Carbohydrates Lecture3

Polysaccharide

Polysaccharides and sácchar means "sugar“ From the Greek:  poly means “many", and  sácchar means "sugar“ Polysaccharides are complex carbohydrates, composed of 10 to up to several thousand monosaccharides arranged in chains. The most common monosaccharides that appear as parts of polysaccharides are glucose, fructose, galactose and mannose. Three main polysaccharides related to the human nutrition include: Starch ─ an energy source and storage form of glucose in plants Glycogen ─ an energy source and a storage form of glucose in the human liver and muscles Cellulose ─ a structural polysaccharide in plants

Polysaccharides Starch ─ an energy source obtained from plants: Starch is (polymer of glucose) a polysaccharide comprising glucose monomers joined by glycosidic bonds. It consists of two types of molecules: amylose and amylopectin. Amylose : is the simplest form of starch; it is the linear polymer of glucose monomers joined in α (1,4) linkages Amylopectin: is the branched form. Branching takes place with α(1→6) bonds occurring every 24 to 30 glucose units, resulting in a soluble molecule that can be quickly degraded

Polysaccharides Starch ─ an energy source obtained from plants: Starch is the most common carbohydrate in the human diet and is contained in many staple foods. The major sources of starch intake worldwide are the cereals (rice, wheat, and maize) and the root vegetables (potatoes and cassava). The enzymes that break down or hydrolyze starch into the constituent sugars are known as amylases. Alpha-amylases are found in plants and in animals. Human saliva is rich in amylase, and the pancreas also secretes the enzyme: ** secreted Alpha-amylases hydrolyse dietary starch into disaccharides and trisaccharides which are converted by other enzymes to glucose to supply the body with energy

Polysaccharides 2. Glycogen ─ an energy source and a storage form of glucose in human Glycogen is the analogue of starch! It is a glucose polymer that functions as energy storage in human. It has a structure similar to amylopectin (a component of starch), but is more extensively branched and compact than starch.

Polysaccharides 2. Glycogen ─ an energy source and a storage form of glucose in human Glycogen is mainly stored in muscles and liver >> it provides the body with a readily available source of energy if blood glucose levels decrease. Glycogen breakdown into glucose: (glycogen -> G1P -> G6P -> Glucose). >> Glycogen Phosphorylase catalyzes breakdown of glycogen into Glucose-1-Phosphate (G1P) >> G1P then into G6P under the action of Phosphoglucomutase enzyme >> G76P into Glucose under the action of glucose phosphatase enzyme

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Polysaccharides 3. Cellulose ─ a structural polysaccharide in plants: Cellulose, a major constituent of plant cell walls, It is a linear chain of several hundred to many thousands of glucose joined with ß (1,4) linkages. Every other glucose is flipped over, due to ß linkages. Cellulose is considered insoluble because it does not bind with water nor change form in the digestive tract.

Polysaccharides 3. Cellulose ─ a structural polysaccharide in plants: Humans are unable to digest cellulose because the appropriate enzymes to breakdown the beta linkages are lacking. Despite the fact that humans (and many other animals) cannot digest cellulose, cellulose is a very important part of the healthy human diet. >> This is because it forms a major part of the dietary insoluble fiber that we know is important for proper digestion. >> insoluble fiber (like cellulose) travels through the digestive system unchanged, >> it helps move waste through your digestive tract, which prevents constipation.

Polysaccharides Three main polysaccharides related to the human nutrition

Carbohydrate Digestion Begins in mouth chewing releases saliva enzyme amylase hydrolyzes starch to maltose Stomach no enzymes available to break down starch acid does some breakdown fibers in starch provide feeling of fullness

Small intestine majority of carbohydrate digestion takes place here. pancreatic amylase reduces carbohydrates to glucose chains or disaccharides. specific enzymes finish the job maltase maltose into 2 glucose sucrase sucrose into glucose and fructose lactase lactose into glucose and galactose

>> then they are transferred to the blood stream. Large intestine Sugars and starches usually take 1-4h to be totally digested and absorbed by small intestine >> then they are transferred to the blood stream. Carbohydrates that are not fully digested or absorbed (fibers), they are eliminated from the body. All undigested carbohydrates move from the small intestine to the large intestine and to the colon, where elimination finally occurs. Fiber can attract water, which softens stool Bacteria ferment some fibers

Carbohydrate Absorption glucose can be absorbed in the mouth. majority absorbed in small intestine. There are two major ways that molecules can be moved across a membrane, and the distinction has to do with whether or not cell energy is used. I. Passive mechanisms (use no energy): simple diffusion: the transportation of molecules across the cell membrane depending on their concentrations gradient (from low to high) such as glucose and galactose. B. facilitated diffusion: is diffusion that is facilitated by a membrane transport channel. These channels are almost always specific for either a certain molecule or a certain type of molecule. Example: FRUCTOSE- (GLUT5) Active transport (requires energy from the cell): Sometimes the body needs to move molecules against their gradient. The transportation of molecules across a specific transmembrane protein such as fructose. Example: Glucose and Galactose-(Na+/glucose cotransporter)

Lactose Intolerance more lactose is consumed than can be digested lactose molecules attract water cause bloating, abdominal discomfort, diarrhea intestinal bacteria feed on undigested lactose to produce acid and gas

Carbohydrate Metabolism When we eat – intake glucose increased therefore, the liver condenses extra glucose to glycogen. When blood glucose falls, the liver hydrolyzes glycogen to glucose 1/3 of body’s glycogen is stored in liver and released as glucose to bloodstream Inadequate supply of carbohydrates ketone bodies (fat fragments) are an alternate energy source during starvation excess ketones can lead to ketosis: imbalance of acids in body when carbohydrates are severely restricted, the liver converts fat into fatty acids and ketone bodies to be used as an alternative energy source in a process called ketosis

Glucose Homeostasis Maintaining an even balance of glucose is controlled by insulin and glucagon insulin moves glucose from the blood glucagon brings glucose out of storage

1 2 3 4 5 6 7 Intestine Fat cell Muscle Pancreas Glucose Insulin When a person eats, blood glucose rises. 1 2 Insulin stimulates the uptake of glucose into cells and storage as glycogen in the liver and muscles. Insulin also stimulates the conversion of excess glucose into fat for storage. 3 4 5 6 7 Blood glucose begins to rise. a The stress hormone epinephrine and other hormones also bring glucose out of storage. Glucose Insulin Glucagon Glycogen Glucagon stimulates liver cells to break down glycogen and release glucose into the blood.a Liver Low blood glucose stimulates the pancreas to release glucagon into the bloodstream. As the body's cells use glucose, blood levels decline. Pancreas Fat cell Muscle High blood glucose stimulates the pancreas to release insulin.

Imbalance diabetes after food intake, blood glucose rises and is not regulated because insulin is inadequate. hypoglycemia blood glucose drops dramatically too much insulin, activity, inadequate food intake, illness.

REFERENCES Murray, R., Bender, D., Botham, K., Kennelly, P., Rodwell, V., Weil., P. (2012) Harper’s illustrated biochemistry, 29th edition. Publisher: McGraw Hill Lange. Nelson, D. and Cox, M. (2008) Lehninger principles of biochemistry, 5th edition. Publisher: W.H. Freeman and company, New York. Harvey, R. and Ferrier, D. (2011) Lippincott’s illustrated reviews: biochemistry, 5th edition. Publisher: Lippincott Williams & Wilkins. Moorthy, K. (2008) Fundementals of Biochemical Calculations, 2nd edition. Publisher: CRC Press. Weil, P.A., (2012) The diversity of the endocrine system. Chapter 41 in HARPER’S Illustrated biochemistry, 29th edition. Edited by: Murray, R.K., Bender, D.A., Botham, K.M., Kennelly, P.J., Rodwell, V.W., Weil, P.A. Sam, A. and Meeran, K. (2009) Lectures notes in endocrinology and diabetes. Publisher: Wiley-Blackwell. Preston, R.R. and Wilson, T.E. (2013) Endocrine system in Physiology. Publisher: Lippincott Williams & Wilkins.