Carbohydrate Metabolism PDF

METABOLISM OF CARBOHYDRATES Lesson 8 METABOLISM OF CARBOHYDRATES • Characteristics, structure and classification of carbohydrates. • Digestion and absorption of carbohydrates. • Cell synthesis and degradation of glucose. • The Cori cycle. Glucose-alanine cycle. • Synthesis and degradation...

METABOLISM OF CARBOHYDRATES Lesson 8 METABOLISM OF CARBOHYDRATES • Characteristics, structure and classification of carbohydrates. • Digestion and absorption of carbohydrates. • Cell synthesis and degradation of glucose. • The Cori cycle. Glucose-alanine cycle. • Synthesis and degradation of glycogen STRUCTURE CHARACTERISTICS • Hydrosoluble molecules • The name means “carbon hydrate”  Their empirical formula (CH2O)n • Monomeric subunits  Monosaccharides  serve as the building blocks of large carbohydrate polymers FUNCTIONS CARBOHYDRATES CLASSIFICATION Monosaccharides Linkage of various monosaccharides Sugars Simple Derivate Simple sugars Aldoses Ketose Aldehyde group Ketone group Oligosaccharides Polysaccharides 2-10 >10 Homopolysaccharides The same monomer Heteropolysaccharides Different monomers MONOSACCHARIDES Number of carbons Name Example Sweet taste  Sugars 3 Trioses Glyceraldehyde Classification according to carbonyl group (C=O) Aldoses  aldehyde group Ketoses  ketone group 4 Tetroses Eritrose 5 Pentoses Ribose 6 Hexoses Glucose, Fructose, Galactose • Soluble in water • • MONOSACCHARIDES OLIGOSACCHARIDES • Between 2 to 10 monosaccharides joined by glycosidic bonds. OLIGOSACCHARIDES • DISACCHARIDES  2 MONOSACCHARIDES Lactose Galactose + Glucose Sucrose (Saccharose)  Glucose + Fructose Maltose  Glucose + Glucose POLYSACCHARIDES • Formed by more than 10 monosaccharides joined by glycosidic bonds. • Differ from each other in : the identity of their recurring monosaccharide units the length of their chains the types of bonds linking the units the degree of branching  Linear or Branched • Types: Homopolysaccharides: A polysaccharide made up of one type of monosaccharide Heteropolysaccharides: A polysaccharide containing more than one type of monosaccharide. POLYSACCHARIDES HOMOPOLYSACCHARIDES  Glucose polymers FUNCTIONS • Storage Forms of Fuel Starch (plants) Branched polymer formed of glucose molecules.  Location Seeds and tubers  Glycogen (animals) More extensively branched and more compact than starch  Location  Liver and muscle cells  • Structural purposes  Cellulose (plants)  Unbranched  Location  cell walls of plants POLYSACCHARIDES HETEROPOLYSACCHARIDES Hyaluronic acid • 2 monosaccharides  alternating residues of D-glucuronic acid and N-acetylglucosamine • Location and function Lubricants in the synovial fluid of joints Give the vitreous humor of the eye its jellylike consistency DIGESTION OF CARBOHYDRATES GLUCOSE HOMEOSTASIS • Glucose is an essential metabolic fuel. • To ensure the continuous supply of glucose to the brain and other tissues, it is important to maintain a constant level of glucose in blood. AFTER A MEAL GLUCOSE HOMEOSTASIS • Glucose is an essential metabolic fuel. • To ensure the continuous supply of glucose to the brain and other tissues, it is important to maintain a constant level of glucose in blood. FASTING CONDITIONS GLYCOLYSIS Glycolysis: The catabolic pathway by which a molecule of glucose is broken down into two molecules of pyruvate • During the sequential reactions of glycolysis, some of the free energy released from glucose is conserved in the form of ATP and NADH • Universal central pathway of glucose catabolism • Location  Cytoplasm PHASES • • Preparatory phase  2 ATP are consumed Payoff phase  4 ATP and 2 NADH are produced FROM EACH MOLECULE OF GLUCOSE • 2 Pyruvates • 2 ATP • 2NADH GLYCOLYSIS GLYCOLYSIS REGULATION  3 key enzymes Activators • Insulin  After a meal • Epinephrine  Under situation of stress Inhibitors • ATP • Glucagon  Fasted condition GLUCONEOGENESIS Gluconeogenesis: The biosynthesis of a carbohydrate from simpler, noncarbohydrate precursors such as oxaloacetate or pyruvate. • “New formation of glucose” from noncarbohydrate precursors • Glucogenic precursors Pyruvate, Glycerol, Lactate, some amino acids • Anabolic pathway. • Location  Mainly in the liver and to a lesser extent in the renal cortex FUNCTION It provides glucose to the tissues when the glycogen stores are depleted: • During periods of fasting • After vigorous exercise. CORI CYCLE After vigorous exercise, lactate produced by anaerobic glycolysis in skeletal muscle returns to the liver and is converted to glucose, which moves back to muscle and is converted to glycogen — a circuit called the Cori cycle GLUCONEOGENESIS • Gluconeogenesis and glycolysis are not identical pathways running in opposite directions, although they do share several steps  7 of the 10 enzymatic reactions of gluconeogenesis are the reverse of glycolytic reactions • Three reactions of glycolysis are essentially irreversible  In gluconeogenesis, the three irreversible steps are bypassed by a separate set of enzymes • ATP (Energy) is required  For each molecule of glucose formed from pyruvate 4 ATP 2 GTP GYCOGEN METABOLISM • Storage form of glucose employed by animals • Highly branched polymer of glucose LOCATION AND FUNCTION • Skeletal muscle Glycogen breakdown in muscle delivers glucose needed for muscle contraction • Liver  Glycogen stored in the liver provides a reservoir that maintains homeostasis of blood glucose throughout the body when dietary glucose is not sufficient GYCOGEN METABOLISM Glycogenolysis: The enzymatic breakdown of stored (not dietary) glycogen. • Catabolic pathway • Epinephrine or glucagon initiates the cascade that activates the pathway • Activated:  Muscle After a sudden burst of physical activity  Liver  Between meals or fasting conditions Glycogenesis: The process of converting glucose to glycogen. • Anabolic pathway • Insulin activates the pathway after a meal GYCOGEN METABOLISM REGULATION OF CARBOHYDRATES METABOLISM Well-fed state Ingestion of glucose Blood glucose Release of insulin Glycogen synthesis Glycolysis Exercise Fasting state Blood glucose Release of glucagon Glycogen breakdown Gluconeogenesis Release of adrenaline Glycogen breakdown Glucolysis Gluconeogenesis in liver