Molecular Structure and Function Polysaccharides as Energy Storage. Biochemistry

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2 1.Objectives Dr. Vijaya Khader Dr. MC Varadaraj To understand how polysaccharides act as energy source To understand the structure and energy generation process from glycogen To understand the structure and energy generation process from Starch To understand the structure and energy generation process from Cellulose 2. Concept Map A road map of polysaccharide to energy 2

3 3. Description 3.1 Polysaccharide, as energy reserve Polysaccharides are the buildings of the monosaccharides, which are cemented by glycosidic bonds. These polysaccharide undergoes their enzymatic breakdown to form simpler sugar, mainly glucose to form biological energy (ATP) and the green energy biofuel. These polysaccharides play central roles in storage of energy as well as the structural backbone formation in plant and animals. If the polysaccharide contains only one type of monomeric unit then it is called homopolymers and if different types of monomeric unit imparts in them, then these polymers are called hetropolymers. There are following three major polysaccahrides which are considered to be the energy blocks : 1. Glycogen (in liver and muscle of animal cell) 2. Starch (in grains) 3. Cellulose (in plant cell wall) Glycogen, an energy reserve polysaccharides A polysaccharide, glycogen, is the most common storage form of glucose in animal cell.glucose molecules are linked with α -1,4-glycosidic bonds in linear chain while synthesis of glycogen and branching is present after 10 glucose units linked with α -1,6-glycosidic bonds (Fig. 1). In humans, glycogen is the secondary long term energy storage material (primary energy storage material is fat of adipose tissue), which is synthesized and stored in liver cells (hepatocytes: up to 8% of its fresh weight) and muscles (1-2% of the muscle mass). Glycogen stored in the liver is a source of glucose mobilized during hypoglycemia (low glucose concentration in blood). Muscle glycogen is stored as an energy reserve for muscle contractions.whereas in plants energy is stored in the form of starch. Glycogen is structurally very similar to amylopectin (a component of starch), but more extensively branched and compact than starch. Fig.1 Structure of glycogen (Source: Strayer s ) 3

4 Function of Glycogen In Liver In liver after the digestion of carbohydrate rich food, the level of blood glucose rises. The excess glucose enters into liver cell (hepatocytes) through the portal vein. At this time pancreas secretes insulin which acts on the hepatocytes to stimulate the action of glycogen synthesizing enzyme glycogen synthase. This enzyme adds the glucose molecules to the chains of glycogen. As the level of glucose begins to fall, insulin secretion is reduced, and glycogen synthesis stops. When there is a need of energy glycogen phosphorylase starts the breakdown of glycogen which make up the desired blood glucose level for next 8 12 hours, and fulfil the energy need of the body. In access of glucose, pancreas also secretes another hormone, glucagon which stimulates both glycogenolysis (the breakdown of glycogen) and gluconeogenesis (synthesis of glucose from noncarbohydrates like pyruvate). In Muscle Muscle cell lack the enzyme glucose-6-phosphatase (an enzyme required to dephosphorylate the glucose 6-phoshphate so that free glucose can escape from cell and enter in to blood stream), therefore the muscle serves as sole energy reserve for muscle s internal use and is not shared with other cells Extraction of energy from Glycogen Fig. 2 Pathway of glycogen breakdown (glycogenolysis) in the liver and energy generation 4

5 Glycogen on breakdown gives glucose through glycogenolysis, which further metabolised through glycolysis in to ATP and NADH (reduced power) Fig.3. Detail metabolism is described in glycogen metabolism. In short Glycogen phosphorylase and inorganic phosphate is required to hydrolyse α(1,4) glycosidic linkage at nonreducing end of glycogen, and debranching enzymes (Amylo-α(1,6)-glucosidase) to remove glucose from α(1,6) branch points. Fig. 3 Glycogen degradation from non-reducing ends 5

6 Fig. 4 Glycogen degradation by debranching enzymes Glucose released after hydrolysis is in phosphorylated and enters in to glycolysis, Tri-carboxalic acid (TCA) cycle and Electron transport system to produce energy (ATP). In liver cells the enzyme Glycogen Phosphorylase is activated by epinephrine and glucagon while it is inhibited by insulin. Besides, this in muscle cells Glycogen Phosphorylase is activated by Epinephrine, AMP and Ca 2+ (through calmodulin) insulin while it is inhibited by insulin and ATP Starch as energy reserve polysaccharide Starch is the most copious storage polysaccharide of green plants except the family Asteraceae where fructan inulin is stored instead of starch. Common starch storage organs of the plants are cereal grains, seeds, tubers and roots. Starch crops not only fulfill the nutritional need of humans and animals but also it is one of major renewable feedstock for first generation Biofuel based biorefineries due to its readily fermentive sugar producing property. 6

7 Starch consists of two glucose based polymers, Fig. 5 Structure of amylase and amylopectin (i) Amylose (unbranched and helical): It constitutes 20-25% (w/w) of total starch of plants. It consists of long and unbranched chains of α-1,4 linked D-glucose residues. These chains possess molecular weight of a few thousand to more than a million (Fig. 5a). (ii) Amylopectin (branched): It is the major component of starch granules and constitute % (w/w) of total starch of plants. The molecular weights of these are up to 100 million. In it D- glucose residues are linked together by α-1,4 linkage to form the linear chains of 6 to >100 residues and these chains are branched by α-1,6-linkages after every 24 to 30 residues. The granular nature of starch is contributed by these. (Fig. 5b) Starch to energy production Starch based crops can be used for production of energy in following ways: Starch to ATP generation in animals The digestion of starch starts in mouth, where the glycosidic linkage is hydrolyzed by α-amylase present in saliva and it produces maltose. The α-amylase enzyme along with starch food passes through esophagus, but as it reaches in stomach they become inactivated due to acidic environment in the stomach. In small intestine the remaining starch material is also converted into maltose by α-amylase secreted by it. Maltose is then further cleaved into glucose (2 molecules) by maltase and passed to blood stream by absorption through the wall of small intestine (Fig. 6). The glucose molecule released from starch crops undergoes the routine cellular metabolic processes to produce the fuel (ATP). 7

8 Starch to Biofuel production Fig. 6 Starch to energy (ATP) generation in animal cell The starch crop undergoes pretreatment and saccharification processes to produce fermentable sugars. In saccharification, starch is hydrolyzed into glucose by chemical or biological processes. In biological saccharification processes amylolytic microorganism or their enzymes (α-amylase and glucoamylase) are used. The fermentable sugars obtained are then exposed to microbial fermentation (commonly yeast) process to produce ethanol (Fig. 7). 8

9 Fig. 7 Schematic presentation of Starch to bioethanol production process Cellulose as energy producing polysaccharides Cellulose is one of the most abundant natural polysaccharide and it generally occurs in lignocellulosic material along with the hemicelluloses and lignin. The lignocellulosic wastes such as wheat bran, sorghum stover, grasses (switchgrass), bamboo etc are trendy cellulose based feedstock for ethanol production. Cellulose is a most abundant unbranched complex homopolysscharide formed by association of several to many thousands β-(1,4)-d, glycosidic linked glucose subunits. Hydrogen-bond and van der waals interactions are involved in the microfibriller stacking of the cellulose chains (Fig. 8). 9

10 Cellulose to Energy production Fig. 8 Structural organization of cellulose in lignocellulose Cellulose to ATP generation in animals The cellulose digesting enzymes are absent in human, therefore it is considered as dietary fiber. But some animals such as termites and ruminants possess cellulase producing microbes such as Trichnympha in their guts, they digest cellulose into glucose and produce energy (ATP) by metabolic processes Cellulose to Biofuel production Cellulose present in plant has been explored to get energy in the form of biofuel to replenish the depleting reserves of petroleum fuel. In order to get energy in the form of ethanol cellulose is being made available to microorganism to ferment from the lignocellulosic complex matrix of plants. To release cellulose from lignocelluloses it has to be retreated so that lignin the hard cover over it can be removed. Following are the sequential steps through which ethanol can be produced from lignocellulosic biomass. The crystalline nature of cellulose, provide hindrance in accessibility of cellulase enzymes and the subsequent conversion of cellulose to glucose. Therefore, an efficient pretreatment process and potential saccharification enzymes are the driving force for the cellulosic ethanol production efficiency (Fig.9). 10

11 Fig. 9 schematic representation of Cellulose to ethanol production process 4. Summary In this lecture we learnt about: Glycogen acts as principal energy storage polysaccharide in the liver and muscle. In the liver, glycogen compensate glucose level for extrahepatic tissues and in muscle, it serves mainly as a ready source of metabolic fuel. Glycogen Breakdown is an enzyme dependent process and Epinephrine and Glucagon Signal the Need for Glycogen Breakdown. Glycogenolysis is the process of glycogen breakdown to release glucose in liver. Starch is the most abundant storage polysaccharide in the plant cell, which can be hydrolysed by amylase in the body to produce glucose to generate energy (ATP). Starch crops can be use for first generation ethanol production by amylase based saccharification and fermentation. But since, starch crops are the major food components for the society; therefore it causes Food Vs. Fuel issue. Cellulose is the most abundant feedstock occupy its existence in the surplus lignocellulosic waste, therefore it solves the starch related issue i.e. Food Vs. Fuel, hence cellulosic feedstocks are the suitable candidate for 2 nd generation Biofuel. The three polysaccharide glycogen, starch and cellulose are the major energy storage polysaccharides for the production of energy i.e. ATP and Biofuel. 11