Carbohydrate Chemistry PDF

D.noor Alhuda.A.Hamzah Msc.in pharmacy sciences 2024-2025 Carbohydrate Chemistry Introduction The carbohydrates are widely distributed both in animal and plant tissues. Chemically, they contain the elements carbon, hydrogen and oxygen. The empirical formula of many simple...

D.noor Alhuda.A.Hamzah Msc.in pharmacy sciences 2024-2025 Carbohydrate Chemistry Introduction The carbohydrates are widely distributed both in animal and plant tissues. Chemically, they contain the elements carbon, hydrogen and oxygen. The empirical formula of many simple carbohydrates is [CH2O]n where n is 3 or more. Hence, the name “carbohydrate”, i.e., hydrated carbon. They are also called “saccharides”. In Greek, saccharon means sugar. Some carbohydrates also contain nitrogen, phosphorus or sulfur. 1 Definition Carbohydrates may be defined chemically as aldehyde or ketone derivatives of polyhydroxy (more than one hydroxy group) alcohols or as compounds that yield these derivatives on hydrolysis Digestion Digestion is the chemical breakdown of large food molecules into smaller molecules that can be used by cell. The breakdown occurs when certain specific enzymes are mixed with food. Metabolism: is the biochemical pathway through which the cells obtain energy Main components of food are: 1-carbohydrates 2-proteins 3-lipids 2 Classification of Carbohydrates Classification of Carbohydrates Depending upon the number of monomeric units present in a molecule, carbohydrates are classified into: 1- Monosaccharides: are those sugars that cannot be hydrolyzed into simpler carbohydrates. They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms, and as aldoses or ketoses, depending upon whether they have an aldehyde or ketone group. 3  The most abundant monosaccharide in nature is six carbon sugar-D-glucose. 2- Disaccharides: are condensation products of two monosaccharide units; examples are maltose and sucrose. 3-Oligosaccharides: are condensation products of three to ten monosaccharides. Most are not digested by human enzymes. E.g., raffinose which is a trisaccharide 4 occurs in legumes, whole grains, cabbage and broccoli etc. Recently it has been found that raffinose oligosaccharide have a beneficial effect on the gut microflora. 4-Polysaccharides: Polysaccharides are polymers consisting of hundreds or thousands of monosaccharide units. They are also called glycans or complex carbohydrates. They may be either linear, (e.g. cellulose) or branched, (e.g. glycogen) in structure. Polysaccharides have high molecular weight and are only sparingly soluble in water. They are not sweetish and do not exhibit any of the properties of aldehyde or ketone group. 5 cellulose are non-starch polysaccharides; they are not digested by human enzymes, and are the major component of dietary fiber. Polysaccharides are of two types 1. Homopolysaccharides (homoglycans) 2. Heteropolysaccharides (heteroglycans). Homopolysaccharides (Homoglycans) When a polysaccharide is made up of several units of one and the same type of monosaccharide unit, it is called homopolysaccharide. The most common homoglycans are: – Starch: is a homopolymer of glucose. It is the most important dietary carbohydrates in plants – Dextrin: are intermediates in the hydrolysis of starch – Glycogen: is the storge polysaccharide in animals. found mostly in liver and muscle; act as a readily available source of glucose for energy within muscle itself. Liver glycogen is concerned with storage and maintenance of the blood glucose. 6 – Inulin: is a polysaccharide of fructose.it is readily soluble in water but it is not hydrolyzed by intestinal enzymes. – Cellulose: is a chief constituent of plant cell wall (dietary fiber) Some homopolysaccharides serve as a storage form of monosaccharides used as fuel, e.g. starch and glycogen, while others serve as structural elements in plants, e.g. cellulose. Heteropolysaccharides (Heteroglycans) They contain two or more different types of monosaccharide units or their derivatives. Heteropolysaccharide present in human beings is glycosaminoglycans (mucopolysaccharides), which are complex carbohydrates containing amino sugars and uronic acid. they may be attached to a protein molecule to form a proteoglycan e.g. – Heparin – Hyaluronic acid – Blood group polysaccharides. Functions of carbohydrates There are many primary functions of carbohydrates in the human body. They are energy production, energy storage, building macromolecules, sparing protein, assisting in lipid metabolism and also involved in detoxification. 7 1-Energy Production The primary role of carbohydrates is to supply energy to all cells in the body. Many cells prefer glucose as a source of energy versus other compounds like fatty acids. Some cells, such as red blood cells, are only able to produce cellular energy from glucose. The brain is also highly sensitive to low blood-glucose levels because it uses only glucose to produce energy and function (unless under extreme starvation conditions). About 70 percent of the glucose entering the body from digestion is redistributed (by the liver) back into the blood for use by other tissues. 2-Energy Storage If the body already has enough energy to support its functions, the excess glucose is stored as glycogen (the majority of which is stored in the muscles and liver). A molecule of glycogen may contain in excess of fifty thousand single glucose units 8 and is highly branched, allowing for the rapid dissemination of glucose when it is needed to make cellular energy. 3- Building Macromolecules Although most absorbed glucose is used to make energy, some glucose is converted to ribose and deoxyribose, which are essential building blocks of important macromolecules, such as RNA, DNA, and ATP. Glucose is additionally utilized to make the molecule NADPH, which is important for protection against oxidative stress and is used in many other chemical reactions in the body. If all of the energy, glycogen-storing capacity, and building needs of the body are met, excess glucose can be used to make fat. Also Serve as structural component, e.g., glycosaminoglycans in humans, cellulose in plants and chitin in insects 4-Sparing Protein When the body requires more glucose than is available, amino acids are used to create glucose. The degradation of proteins, particularly from muscle tissue, is necessary for this process because there is no storage molecule for amino acids. Having enough glucose essentially prevents the body from breaking down proteins to produce the glucose it needs. 5-Lipid Metabolism As blood-glucose levels rise, the use of lipids as an energy source is inhibited. Thus, glucose additionally has a “fat-sparing” effect. This is because an increase in blood glucose stimulates release of the hormone insulin, which tells cells to use glucose (instead of lipids) to make energy. 9 Adequate glucose levels in the blood also prevent the development of ketosis. Ketosis is a metabolic condition resulting from an elevation of ketone bodies in the blood. Ketone bodies are an alternative energy source that cells can use when glucose supply is insufficient, such as during fasting. Ketone bodies are acidic and high elevations in the blood can cause it to become too acidic. This is rare in healthy adults, but can occur in alcoholics, people who are malnourished, and in individuals who have Type 1 diabetes. The minimum amount of carbohydrate in the diet required to inhibit ketosis in adults is 50 grams per day. 5- Ading in digestion and increase intestinal movement for example (fiber). 6- Detoxification Carbohydrates are also involved in detoxification, e.g., glucuronic acid. Glucuronic acid is a type of sugar derived from glucose. It is commonly involved in the detoxification and removal of foreign substances from the body. Glucuronic acid can also form complexes with some drugs and hormones, making them more soluble and easier to excrete 10 Structure of glucose Physiologically and biomedically, glucose is the most important monosaccharide. The structure of glucose can be represented in the following ways (Figure 2.1): 1. The straight chain structural formula (Fisher projection). 2. Cyclic formula (Ring structure or Haworth projection( 3- glucose chair form Monosaccharide in solution is mainly present in ring form. 11 In solution, aldehyde (CHO) or ketone (C=O) group of monosaccharide react with a hydroxy (OH) group of the same molecule forming a bond hemiacetal or hemiketal respectively. ISOMERISM Isomer definition: The compounds possessing identical molecular formula but different structures are referred to as isomers. The phenomenon of existence of isomers is called isomerism. (Greek ‘isos’ means equal, ‘meros’ means parts). The important types of isomerism exhibited by sugar are as follows: 1. Ketose-aldose isomerism 2. D and L isomerism 3. Optical isomerism 4. Epimerism 5. Anomerism (α and β Anomerism) 12 Types of isomerism: Ketose-Aldose isomerism Glucose and fructose are isomers of each other having the same chemical (molecular) formula C6H12O6, but they differ in structural formula with respect to their functional groups. There is a keto group in position two of fructose and an aldehyde group in position one of glucose. This type of isomerism is known as ketose-aldose isomerism. Optical Isomerism 13 The presence of asymmetric carbon atoms exhibits optical activity on the compound. Optical activity is the capacity of a substance to rotate the plane polarized light passing through it. When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right and is said to be dextrorotatary (d) or (+) or to the left and is said to be, levorotatory (l) or (-). When equal amount of d and l isomers are present, the resulting mixture has no optical activity. Since the activity of each isomer cancel one another, such a mixture is said to be a racemic or dl mixture. Enantiomers (D and L isomerism) D and L isomerism depends on the orientation of the H and OH groups around the asymmetric carbon atom adjacent to the terminal primary alcohol carbon, e.g. carbon atom number 5 in glucose determines whether the sugar belongs to D or L isomer. When OH group on this carbon atom is on the right, it belongs to D-series, when it is on the left, it is the member of the L-series. D and L isomers are mirror images of each other. These two forms are called enantiomers. 14 Epimerism When two monosaccharides differ from each other in their configuration around a single asymmetric carbon (other than anomeric carbon) atom, they are referred to as epimers of each other. For example, galactose and mannose are two epimers of glucose. They differ from glucose in the configuration of groups (H and OH) around C-4 and C-2 respectively. Galactose and mannose are not epimers of each other as they differ in configuration at two asymmetric carbon atoms around C-2 and C-4 15 Anomerism α- and β-anomers: These are cyclic forms of monosaccharides that differ in the orientation of the -OH group on the anomeric carbon (the one that was the carbonyl carbon before cyclization). The predominant form of glucose and fructose in a solution are not an open chain. Rather, the open chain form of these sugar in solution cyclizes into rings. An additional asymmetric center is created when glucose cyclizes. Carbon-1 of glucose in the open chain form, becomes an asymmetric carbon in the ring form and two ring structures can be formed. These are: α-D-glucose β-D-glucose. The designation α means that the hydroxyl group attached to C-1 is below the plane of the ring, The designation β means that it is above the plane of the ring. The C-1 carbon is called the anomeric carbon atom and so, α and β forms are anomers. 16 Pyranose and furanose ring structures The ring structure of monosaccharide are similar to the ring structure of either pyran (a six membered ring) so called glucopyranose or similar to furan (a five membered ring) so called glucofuranose. However, in case of glucose, the six membered glucopyranose is much more stable than the glucofuranose ring. In the case of fructose, the more stable form is fructofuranose. 17 MUTAROTATION: Mutarotation is defined as the change in the specific optical rotation by the interconversion of α and β forms of D-glucose to an equilibrium mixture. In water, α-D-glucopyranose and β-D-glucopyranose interconvert through the open chain form of the sugar. This interconversion was detected by optical rotation. 18 The specific rotation [α]D, of the α and β anomers of D-glucose are +112o and +18.7o. When crystalline sample of either anomers is dissolved in water, specific rotation [α]D changes with time until an equilibrium value of + 52.7o is attained. This change called mutarotation, results from the formation of an equilibrium mixture containing about one-third α-anomers and two-thirds β-anomers. Very little of the open chain form of glucose is present (