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I am happy to complete the first edition of this book on the Biochemistry of carbohydrates to the level of expectations. Writing a textbook is timeconsuming, and completing the book would take much longer if I covered all biomolecules. Therefore, I first thought of concentrating on carbohydrates, the most important energy-rich biomolecule, having a multifunctional role even as one of the components of nucleic acids, which are crucial for reproduction and expressing various genotypic and phenotypic characteristics. Carbohydrates are found universally, are the primary energy source, and a very vast topic of biochemistry as they are found in making clothes, many industrial applications, and are modified to prepare medicines and many important substitutes. The textbook includes chemistry, structure, source, metabolism, energy, and carbohydrate-related disorders. The book is an extract of hard work over the years, what I learned from my teachers and mentors, discussions with the other teachers, interactions with many students, research scholars, and as a researcher publishing International and National research papers, and as a Guide to postgraduate research students, and reviewing many articles. Understanding the importance of carbohydrates is taught at various colleges and universities offering bachelor's and postgraduate programs in Basic sciences, Pharmacy, Agriculture, Home Science, Nursing, Veterinary, and Medical branches. This book will also be helpful to students, faculty members, and research workers engaged in different fields. It also has precious references for research workers. Further, even nontechnical readers are eager to gain technical knowledge in straightforward language and understand the science of carbohydrates. Attempts have been made to incorporate the technically and grammatically updated information. The figures in the book are simple to understand for all students and technical staff working in various disciplines. All suggestions from readers are welcome for improvement.

1.1 Carbohydrates Biochemistry Carbohydrates are essential for life, both in plants and in animals. They are organic chemical compounds made of C, H, and O. These elements get arranged in different ways so that several simple, and stored complex compounds (starch in plants and glycogen in animals) are available in abundance on the Earth. They are involved in enormous biological and nonbiological functions. Their biological functions, found in all living things, are the chief energy source for men and animals, even for herbivorous fishlike carp and rohu fingerlings. Carbohydrates are also essential structural components like nucleic acids, containing genetic information. One must recognize the protective function in cells, plants, and human health management. Even bacteria form a glycocalyx and biofilm for protection. Knowledge about their involvement in blood groups helped to identify universal donors, universal recipients, and genuine donors for the recipient to save a life. Carbohydrates are found as a storage structure for energy in emergency conditions. Upon combustion, they yield carbon dioxide and water. Autotrophs (plants) synthesize them from carbon dioxide and water to grow, reproduce, and store, but heterotrophs (humans and other animals) use them, too. In plants, they occur as fibers, supporting structures, and simple sugars. The plant carbohydrates are used for preparing adhesives, fermentation, foods, and papers. Cotton fibers are used to make clothes to cover the body and protect humans and animals against adverse climatic conditions.

Carbohydrates are the most abundant class of organic compounds found in living organisms. Carbohydrates are also called sugars and are found in plants and animals. Plants synthesize them through photosynthesis (endothermic) by fixing carbon, hydrogen, and oxygen in the presence of solar energy and chlorophyll pigments. nCO2 + nH2O + energy → CnH2nOn + nO2 Carbohydrates are stored as starch in plants as a source of energy. They are also stored in animals as glycogen in muscles and the liver. They are also found in skeletal structures of cell walls in plants (cellulose), bacteria (peptidoglycans), insects, and crustaceans. This stored food is consumed by animals and man, and upon their oxidation in the body, energy is released for various metabolic activities (work, growth, and various productive functions). Compositionwise, plants are more prosperous in carbohydrates (30%) than animals (1%), with water at 60% and ash at 4%. Plants are poor in proteins (5%) and lipids (1%) as compared to proteins (20%) and lipids (15%) in animals. 2.1 Introduction to Carbohydrates Carbohydrates or saccharides (sakcharon=sugar or sweetness) are a chemical term applied to a large group of substances, which are organic compounds containing elements Carbon, Hydrogen, and Oxygen in a ratio of 1:2:1. Hydrogen and Oxygen atoms are in the ratio of two to one as in water molecules. As the ratio of hydrogen to oxygen is like H2O, they are also known as “hydrates of carbon” and are represented as (CH2O)n. But there is an exception to this general formula. Deoxyribose (C5H1OO4) and rhamnose (C6H12O5) do not have the required ratio of 2:1 for hydrogen to oxygen. On the other side, some compounds possess this required ratio but are not carbohydrates, e.g., formaldehyde (CH2O), acetic acid (C2H4O2), and lactic acid (C3H6O3). Further, some carbohydrates have nitrogen(glucosamine), phosphorus, and sulfur but do not satisfy the above general formula. The simplest sugars are called monosugars or monosaccharides. There are two main functional groups in

Hydrates carbons (saccharides) found in nature (animals and plants) are classified into three groups based on the number of monosaccharides found or released upon acid hydrolysis. Monosaccharides the groups of carbohydrates. 3.1 Formulation of Monosaccharides The molecular formula of any monosaccharide (mono=one and saccharide=sugar) is Cn(H2O)n or (CH2O)n. Rhamnose and Deoxyribose are exceptions to the general formula of carbohydrates. Acetic acid (C2H4O2) and formaldehyde (CH2O) are not carbohydrates but have the formula Cn(H2O)n or (CH2O)n. Fitting and Baeyer proposed the formula CHO-(CHOH)4-CH2OH, which indicates the presence of aldehyde and five hydroxyl groups. More than one monosaccharide molecule unites to form disaccharides, oligosaccharides, and polysaccharides. Monosaccharides, being simpler units, cannot be broken down further. Generally, they have two to ten carbon atoms per molecule (dioses to decoses). The monosaccharides may have either aldehyde or ketone groups. The one possible aldodiose (CH2OH.CHO) is glycolaldehyde (2-hydroxyethanal), but ketodiose is not possible since there are only two carbons. The glucose molecular formula C6H12O6 represents the number of C, H, and O atoms but not groups. However, the proposed formula, CHO-(CHOH)4-CH2OH, indicates the presence of an aldehyde and five hydroxyl groups. These simple sugars or monosaccharides have carbon backbones linked by single bonds with hydrogen and hydroxyl groups attached in an unbranched manner.

Oligosaccharides comprise 2 to 10 units of monosaccharide molecules, which are released upon hydrolysis. The oligosaccharides are classified depending upon the number of monosaccharide units, e.g., Disaccharides (two monosaccharide units), Trisaccharides (three monosaccharide units), Tetrasaccharides (four monosaccharide units). Disaccharides are the most common oligosaccharides found in nature. 4.1 Disaccharides Disaccharides are formed when glycosidic linkages covalently bond two monosaccharides together. A glycosidic bond is formed when the hydroxyl group on one of the sugars reacts with the monomeric carbon on the second sugar. A glycosidic bond can be alpha or beta, depending on the steric configuration at carbon one on the monosaccharide unit. During the union of two monosaccharide units, water molecules are eliminated (dehydration reaction or condensation reaction, or dehydration synthesis), and the formation of a glycosidic bond occurs, through which the units are linked via an oxygen bridge.

Polysaccharides comprise several monosaccharide units (more than ten) linked by glycosidic bonds; therefore, they are high molecular weight carbohydrates. They are also called polyanhydrides of simple sugars. The most common monosaccharide unit of polysaccharides is D-glucose. The polysaccharides may differ in their monosaccharide units, molecular weight, chain nature, glycosidic bond types (alpha, beta), and linkages (1-2, 1-3, 1-4, or 1-6). They are tasteless, colorless, amorphous powders that are a little water-soluble. Some may form colloidal solutions. The polysaccharides provide structural stability (structural polysaccharides) to cells, prevent cells and tissues from drying out due to their water-binding properties, and reserve food for the future. Reserved polymeric carbohydrates are osmotically less active and can be stored in large quantities within the cell. The right and left end of polysaccharides, respectively, are called the reducing and non-reducing ends. Based on the functional aspects, they are grouped in two ways. • The first way depends on its digestibility—1. Nutrient polysaccharides are (digestible) starch, dextrin, and glycogen, and 2. Structural polysaccharides are (indigestible), chitin and cellulose. • The second way, depending upon the constituent monosaccharide units, they are 1. Homoglycans (same units), and 2. Heteroglycans (different units).

6.1 Metabolism Dietary components like carbohydrates, lipids, and proteins are continuously broken down and synthesized through various chemical reactions (exergonic or endergonic) that are concerned with energy. These processes not only break them down but also interconvert them for their synthesis. Dietary components, when utilized, are known as nutrients the body. The number of reactions occurring in living cells maintains the body cells and fluid’s homeostasis and dynamic status. To keep the dynamic state of a cell, many reactions are going on to synthesize or break down the different types and sizes of molecules. The total of these reactions is known as metabolism. The metabolism may be anabolic or catabolic. Further, it may be aerobic or anaerobic, depending on oxygen availability. These four terms are explained below: I. Aerobic and Anaerobic Metabolism • Aerobic metabolism-- In aerobic metabolism, the carbohydrates (substrate) in the presence of oxygen are metabolized to release energy, carbon dioxide, and water. The released energy is converted to ATP by two processes in the mitochondria (the Krebs cycle and oxidative phosphorylation). In oxidation, the electrons released from a glucose molecule are transferred to reduced NAD+ and FAD. The oxygen (electron acceptor) is reduced to water. Water formation [from H2(g) and O2(g)] is an exothermic reaction. The energy released from the chemical bond of glucose is captured as ATP, and heat is released.

Glycolysis involves a series of reactions to extract energy from glucose to release two molecules with three carbons called pyruvate. The process is not dependent on oxygen but can occur under aerobic and anaerobic conditions, releasing different end products. Under aerobic conditions, the pyruvate formed is then oxidized in the mitochondria to CO2 and H2O. The energy released in glycolysis is in the form of ATP (High Energy Bond). All the reaction steps occur in the cytoplasm, as it is the extramitochondrial pathway. In anaerobic conditions and the absence of mitochondria (RBC), the end product of glycolysis is lactic acid. The net reaction of Glycolysis (anaerobic) may be summarized as under Glucose + 2ADP + 2Pi → 2 Lactate + 2ATP 7.1 Steps in EMP Cycle or Glycolytic Pathway Table 7.1: Steps and enzymes of glycolysis or EMP cycle.

The Hans Krebs cycle, known by different names like the citric acid or tricarboxylic acid cycle, was named after the scientist Hans Krebs. The citric acid cycle is named after the citrate product formed in the cycle. The name tricarboxylic acid is given as citrate itself, which is a tricarboxylic acid due to the presence of three carboxyl functional groups (-COOH). The tricarboxylic acid (TCA) cycle occurs in aerobic conditions and within mitochondrial ETC, involving various sequential oxidoreductase reactions. The cycle requires Acetyl-CoA (C2 molecule) and oxaloacetate molecule (C4 molecule) to begin with the formation of a citrate molecule (C6 molecule). Acetyl-CoA is generated from the pyruvate (glycolysis), the fatty acids (lipolysis), or by oxidative deamination of specific amino acids. The complete oxidation of a glucose molecule results in the end products like energy (ATP), CO2 and H2O. A molecule of Acetyl-CoA has two carbons. It is formed from a three-carbon pyruvic acid molecule generated from the glycolysis of a sixcarbon glucose molecule. Acetyl-CoA is the substrate for this cycle and is an energy-rich molecule having a thioester bond between coenzyme A and acetic acid. 8.1 Formation of acetyl-CoA from Pyruvic Acid Pyruvate is obtained by glycolysis or the oxidation of glucogenic amino acids. The pyruvic acid produced in the cytoplasm crosses the inner membrane of the mitochondrion to enter the mitochondrial matrix, where it undergoes oxidative decarboxylation, forming Acetyl-CoA. Acetyl-CoA carries (carrier molecule) the acetyl group to the citric acid cycle. It also links various metabolic products with the TCA cycle. Such products can be from the EMP cycle, beta-oxidation, and oxidative deamination of amino acids. Oxidative decarboxylation of pyruvic acid needs a complex enzyme system known as pyruvate dehydrogenase

This alternative pathway for glucose oxidation has enzymes in the cytosol through which glucose is degraded to carbon dioxide. It is also called a shunt because carbon atoms from glucose 6-phosphate take a long route before they proceed down the EMP or glycolytic pathway. This pathway is anabolic in nature as it produces many intermediates. In this pathway, two NADPH are generated from one glucose 6-phosphate molecule. No ATP is generated or consumed. Overall, this pathway needs three glucose-6-phosphate molecules to complete all the reactions. Therefore, one cycle run produces 2 mol of fructose-6- phosphate, 1 mol of glyceraldehyde-3-phosphate, and 3 mol of carbon dioxide from 3 mol of glucose-6-phosphate with the reduction of 6 mol of NADP+ to NADPH. Overall reactions of the HMP pathway are as follows: 3 Glucose-6-P + 6 NADP+→ 3 ribulose-5-P + 3 CO2 + 6 NADPH 3 Ribulose-5-P → 2 xylulose-5-P + Ribose-5-P 2 Xylulose-5-P + Ribose-5-P → 2 fructose-6-P + Glyceraldehyde-3-P

The uronic acid pathway is a minor and alternative pathway for glucose oxidation and for the production of uronic acid sugar. Uronic acids of sugars are formed when the terminal carbon’s (C6) hydroxyl group (CH2OH) of sugar gets oxidized to a carboxylic acid. Uronic acid names depend on the name of the parent sugars. Uronic acid of glucose is glucuronic acid, hexose uronic acids are hexuronic acids, and pentose uronic acids are named penturonic acids. The uronic acid pathway is located in the cytosol of the liver and adipose tissue. The glucuronic acid should not be confused with gluconic acid (aldonic acid) and glucaric acid (aldaric acid or dicarboxylic acid). Gluconic acid (aldonic acid or “onic” acid) is obtained from D-glucose by oxidizing its aldehyde group at C1 to a carboxyl group. Glucaric acid (aldaric or dicarboxylic acid) is formed upon oxidation of both the terminal hydroxyl groups (C1 and C6) and the aldehyde. They have two carboxylic acid groups. This pathway yields pentoses & ascorbic acid or vitamin C (not in primates & guinea pigs) from glucose.

Plants, with the help of photosynthesis, convert simple molecules (like carbon dioxide and water) into more complex molecules (anabolism) through various chemical reactions. Animals synthesize more complex molecules (anabolism) from monomer units through multiple chemical reactions. Anabolism refers to a biosynthesis process involving the sequences of biochemical reactions by which complex molecules are formed in living cells from available monomer units. Anabolic processes include the biosynthesis of complex biomolecules like carbohydrates, proteins, and lipids. The anabolic processes require energy from high-energy bond compounds like ATP. The anabolic processes related to the biosynthesis of carbohydrates include (1) Gluconeogenesis and (2) Glycogenesis 11.1 Gluconeogenesis Gluconeogenesis is the synthesis of new glucose molecules from noncarbohydrate molecules. The non-carbohydrate molecules that are used for gluconeogenesis are derived from glycolysis, lipolysis, and proteolysis. Examples are pyruvate, lactate, glycerol, and glucogenic amino acids. The liver and kidneys are the preferred organs for the gluconeogenesis process. It also occurs to some extent in the brain. Blood glucose levels are maintained at physiological concentrations through the process of gluconeogenesis in humans and ruminants. The blood glucose drops below normal levels during various conditions like fasting, starvation, pregnancy, lactation, fever, and low carbohydrate diets.

Glycogenesis helps to maintain blood glucose, as it is a primary source of energy for every cell. An inadequate supply of glucose to the body may lead to the failure of the vital organs. Glucose, when in excess, is synthesized into glycogen and stored (glycogenesis), mainly in the liver and muscles (including the uterus). It is also found in other tissues, such as the kidney, brain, heart, and adipose tissues. The stored liver glycogen is broken down (glycogenolysis) into blood glucose when needed. Still, the glycogen stored in muscle is broken down to glucose 1-phosphate and converted to blood glucose later. Either the synthesis or breakdown pathway is primarily active at a time in tissues. Glycogen synthesis and its breakdown are not reversible processes. The percentage of glycogen stored in skeletal muscles is less ( 1% to 2%) than in the liver (10%). Still, the total quantity of glycogen is greater in muscles due to their greater mass than an animal’s total liver mass. Muscle mass, on average, ranges from 40 to 45 % of body weight for the most mature mammals. Glycogen synthesis (glycogenesis) is an anabolic process forming a branched homopolysaccharide made of several chains of alpha D-glucose units linked by α-1,4 glycosidic bonds in a linear chain and nearly every ten glucose residues; the chain branches off via α-1,6 glycosidic linkages. The α-glycosidic bonds give rise to a helical polymer structure. The number of glucose units in a single glycogen molecule varies (1,700 to -600,000 units). The glycogen granules are formed on the glycogenin protein in the center, so it acts as a primer, and their number determines the total number of glycogen particles.

Glycogen breakdown is known as glycogenolysis. This breakdown occurs in the cytoplasm of cells of the organs, wherein glycogen is stored. The end product of glycolysis is glucose in the liver, which is glucose-1-phosphate in skeletal muscles. Glycogen is a polysaccharide synthesized from glucose monomers. Glycogen is stored in the cell instead of glucose because glucose requires more water, whereas glycogen can be stored with much less water. Most of the stored glycogen is in the liver (5% of body weight) and muscle (1 to 2 % of body weight). Muscle and liver glycogen undergo glycolysis for different purposes. Muscle glycogen provides energy for muscular activity, whereas liver glycogen helps to maintain the blood glucose level. Glycogen contains many glucose residues (up to 120,000) linked by alpha 1-4 glycosidic bonds, forming a straight chain. Glycogen has branches at alpha 1-6 glycosidic bonds at every ten glucose residues. At the reducing end, the carbon one of glucose is free, whereas it is not accessible in the case of non-reducing ends.

Carbohydrates (glucose) are the primary energy source for a body; when digested, glucose is the end product in human beings and simple stomach animals, but it yields volatile fatty acids in ruminants. Precise regulation of levels of blood sugars/glucose in the body is the net result of various processes like digestion, absorption, and metabolism operating under the interaction of various a) enzymes and b) hormones secreted by the different organs of the body. An abnormal secretion of any of the enzymes or hormones associated with the metabolism of carbohydrates disrupts the precise regulation of blood sugar, leading to various disorders. 14.1 Enzyme Deficiency and Disorders in Carbohydrate Metabolism Usually, various enzymes are involved in the breakdown of carbohydrates, and if their secretion is insufficient, missing, or does not work correctly, it leads to different disorders. I. Glycogen storage diseases (GSDs, glycogenosis, and dextrinosis) Glucose is needed by the cells for the generation of energy and other cellular activities. If glucose happens to be in excess, it is stored as glycogen, mainly in the liver and muscles, for future needs. The stored glycogen can be converted back to glucose when the need arises. Many enzymes are related to this interconversion of glucose and glycogen. Deficiencies of one or more enzymes may lead to abnormal glycogen metabolism, leading to glycogen accumulation of normal or abnormal chemical structure in tissue, collectively called glycogen storage diseases (GSDs). Roman numerals identify different types of glycogen storage conditions. They are also classified under various categories like inherited metabolic diseases, autosomal recessive type, and X-linked type (VIII/IX type).

The number of hormones secreted by the body exhibits control over the physiological regulation of glucose sugar level (homeostasis) in the manner needed by the body. Depending upon the situation, hormones may increase or decrease the level of glucose sugar present in the blood or tissues. The glucose is oxidized to yield energy, carbon dioxide, and water. Glucose, more than the body’s needs, is stored or converted mainly to glycogen and, to some extent, protein and fat. If glucose is less than the need of the body, its denovo synthesis takes place from amino acids or fatty acids, or is formed from the breakdown of stored glycogen. Various hormones are secreted by multiple endocrine glands to control the blood glucose level. If it fails, it leads to hypoglycemia or hyperglycemia conditions. Gluconeogenesis is not developed at birth in the case of a baby pig. Therefore, hypoglycemia in baby pigs is observed within 2 days of age if starved, leading to hypoglycemia, weakness, confusion, and death. 15.1 The Hormones Involved in Homeostatic Regulation of Blood Glucose Level The hormones influencing carbohydrate metabolism are explained below: