₂O) + Carbon (C). So it is formula shows that one molecule of water per carbon atom [Cn(H₂O)n], n = Number of atoms All carbohydrates are not sugar but all sugar is carbohydrates e.g. Glycogen, Insulin. Sugar example: Glucose, Fructose, Sucrose, Lactose and Maltose.

5. 3. Classification On the basis of taste Sugar/Simple: Soluble in water, Sweet taste (Glucose, Fructose, Lactose) Non Sugar/complex: Insoluble in water and tasteless (Starch, Cellulose) On the basis of Reduction Test Reducing sugar: A sugar that is capable of reducing a mild Oxidizing agent such as Fehling, Tollens reagent [All monosaccharides, Many disaccharides (Maltose, Lactose)] Non Reducing Sugar: A sugar that is not capable to reduce a mild oxidizing agent like fehling, tollens reagents [All polysaccharides, Few disaccharides (Sucrose) On the basis of hydrolysis Monosaccharide: Polyhydroxy aldehyde or polyhydroxy ketone which cannot be hydrolysed (Example – Glucose, Fructose, Galactose) Oligosaccharide: When 2-10 monosaccharides join together by O-Glycosidic Linkage form oligosaccharides. Oligosaccharides can be classified into many groups.

6. Classification of Oligosaccharides Disaccharides : When 2 monosaccharides join together by O-Glycosidic bond called disaccharide. Example: Lactose = Glucose + Galactose Maltose = Glucose + Glucose Sucrose = Glucose + Fructose Trisaccharides : On hydrolysis, we get 3 monosaccharides. Example: Robinose: Glucose + rhamnose + monosaccharides Raffinose: Glucose + Fructose + Galactose

7. Tetrasaccharide : On hydrosis, we get 4 monosaccharides (Stachyose = 4*glucose). Polysaccharide: When more than 10 monosaccharides join together by O-Glycosidic linkage form polysaccharide. Polysaccharides are divided into 2 groups: homopolysaccharide : On hydrolysis, all monosaccharides are identical (Example: Starch, Glycogen, Cellulose, Chytin) Heteropolysaccharide : On hydrolysis, we get different monosaccharides (Example: Mucopolysaccharide)

8. 4. Identification Test Carbohydrate identification test name is Molisch’s Test. Reducing sugar identification test --- ( Benedict’s Test, Fehling Test ).

9. Benedict’s Test Benedict’s Test is used to test for simple carbohydrates. The Benedict’s test identifies reducing sugars (monosaccharides and some disaccharides), which have free ketone or aldehyde functional groups. Benedict’s solution can be used to test for the presence of glucose in urine. Some sugars such as glucose are called reducing sugars because they are capable of transferring hydrogens (electrons) to other compounds, a process called Reduction . Principle of Benedict’s Test When Benedict’s solution and simple carbohydrates are heated, the solution changes to orange red/brick-red. This reaction caused by the reducing property of simple carbohydrates. The Copper (ii) ions in the Benedict’s solution are reduced to Copper (i) ions, which causes the colour change. The red Copper (i) Oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed. As the concentration of reducing sugar increases, the nearer the final colour is to brick-red and the greater the precipitate formed. Sometimes, a brick red solid copper oxide precipitates out of the solution and collects at the bottom of the test tube.

10. Composition and Preparation of Benedict’s Solution Benedict’s solution is deep-blue alkaline solution used to test for the presence of the aldehyde functional group (-CHO). Anhydrous sodium carbonate = 100 gm: This provides the alkaline conditions which are required for the redox reaction. Sodium citrate – 173 gm: This complexes with Copper (ii) ions so that they do not deteriorate to Copper (i) ions during storage. Copper (ii) sulfate pentahydrate = 17.3 gm. One litre of Benedict’s solution can be prepared from 100 g of anhydrous sodium carbonate, 173 g of sodium citrate and 17.3 g of copper (ii) sulfate pentahydrate.

11. Procedure of Benedict’s Test Approximately 1 ml of sample is placed into a clean test tube. 2 ml (10 drops) of Benedict’s reagent (CuSO₄) is placed in the test tube. The solution is then heated in a boiling water bath for 3-5 minutes. Observe for colour change in the solution of test or precipitate formation. Note : Complex carbohydrates such as starches DO NOT react positive with Benedict’s test unless they are broken down through heating or digestion.

12. Result Interpretation of Benedict’s Test If the colour upon boiling is changed into green, then there would be 0.1-0.5% sugar in solution. If it changes colour to yellow, then 0.5-1% sugar is present. If it changes to orange, then it means that 1-1.5% sugar is present. If colour changes to red, then 1.5-2.0% sugar is present. And if colour changes to brick red, it means that more than 2% sugar is present in solution. Positive Benedict’s Test : Formation of a reddish precipitate within three minutes; reducing sugars present e.g. Glucose. Negative Benedict’s Test : No colour change (Remains Blue); reducing sugars absent e.g. Sucrose.

13. 5. Importance of Carbohydrates Provide energy to the body (muscle, tissue, brain). Prevent the breakdown of proteins for energy. Reduce breakdown of fatty acids and prevent ketosis. Store energy in the form of Starch (in plants), Glycogen (in animals and Humans).

14. 6. Carbohydrate Metabolism and Biosynthesis of Glycogen The goal of glycolysis, glycogenolysis, and the citric acid cycle is to conserve energy as ATP from the catabolism of carbohydrates. If the cells have sufficient supplies of ATP, then these pathways and cycles are inhibited. Under these conditions of excess ATP, the liver will attempt to convert a variety of excess molecules into glucose and/or glycogen.

15. Glycogenesis Glycogenesis is the formation of glycogen from glucose. Glycogen is synthesized depending on the demand for glucose and ATP (energy). If both are present in relatively high amounts, then the excess of insulin promotes the glucose conversion into glycogen for storage in liver and muscle cells. In the synthesis of glycogen, one ATP is required per glucose incorporated into the polymeric branched structure of glycogen. actually, glucose-6-phosphate is the cross-roads compound. Glucose-6-phosphate is synthesized directly from glucose or as the end product of gluconeogenesis.

16. Glycogenolysis In glycogenolysis, glycogen stored in the liver and muscles, is converted first to glucose-1- phosphate and then into glucose-6-phosphate. Two hormones which control glycogenolysis are a peptide, glucagon from the pancreas and epinephrine from the adrenal glands . In short, Glycogenolysis is the biochemical pathway in which glycogen breaks down into glucose-1-phosphate and glucose. Glucagon is released from the pancreas in response to low blood glucose and epinephrine is released in response to a threat or stress. Both hormones act upon enzymes to stimulate glycogen phosphorylase to begin glycogenolysis and inhibit glycogen synthetase (to stop glycogenesis ). Glycogen is a highly branched polymeric structure containing glucose as the basic monomer. First individual glucose molecules are hydrolyzed from the chain, followed by the addition of a phosphate group at C-1. In the next step the phosphate is moved to the C-6 position to give glucose 6-phosphate, a cross road compound. Glucose-6-phosphate is the first step of the glycolysis pathway if glycogen is the carbohydrate source and further energy is needed. If energy is not immediately needed, the glucose-6-phosphate is converted to glucose for distribution in the blood to various cells such as brain cells.

17. Gluconeogenesis Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as pyruvate, lactate, glycerol, and glucogenic amino acids. The vast majority of gluconeogenesis takes place in the liver and, to a smaller extent, in the cortex of kidneys. This process occurs during periods of fasting, starvation, or intense exercise and is highly endergonic. Gluconeogenesis is often associated with ketosis.

18. G lycolysis In glycolysis pathway, glucose is converted to pyruvate (aerobic condition) or lactate (anaerobic condition), along with production of a small quantity of energy . In simple definition, glycolysis is the cellular degradation of the simple sugar glucose to yield pyruvic acid and ATP as an energy source. Site of reaction : All the reaction steps take place in the cytoplasm . Importance of the glycolysis pathway : It is the only pathway that is taking place in all the cells of the body. Glycolysis is the only source of energy in erythrocytes. In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles. The glycolytic pathway may be considered as the preliminary step before complete oxidation. The glycolytic pathway provides carbon skeletons for synthesis of nonessential amino acids as well as glycerol part of fat. Most of the reactions are reversible.

19. Control of Blood Glucose One of the most important and tightly regulated responses in the human body is the concentration of blood glucose (blood sugar). Glucose is the major breakdown product of cellular metabolism. As such, it is required both as an energy source and as a source of carbon for making organic molecules. Blood glucose concentrations are regulated by negative feedback pathways that are modulated by two separate hormones: insulin and glucagon. Both of these hormones are produced in special cells called islet cells, or islets of Langerhans – are found in clusters throughout the pancreas. Islet cells make up a very small percentage of the pancreas (about 1-2%); the remainder of the organ is an exocrine gland producing digestive enzymes and bicarbonate ion. This tiny number of endocrine cells is exceedingly important. Each islet contains two kinds of cells: alpha cells, which produce glucagon, and beta cells, which produce insulin.

20. Insulin vs. Glucagon Normally , blood glucose concentrations in human blood should range between 70-110 milligrams (mg/ml). Insulin and glucagon operate in an antagonistic (opposing) manner. The result is a precise control of blood glucose levels within this range. The insulin pathway is activated when blood glucose levels are too high. High blood glucose levels (e.g., occurring after the stomach has digested a food high in sugar) stimulate beta cells in the pancreas to release insulin. Insulin causes an increased uptake of glucose from the blood; promotes conversion of glucose into triglycerides in the liver, fat and muscle cells; and increases the cellular rate of glycolysis – breaking glucose into smaller components that can be used for synthesis of other compounds. The glucagon pathway is activated when blood glucose levels are too low. Low blood glucose levels (e.g., due to exercise combined with not eating for several hours) stimulate the alpha cells in the pancreas to produce glucagon. Glucagon causes the liver to convert stored glycogen into glucose, then release the glucose into the blood (a process called glycogenolysis). The two hormones, insulin and glucagon, each regulate the other. A decrease in insulin (as well as low glucose levels) stimulates the secretion of glucagon, while an increase in insulin (as well as an increase in blood glucose) suppresses glucagon secretion. This results in a continuous cycle, with insulin and glucagon constantly monitoring blood glucose levels and regulating their secretion to maintain these levels as nearly constant as possible. The main function of insulin is removal of excess blood glucose. Because all cells use glucose as an energy source and as a raw material for making other organic compounds, all cells except brain cells are targets for insulin. Since the function of glucagon is opposite that of insulin, it stimulates the addition of glucose to the bloodstream. Thus, it targets cells with high concentrations of energy stored as glycogen, including the liver and skeletal muscles. It also stimulates glucose production from fats, so adipose tissue cells are another target of glucagon.

21. Lactose intolerance Lactose intolerance is a common digestive problem where the body is unable to digest lactose, a type of sugar mainly found in milk and dairy products. The body digests lactose by using an enzyme called lactase to break down lactose into two simpler sugars called glucose and galactose, which can then be easily absorbed into the bloodstream. Enzymes are proteins that cause chemical reactions to occur. In cases of lactose intolerance, the body does not produce enough of the lactase enzyme so lactose stays in the digestive system, where it is fermented by bacteria (in the same way that yeast is fermented to produce beer). It’s this fermentation process that causes the symptoms associated with lactose intolerance . Symptoms of lactose intolerance include; a bloated stomach, flatulence (wind) and diarrhea. L imiting intake of foods and drinks containing lactose is the main treatment for lactose intolerance. Levels of lactase often fall as people grow older and some health conditions can also reduce the production of lactase . Diabetes Mellitus Diabetes mellitus (often referred to simply as diabetes) is a group of metabolic diseases characterized by high blood glucose levels. The term comes from two Greek words: “diabetes” comes from a verb that means “to pass through” and refers to the frequent, copious urination that is a characteristic of the disease; the word “meli” is Greek for “honey” so the term “mellitus” refers to the presence of high levels of glucose (sugar) in the blood. In addition to urination, other classic symptoms of diabetes are increased thirst and hunger. The diabetic’s blood contains more glucose than can be taken up by the cells so this excess glusose is therefore released in the urine (a diagnostic characteristic of diabetes is sugar in the urine). The presence of sugar results in more water being drawn into the urine to balance the osmotic pressure, leading to copious urination .

22. Questions-Answers What is Glycosidic Linkage? The covalent bond linking the anomeric carbon of a carbohydrate with alcoholic OH group of some other molecule is O-Glycosidic Linkage. What is dextrose? The dextrorotatory form of glucose is called as dextrose or D-Glucose. 3) Why is sucrose called as non reducing sugar? No free anomeric carbon is available in sucrose because in sucrose, glycosidic bond is made of C1 of glucose & C2 of fructose and sucrose is not capable of reducing a mild oxidizing agent like tollense or fehling reagent --- that is why sucrose is called as non reducing sugar. Lactose is known as ----- Malt sugar Cane sugar Milk sugar None

23. Note: Lactose = Milk sugar Maltose = Malt sugar Sucrose = Cane sugar Classify carbohydrate Define Glycolysis, Glycogenesis, Glycogenoloysis and Gluconeogenesis. Write down the example of polysaccharides = Starch, Cellulose, Chytin, Glycogen.