College of Science Department of Biochemistry

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1 Dr. Abir Alghanouchi College of Science Department of Biochemistry

2 Carbohydrates Carbohydrates are called carbohydrates because they are essentially hydrates of carbon(i.e. they are composed of carbon and water and have a composition of (CH 2 O) n. The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram No single carbohydrate is essential, but carbohydrates do participate in many required functions in the body.

3 Monosaccharides Do not need hydrolysis before absorption Very little in most feeds Di-and poly-saccharides Relatively large molecules Must be hydrolyzed prior to absorption Hydrolyzed to monosaccharides Only monosaccharides can be absorbed

4 Three types of monosaccharides fructose glucose Galactose* * join together to make three types of disaccharides sucrose maltose lactose (fructose-glucose) (glucose-glucose) (glucosegalactose) * Galactose does not occur in foods singly but only as part of lactose

5 Mouth: o Digestion of CHO begins in the mouth o During mastication, salivary alpha amylase: Breaks starches down to maltose, dextrins, isomaltase, maltriose Plays only a small role in breakdown because of the short time food is in the mouth Optimum ph 6.7 Requires Cl- for its activity o The chemical digestion of carbohydrates, which begins in the oral cavity, is terminated due to a decrease in ph CHO digestion stops in the stomach because the high acidity inactivates salivary alpha amylase

6 Pancreas ofurther digestion by pancreatic enzymes occurs in the small intestine: when the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas oat alkaline ph pancreatic alpha amylase continues the starch digestion: Hydrolyzes alpha 1-4 linkages between glucose residues Major importance in hydrolyzing starch and glycogen to maltose Polysaccharides Amylase Disaccharides (maltose, isomaltose)

7 The final digestive processes occur at the small intestine and include the action of several disaccharidases. These enzymes are secreted through and remain associated with the brush border of the intestinal mucosal cells. Disaccharides Brush Border Enzymes Monosaccharides

8 Sucrose Maltose Sucrase Maltase Glucose + Fructose Glucose + Glucose Isomaltose Lactose Isomaltase Lactase Glucose + Glucose Glucose + Galactose

Figure: Steps of Carbohydrate digestion Mouth Starch Lactose Succrose Cellulose α-amylase stomach Starch-dextrins Isomaltose Maltose Lactose Succrose Cellulose Low ph stops action of salivary

9 Figure: Steps of Carbohydrate digestion Mouth Starch Lactose Succrose Cellulose α-amylase stomach Starch-dextrins Isomaltose Maltose Lactose Succrose Cellulose Low ph stops action of salivary α-amylase pancreas liver HCO3 Cellulose small intestine Fructose Galactose glucose Pancreatic α-amylase Isomaltose Maltose Lactose Succrose Mucosal cell membranebound enzymes (isomaltase, maltase, lactase, Sucrase)

through circulation In liver, galactose and fructose are

10 Monosaccharides, the end product of CHO digestion, enter the capillaries of the intestinal villi Distributed to tissue through circulation In liver, galactose and fructose are converted to glucose Monosaccharides travel the liver via the portal vein

11 Carbohydrate absorption (cond ) Extra glucose is stored as glycogen in the liver and skeletal muscles Insulin is not required for the uptake of glucose by the intestinal cells o Absorption of glucose= 100% (taken as standard) o Absorption of galactose = 110% o Absorption of fructose = 43% The maximal rate of glucose absorption from intestine is 120gm/hr

12 In the cell membrane of the intestinal cells, there is a mobile carrier Mechanisms of absorption A- Active transport: protein called sodium dependant glucose transporter(sgl T-1). It transports glucose to inside the cell using energy. The energy is derived from sodium-potassium pump. This transporter has 2 separate sites: one for sodium and the other for glucose. It transports them from the intestinal lumen across cell membrane to the cytoplasm. Then both glucose and sodium are released into the cytoplasm allowing the carrier to return for more transport of glucose and sodium.

13 Mechanisms of absorption

14 The sodium is transported from high to low concentration (with concentration gradient) and at the same time causes the carrier to transport glucose against its concentration gradient (from lower to higher concentrations) allowing for greater accumulation of glucose on one side of the membrane than on the other. The Na+ is expelled outside the cell by sodium pump which needs ATPas a source of energy. The reaction is catalyzed by an enzyme called "Adenosine triphosphatase (ATPase)". Active transport is much more faster than passive transport. Insulin increases the number of glucose transporters in tissues containing insulin receptors e.g. muscles and adipose tissue.

15 Glucose enter cells by two ways: 1- Insulin independent transport system: o Not require insulin for glucose uptake o Mediated through carrier protein o Present in brain, RBCs, hepatocytes, intestinal mucosa, renal tubules and cornea 2- Insulin-dependent transport system (Require insulin) muscles and adipose tissue

16 B. Passive transport (facilitated diffusion): Sugars pass with concentration gradient i.e. from high to low Concentration Passive process, it needs no energy. It occurs by means of a sodium independent facilitative transporter (GLUT-5). Glucose binds to receptor on carrier protein; Latter changes shape then releases solute on other side of membrane

17 Fructose and pentoses are absorbed by this mechanism. Glucose and galactosecan also use the same transporter if the concentration gradient is favorable. C. There is also sodium independent transporter (GLUT-2), That is facilitates transport of sugars out of the cell i.e. to circulation.

18 Summary of types of functions of most important glucose transporters: Function Site SGLT-1 GLUT -5 GLUT - 2 Absorption of glucose by active transport (energy is derived from Na + - K + pump) Fructose transport and to a lesser extent glucose and galactose. Transport glucose out of intestinal and renal cells circulation Intestine and renal tubules. Intestine and sperm - Intestine and renal tubule -βcells of islets (pancreas)-liver

19 Fate of absorbed sugars: Monosaccharides (glucose,galactose and fructose) resulting from carbohydrate digestion are absorbed and undergo the following: Uptake by tissues (liver): After absorption the liver takes up sugars, where galactose and fructose are converted into glucose. Glucose utilization by tissues: Glucose may undergo one of the following fate:

20 1. Oxidation: through Major pathways (glycolysis and Krebs' cycle)for production of energy. Hexose monophosphate pathway: for production of ribose, deoxyribose and NADPH + H + Uronic acid pathway, for production of glucuronic acid, which is used in detoxification and enters in the formation of mucopolysaccharide.

21 2. Storage: o As glycogen (glycogenesis) in the liver and muscles mainly. o As TG (lipogenesis) in adipose tissues 3. Conversion: to substances of biological importance: o Ribose, deoxyribose RNA and DNA. o Amino sugars o Non essential amino acids o Fatty acids o Fructose nutrition of sperms. o Glucouronic acid o Galactose essential for formation of lactose, glycolipids, mucopolysaccharides

22 4. Excretion of glucose in urine owhen blood glucose level exceeds certain limit, it will pass to urine This will occur when blood glucose level is above othis will occur when blood glucose level is above 180mg/dl and this is known as glucosuria

23 ate%20metabolism.ppt

Digestion of Carbohydrates. BCH 340 Lecture 2

Digestion of Carbohydrates. BCH 340 Lecture 2 Digestion of Carbohydrates BCH 340 Lecture 2 Carbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of