Carbohydrates Lecture2
Disaccharides
Consist of two monosaccharides covalently bound to each other. All of which are isomers with the molecular formula C 12 22 O 11. The differences in these disaccharides are due to atomic arrangements within the molecule. In disaccharides the chemical bond that joins the two monosaccharide units is called a glycosidic bond. Glycosidic bond characteristics: Is readily hydrolyzed by acid. It resists cleavage by base.
Glycosidic Bonds The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond: R-O + O-R' R-O-R' + 2 O
Naming glycosidic bonds Glycosidic bonds between sugars are named According: 1. To the numbers of the connected carbons 2. To the position of the anomeric hydroxyl group of the sugar involved in the bond. If this anomeric hydroxyl group is in the α configuration, the linkage is an α-bond. If it is in the β configuration, the linkage is a β-bond.
Major Disaccharides 1. Maltose, it is a disaccharide with an a(1 4) glycosidic link between C1 - C4 O of 2 glucoses. It is the a anomer (C1 O points down). Maltose is a sugar not naturally found in high quantities in the food supply, but your body can generate it when you digest starchy foods. Although maltose is a small sugar, it is actually made of two smaller glucose sugars. Maltose is a bit too big for people to absorb, so your body must break it down into glucose in order to transport it to other parts of the body for use as fuel and for storing energy. 6 C 2 O 6 C 2 O 4 5 O O 1 4 5 O O 1 O 3 2 O 3 2 O O maltose O
2. Cellobiose, a product of cellulose breakdown, is the otherwise equivalent b anomer (O on C1 points up). The b(1 4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other. 6 C 2 O 6 C 2 O 4 O 5 O 3 O 2 1 O 4 5 O 3 O 2 1 O O cellobiose O
Sucrose (table sugar) is composed of glucose & fructose, with α(1 2) linkage from the anomeric O of glucose; (O points down from ring). It is known as beet sugar, cane sugar, table sugar. In humans and other mammals, sucrose is broken down into its constituent monosaccharides, glucose and fructose, by sucrase, which are located in the membrane of the microvilli lining the duodenum. The resulting glucose and fructose molecules are then rapidly absorbed into the bloodstream.
4. Lactose (milk sugar), is composed of galactose & glucose, with b(1 4) linkage from the anomeric O of galactose. It is found in milk and other dairy products. It makes up around 2 8% of milk (by weight).
ydrolysis Addition of WATER to a compound to split it into smaller subunits or by boiling with dilute acid. Also called chemical digestion. Example: disaccharide + 2O monosaccharide + monosaccharide
Disaccharides major characteristics: just like monosaccharide; they are white crystalline solids at room temperature and are soluble in water. Sucrose very soluble in water Maltose fairly soluble Lactose only slightly soluble The disaccharides, owever, even though they are soluble in water, they are too large to pass through cell membranes.
Oligosaccharides
oligosaccharide From the Greek: olígos means "a few", and sácchar means "sugar It is a saccharide polymer containing a small number (up to ten) of simple sugars (monosaccharides). A large number of oligosaccharides have been prepared by partially breaking down more complex carbohydrates (polysaccharides). Most of the few naturally occurring oligosaccharides are found in plants.
oligosaccharide Oligosaccharides, except maltotriose, are indigestible, which means humans lack enzymes to break them down in the small intestine. >> so they reach the large intestine, where beneficial colonic bacteria break them down (ferment) to absorbable nutrients, which provide some energy about 2 Calories (kilocalories) per gram in average >> Certain breakdown products of oligosaccharides namely short-chain fatty acids (SCFAs) may have beneficial effect on large intestinal lining. Most oligosaccharides act as a soluble fiber, which may help prevent constipation. Ingestion of large amount of oligosaccharides can result in abdominal bloating and excessive gas (flatulence).
oligosaccharide Example: Raffinose It is a tri-saccharide found in many plants, consists of galactose, glucose and fructose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains.
oligosaccharide Raffinose can be hydrolyzed to D-galactose and sucrose (glucose+fructose) by the enzyme α-galactosidase (α-gal). umans do not possess the α- GAL enzyme in their digestive tract to break down Raffinose!! Therefore, these oligosaccharides pass undigested through the stomach and upper intestine. In the lower intestine, they are fermented by gas-producing bacteria that do possess the α- GAL enzyme and make carbon dioxide, methane or hydrogen leading to the flatulence commonly associated with eating beans and other vegetables.