Carbohydrates 1 VP 2024 PDF

CARBOHYDRATES Carbohydrates (CH2O)n (3,4,5,6,7,8,9) (= saccharides) The most abundant organic molecules in nature Functions: Main energy source for most animals Storage form of energy (in animals = glycogen) Cell membrane components (glycoproteins – cell communication) Structural components (cell wa...

CARBOHYDRATES Carbohydrates (CH2O)n (3,4,5,6,7,8,9) (= saccharides) The most abundant organic molecules in nature Functions: Main energy source for most animals Storage form of energy (in animals = glycogen) Cell membrane components (glycoproteins – cell communication) Structural components (cell walls of plants, bacteria, fungi, algae; exoskeleton of insects) From: Alberts. Molecular Biology of the Cell CARBOHYDRATE CLASSIFICATION All carbohydrates consist of monosaccharides (simple sugars) Can be organized according to the number of carbon atoms C From: Harvey & Ferrier. Biochemistry CARBOHYDRATE CLASSIFICATION Can also be organized depending on the type of carbonyl group they contain Aldo- KetoFrom: Harvey & Ferrier. Biochemistry From:Alberts. Molecular Biology of the Cell CARBOHYDRATE ISOMERS Isomers = same chemical formula; different structures/ spatial arrangement of atoms Examples: fructose, glucose, mannose and galactose (C6H12O6) C6H12O6 fructose Alpha- and beta-isomers position change of the carbon that carries the aldehyde/keto group CARBOHYDRATE ENANTIOMERS ENANTIOMERS are special types of isomers o Mirror images of each other o Two parts are assigned as a D- and an L-sugar o Most sugars found in nature are D-isomers  D-isomers: the – OH group on the asymmetric carbon farthest from the carbonyl group (CHO) is on the right, in L- isomers, it is on the left Most enzymes responsible for degradation of carbohydrates are specific for either isomer Isomerases are enzymes capable of interconvert D- and L-isomers Asymmetric carbons are shown in green MONOSACCHARIDE JOINING Monosaccharides can be joined to form disaccharides, oligosaccharides and polysaccharides The bonds that link sugars: glycosidic bonds  Disaccharides = 2 monosaccharides  Oligosaccharides = 3-10 monosaccharide  Polysaccharides: > 10 monosaccharide (up to hundreds!) What is the name of this product? LACTOSE MONOSACCHARIDE JOINING Important disaccharides: LACTOSE (galactose + glucose) SUCROSE (glucose + fructose) MALTOSE (glucose + glucose) Important polysaccharides: Branched Glycogen (animal) Starch (plant amylose, amylopectin) Cellulose C A R B O H Y D R AT E S Lactose CARBOHYDRATES LINKED TO NONCARBOHYDRATES Carbohydrates can be attached to non- carbohydrates via glycosidic bonds → glycosides:  Carbs + purine and pyrimidine → Nucleic bases  Carbs + aromatic rings → bilirubin and steroids  Carbs + proteins → Glycoproteins/proteoglycans  Carbs + Lipids → Glycolipids Mucin (glycoprotein) O-glycosidic bond polysaccharide to a protein backbone From: Alberts. Molecular Biology of the Cell Sugar derivatives = replacement of a single OH group by another group I.e., Glucosamine = aminosugar → common monosaccharide in many polysaccharides (cartilage, chitin) Amino group DIGESTION and ABSORPTION  Digestion is the process of breaking down complex nutrients into simple molecules  Absorption is the process of transporting those simple molecules across the intestinal epithelium.  Absorption cannot occur if food is not digested!!  Digestion is fruitless if the digested nutrients cannot be absorbed!! DIETARY CARBOHYDRATE DIGESTION In most omnivores (i.e., humans, pigs, rats) digestion of carbohydrates begins in the mouth  Enzyme: salivary alpha-amylase = ptyalin  Plays a small role overall Carbohydrate digestion happens mainly in small intestine Pancreatic and enterocyte hydrolases (glycosidases)  brake glycosidic bonds From: Harvey & Ferrier. Biochemistry DIETARY CARBOHYDRATE DIGESTION The final products of carbohydrate digestion are monosaccharides: ✓ glucose ✓ fructose ✓ galactose  Monosaccharides are then absorbed by enterocytes (epithelial cells which line the From: Harvey & Ferrier. Biochemistry inner surface of the small and large intestines) DIETARY CARBOHYDRATE DIGESTION Main dietary polysaccharides:  Starch (plant) and glycogen (animal)  Mastication helps break down material Mammals generally do not have the enzymes necessary to break down cellulose β (1→4) glycosidic bonds Ruminants & other herbivores have bacteria in their digestive tracts, which produce enzymes to digest cellulose From: Harvey & Ferrier. Biochemistry DIETARY CARBOHYDRATE DIGESTION Final digestive processes and absorption occur at the mucosal lining ➔ in the duodenum and upper jejunum Absorption of the monosaccharide products of carbohydrate digestion by enterocytes From: Harvey & Ferrier. Biochemistry GLUT = glucose transporter SGLT-1 = sodium (Na+)-dependent glucose cotransporter DIETARY CARBOHYDRATE DIGESTION Major dietary carbohydrates:  starch  glycogen  sucrose (saccharose)  lactose  Salivary amylase starts digestion  Further digestion of carbohydrates is achieved by pancreatic enzymes (in lumen of small intestine) From: Silbernagl. Physiology DIETARY CARBOHYDRATE DIGESTION  Digestion is finished by enzymes synthesized by the intestinal mucosa maltase isomaltase lactase sucrase  Absorption of carbohydrates (monosaccharides) takes place in the duodenum and upper jejunum by: Na+- dependent transport mechanisms (SGLT1) Facilitated transport (GLUT5 and GLUT2) From: Silbernagl. Physiology Cunningham’s textbook of Veterinary Physiology From: Harvey & Ferrier. Biochemistry ABNORMAL DEGRADATION OF DISACCHARIDES Abnormal degradation of disaccharides  passage of disaccharides into the large intestine ↑ osmotic activity, bacterial fermentation → abdominal cramps, diarrhea, flatulence  Reasons: genetic, intestinal diseases, malnutrition, pharmaceuticals/drugs that injure the mucosa, lactose intolerance, age-related reductions in enzyme activities From: Harvey & Ferrier. Biochemistry