Carbohydrate Principles of Biochemistry PDF

PRINCIPLES OF BIOCHEMISTRY  Carbohydrate The term carbohydrate was originally used to describe compounds that were literally “hydrates of carbon” because they had the empirical formula CH2O. In recent years, carbohydrates have been classified on the basis of their structures, not their formulas....

PRINCIPLES OF BIOCHEMISTRY  Carbohydrate The term carbohydrate was originally used to describe compounds that were literally “hydrates of carbon” because they had the empirical formula CH2O. In recent years, carbohydrates have been classified on the basis of their structures, not their formulas. They are now defined as polyhydroxy aldehydes and ketones. Among the compounds that belong to this family are cellulose, starch, glycogen, and most sugars.  Carbohydrate (CH2O)n Organic compounds consisting of C, H and O (~ 1:2:1) Function: 1) Immediate energy and carbon source (simple carbohydrates: mono/disaccharides) 2) Energy and carbon storage compound (long chain carbohydrates with partial crystalline structure: starch) 3) Structural components in plant cells (long chain carbohydrates, crystalline structure: cellulose)  The 3 classes of carbohydrates Monosaccharide Carbohydrate Disaccharide Polysaccharide  Monosaccharides The monosaccharides are white, crystalline solids that contain a single aldehyde OR ketone functional group. They are subdivided into two classes: aldoses and ketoses on the basis of whether they are aldehydes or ketones. They are also classified as a triose, tetrose, pentose, hexose, or heptose on the basis of whether they contain three, four, five, six, or seven carbon atoms With only one exception, the monosaccharides are optically active compounds. Do you know which one?  Glyceraldehyde D-Glyceraldehyde L-Glyceraldehyde Although both D and L isomers are possible, most of the monosaccharides found in nature are in the D configuration. Structures for the D and L isomer of the simplest aldose (3C), glyceraldehyde, are shown above. The structures of many monosaccharides were first determined by Emil Fischer in the 1880s and 1890s and are still written according to a convention he developed. The Fischer projection represents what the molecule would look like if its three-dimensional structure were projected onto a piece of paper. By convention, Fischer projections are written vertically, with the aldehyde or ketone at the top. The -OH group on the second-to-last carbon atom is written on the right side of the skeleton structure for the D isomer and on the left for the L isomer.  Fischer Projection D-Glyceraldehyde L-Glyceraldehyde Fischer projections for the two isomers of glyceraldehyde The -OH group on the second-to-last carbon atom is written on the right side of the skeleton structure for the D isomer and on the left for the L isomer.  Fischer projection for common monosaccharides (Aldose) D-Arabinose D-Galactose D-Ribose D-Xylose D-Glucose D-Mannose All carbon atoms except C1 in monosaccharides have an -OH group  Fischer projection for common monosaccharides (Ketose) D-Ribulose D-Fructose Glucose (aldose) and fructose (ketose) have the same formula: C6H12O6. Glucose is the sugar with the highest concentration in the bloodstream; fructose is found in fruit and honey.  Monosaccharides: Reducing sugar Both the functional groups of aldehyde and ketone have reducing property So, all monosaccharides are reducing sugars Benedict test: To detect the presence of reducing sugar Copper (II) sulphate BLUE ORANGE Copper (I) sulphate  Cyclic monosaccharide (6C) Pyranose OH If the carbon chain is long enough, the hydroxyl at one end of a monosaccharide can attack the carbonyl group at the other end to form a cyclic compound. When a six-membered ring is formed, the product of this reaction is called a pyranose, shown in the figure above.  Cyclic monosaccharide (5C) Furanose + D-Ribose α-D-Ribofuranose β-D-Ribofuranose The reactions that lead to the formation of a pyranose or a furanose are reversible. Thus, it doesn‘t matter whether we start with a pure sample of α-D- glucopyranose or β-D-glucopyranose. Within minutes, these anomers are interconverted to give an equilibrium mixture that is 63.6% of the β- anomer and 36.4% of the α-anomer. The 2:1 preference for the β-anomer can be understood by comparing the structures of these cyclic molecules. In the β-anomer, all of the bulky -OH or -CH2OH substituents lie more or less within the plane of the six-membered ring. In the α-anomer, one of the -OH groups are perpendicular to the plane of the six-membered ring, in a region where it feels strong repulsive forces from the hydrogen atoms that lie in similar positions around the ring. As a result, the β-anomer is slightly more stable than the α-anomer.  Cyclic monosaccharide (5C) Furanose CH2OH + D-Fructose α-D- β-D- Fructofuranose Fructofuranose β anomer is slightly more stable! (Ratio of α : β is 1:2)  Characteristics of monosaccharide All monosaccharides: Sweet taste Dissolves easily in water Form white crystals  Functions of monosaccharide Triose: Important intermediates of respiration and photosynthesis processes Pentose: Ribose & deoxyribose are components of nucleotides. Coenzyme components eg. NAD, NADP, FAD, involved in H transfer. ATP component. Hexose: Most important respiration substrate (Glucose). Energy and carbon source for animal and plant cells.  Ribose & Deoxyribose 3C 6C 5C  Structure of glucose D/L isomer? Aldose/Ketose?  Disaccharides (C12H22O11) Maltose Maltose, or malt sugar, which forms when starch breaks down, is an important component of the barley malt used to brew beer.  Disaccharide: Lactose Lactose, or milk sugar, is a disaccharide found in milk. Very young children have a special enzyme known as lactase that helps digest lactose. As they grow older, many people lose the ability to digest lactose and cannot tolerate milk or milk products. Because human milk has twice as much lactose as milk from cows, young children who develop lactose intolerance while they are being breast-fed are switched to cows' milk or a synthetic formula based on sucrose.  Disaccharide: Sucrose The substance most people refer to as “sugar” is the disaccharide sucrose, which is extracted from either sugar cane or beets. Sucrose is the sweetest of the disaccharides. It is roughly three times as sweet as maltose and six times as sweet as lactose. In recent years, sucrose has been replaced in many commercial products by corn syrup, which is obtained when the polysaccharides in cornstarch are broken down. Corn syrup is primarily glucose, which is only about 70% as sweet as sucrose. Fructose, however, is about two and a half times as sweet as glucose. A commercial process has therefore been developed that uses an isomerase enzyme to convert about half of the glucose in corn syrup into fructose. This high-fructose corn sweetener is just as sweet as sucrose and has found extensive use in soft drinks.  Polysaccharides (C6H10O5)n Polysaccharides are large molecules composed of individual monosaccharide units. A common plant polysaccharide is starch, which is made up of many glucoses. Two forms of polysaccharide, amylose and amylopectin makeup what we commonly call starch. The formation of the glycosidic bond by condensation (the removal of water from a molecule) allows the linking of monosaccharides into disaccharides and polysaccharides. Glycogen is an animal storage product that accumulates in the vertebrate liver.  Starch -1,6 glycosidic linkage -1,4 glycosidic linkage 2 types of polymers in starch: amylose (linear chain of glucose) & amylopectin (branched chain of glucose)  Structure of starch, glycogen & cellulose STARCH & GLYCOGEN The -OH substituent that serves as the primary link between α-glucopyranose rings in starch and glycogen is oriented perpendicular to the plane of the six- membered ring As a result, the glucopyranose rings in these carbohydrates form a structure that resembles the stairs of a staircase. CELLULOSE The -OH substituent that links the β-glucopyranose rings in cellulose lies in the plane of the six-membered ring. This molecule, therefore, stretches out in a linear fashion. This makes it easier for strong hydrogen bonds to form between the –OH groups of adjacent molecules.This, in turn, gives cellulose the rigidity required for it to serve as a source of the mechanical structure of plant cells. Cellulose and starch provide an excellent example of the link between the structure and function of biomolecules. At the turn of the last century, Emil Fischer suggested that the structure of an enzyme is matched to the substance on which it acts, in much the same way that a lock and key are matched. Thus, the amylase enzymes in saliva that break down the -linkages between glucose molecules in starch, cannot act on the -linkages in cellulose. Most animals cannot digest cellulose because they don‘t have an enzyme that can cleave -linkages between glucose molecules. Cellulose in their diet, therefore, serves only as fiber, or roughage. The digestive tracts of some animals, such as cows, horses, sheep, and goats contain symbiotic bacteria that have enzymes that cleave these -linkages, so these animals can digest cellulose.  Cellulose & Chitin plant structural polysaccharide animal structural polysaccharide  Cellulose Cellulose is a polysaccharide found in plant cell walls. Cellulose forms the fibrous part of the plant cell wall. In terms of human diets, cellulose is indigestible, and thus forms an important, easily obtained part of dietary fiber. As compared to starch and glycogen, which are each made up of mixtures of  and  glucose, cellulose (and the animal structural polysaccharide chitin) are made up of only  glucose. The three-dimensional structure of the structural polysaccharides is thus constrained into straight microfibrils by the uniform nature of the glucose, which resists the actions of enzymes (such as amylase) that break down storage polysaccharides (such as starch).  Alfa-amylase Beta-amylase  Why is sugar sweet? The tongue contains many flavor receptors, some of which are specific for sugar molecules. When sugar hits the tongue, the molecules bind to the sugar receptors, and the receptor triggers a nerve impulse that travels to the brain and says, "sweet." As these sugar receptors only recognize a part of the sugar molecule, they are easily fooled by other molecules which have this piece of the sugar molecule; this is why saccharine and aspartame taste sweet. But why go to the trouble of telling your brain, "sweet," whenever you eat sugar? Sugar is the simplest form of carbohydrate and, as such, is the best source of energy for the body to consume. So, the taste system was developed to tell the brain when "good food" was around, so that you would know to eat lots of it. We describe the brain’s view of "sugar is good" as "sweet".  Artificial sweeteners/sugar substitutes Structure of saccharin Structure of aspartame Structure of sucralose 300 times sweeter than sucrose! 180 times sweeter than sucrose! 600 times sweeter than sucrose!