Carbohydrates

What are Carbohydrates?

Carbohydrates are the most abundant organic compounds in the plant world. They act as storehouses of chemical energy (glucose, starch, glycogen); are components of supportive structures in plants (cellulose), crustacean shells (chitin), and connective tissues in animals (acidic polysaccharides); and are essential components of nucleic acids (D-ribose and 2-deoxy-D-ribose). Carbohydrates make up about three fourths of the dry weight of plants. Animals (including humans) get their carbohydrates by eating plants, but they do not store much of what they consume.

• Carbohydrates are the most abundant class of organic compounds found in living organisms. They originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll. Carbohydrates (glycans) have the following basic composition: (CH2O)n or H - C - OH Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. Disaccharides - 2 monosaccharides covalently linked. Oligosaccharides – 3-12 monosaccharides covalently linked. Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units. Monosaccharides Aldoses (e.g., glucose) have Ketoses (e.g., fructose) have an aldehyde group at one end. a keto group, usually at C2. An asymmetric carbon atom (chiral carbon) is a carbon atom that is attached to four different types of atoms or four different groups of atoms. An aldehyde can react with an alcohol to form a hemiacetal. A ketone can react with an alcohol to form a hemiketal.
  

Monosaccharides (CnH2nOn) Classified by the number of carbon atoms:

• 3-C triose

• 4-C tetrose

• 5-C pentose nutritionally important

• 6-C hexose
  

Sugars that contain four or more carbons exist primarily in cyclic form

• Pentoses (5C)

• Xylose and arabinose

• Component in hemicellulose, glycoproteins

• Ribose

• Found in every living cell

• Found in compounds involved in metabolism:

• ATP/ADP

• Riboflavin

• DNA/RNA

• Hexoses (6C)

• Glucose

• Component of starch, cellulose, and glycogen

• Major end-product of carbohydrate digestion in monogastrics

• Primary form of sugar used for energy
  

Glucose, fructose, and galactose are among the most important monosaccharides in living organisms Fructose

• 75% of sugars in honey

• Found in fruits and cane sugar

Galactose

• Component of milk sugar (lactose)

• May be metabolized to glucose

Mannose

• Found after hydrolysis of plant mannosans and gums; legumes Disaccharides • 3 main disaccharides-sucrose maltose lactose • All are isomers with molecular formula C12H22O11. • On hydrolysis they yield 2 monosaccharide. • Soluble in water • Even though they are soluble in water, they are too large to pass through the cell membrane.
  

Structure of Disaccharides • Formed by combination of 2 monosaccharides. • Bonds between 2 monosaccharide are known as Glycosidic bond.  Maltose, a cleavage product of starch (e.g., amylose), is a disaccharide with an a(1 4) glycosidic link between C1 - C4 OH of 2 glucoses.  Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. Because the configuration at the anomeric C of glucose is b (O points down from ring), the linkage is b (12). • Lactose, milk sugar, is composed of galactose & glucose, with b(14) linkage from the anomeric OH of galactose. Oligo- and Poly-saccharides Oligosaccharide  Chain of 3–10 sugar molecules Polysaccharide  Chain of 10+ sugar molecules

Polysaccharides • Homoglycans - homopolysaccharides containing only one type of monosaccharide • Heteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharide • Lengths and compositions of a polysaccharide may vary within a population of these molecules Homopolysaccharides

STARCH Starch or amylum is a carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as an energy store. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin by weight. Amylopectin is a glucose polymer with mainly a(14) linkages, but it also has branches formed by a(16) linkages. Amylose is a glucose polymer with a(14) linkages. Glycogen Animal storage form of carbohydrate • found in LIVER and MUSCLE • Humans store ~ 100g in liver; ~ 400g in muscle – negligible source of carbohydrate in the diet (meat) Glucoses are linked together linearly by α(1→4) glycosidic bonds from one glucose to the next. Branches are linked to the chains from which they are branching off by α(1→6) glycosidic bonds. Glycogen has more a(16) branches. The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants. Cellulose Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(14) linkages. Mammals lack any enzyme that hydrolzes the b(14) bonds and so cannot digest cellulose. Other polysaccarides • Dextrans- Dental plaque • Chitin - Exoskeletons of insects,spiders and cells of fungi HETEROPOLYSACCHARIDES Heteropolysaccarides contain two or more different kind of monosaccharides. Usually they provide extracellular support for organisms of all kingdoms: the bacteria cell envelope, or the matrix that holds individual cells together in animal tissues, and provides protection, shape and support to cells, tissues and organs. 1- Glycosaminoglycans 2- Glycoproteins 3- Glycolipids Glycosaminoglycans (Mucopolysaccharides) Glycosaminoglycans (GAGs) or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit. Involved in a variety of extracellular functions; chondroitin is found in tendons, cartilage and other connective tissues. Glycoproteins • Proteins that contain covalently-bound oligosaccharides, either to serine (O-Glycosidic linkage) or asparagine (N-glycosidic linkage) Digestion of Carbohydrates  Monosaccharides  Do not need hydrolysis before absorption  Very little (if any) in most feeds  Di- and poly-saccharides  Relatively large molecules  Must be hydrolyzed prior to absorption Hydrolyzed to monosaccharides Only monosaccharides can be absorbed Digestion of carbohydrate by salivary α -amylase:

In the mouth: A. This enzyme is produced by salivary glands. Its optimum pH is 6.7. B. It is activated by chloride ions (cl-). C. It acts on cooked starch and glycogen breaking α 1-4 bonds, converting them into maltose [a disaccharide containing two glucose molecules attached by α 1-4 linkage]. α –amylase cannot hydrolyze α (1-6) bonds. Both starch and glycogen also contain 1-6 bonds, the resulting digest contains isomaltose [a disaccharide in which two glucose molecules are attached by 1-6 linkage]. Because food remains for a short time in the mouth, digestion of starch and glycogen may be incomplete and gives a partial digestion products called: starch dextrins (amylodextrin, erythrodextrin and achrodextrin). Therefore, digestion of starch and glycogen in the mouth gives maltose, isomaltose and starch dextrins.

ln the stomach:

Carbohydrate digestion stops temporarily due to the high acidity which inactivates the salivary - amylase. Digestion of carbohydrate by the pancreatic - amylase in the small intestine. A. α-amylase enzyme is produced by pancreas and acts in small intestine. Its optimum pH is 7.1. B. It is also activated by chloride ions. C. It acts on cooked and uncooked starch, hydrolysing them into maltose and isomaltose.

Final carbohydrate digestion by intestinal enzymes: A. 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.

Digestion in Small Intestine is mediated by enzymes synthesized by cells lining the small intestine (brush border) B. The disaccharidases include: 1. Lactase (β-galactosidase) which hydrolyses lactose into two molecules of glucose and galactose: 2. Maltase ( α-glucosidase), which hydrolyses maltose into two molecules of glucose: 3. Sucrase (α-fructofuranosidase), which hydrolyses sucrose into two molecules of glucose and fructose: 4. α - dextrinase (oligo-1,6 glucosidase or isomaltase) which hydrolyze (1 ,6) linkage of isomaltose. Cellulose contains β(1-4) bonds between glucose molecules. In humans, there is no β (1-4) glucosidase that can digest such bonds. So cellulose passes as such in stool. Cellulose helps water retention during the passage of food along the intestine

 producing larger and softer feces

 preventing constipation. Nutrient Absorption  Active transport for glucose and galactose  Sodium-glucose transporter 1 (SGLT1)  Dependent on Na/K ATPase pump  Facilitated transport for fructose Mechanisms of absorption:

A. Active transport:

1. Mechanism of active transport: a) In the cell membrane of the intestinal cells, there is a mobile carrier 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. The 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. 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. The Na+ is expelled outside the cell by sodium pump. Which needs ATP as 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.

2. Inhibitors of active transport: a) Ouabin (cardiac glycoside): Inhibits adenosine triphosphatase (ATPase) necessary for hydrolysis of ATP that produces energy of sodium pump. b) Phlorhizin; Inhibits the binding of sodium in the carrier protein.

B. Passive transport (facilitated diffusion): Sugars pass with concentration gradient i.e. from high to low concentration. It needs no energy. It occurs by means of a sodium independent facilitative transporter (GLUT -5). Fructose and pentoses are absorbed by this mechanism. C. Sodium – independent transporter (GLUT-2):

Facilitates transport of sugars out of the intestinal mucosal cell i.e. to portal circulation. Summary  Polysaccharides broken down to monosaccharides  Monosaccharides taken up by active transport or facilitated diffusion and carried to liver  Glucose is transported to cells requiring energy  Insulin influences rate of cellular uptake