Carbohydrates (II) Reactions of Monosaccharides PDF

Carbohydrates (II) Reactions of Monosaccharides Hemiacetals and Hemiketals The dominant form of monosaccharides with 5 or more C atoms is cyclic Hemiacetals and hemiketals are formed from the reaction between two functional groups: aldehyde or ketone and alcohol...

Carbohydrates (II) Reactions of Monosaccharides Hemiacetals and Hemiketals The dominant form of monosaccharides with 5 or more C atoms is cyclic Hemiacetals and hemiketals are formed from the reaction between two functional groups: aldehyde or ketone and alcohol FUNDAMENTALS OF BIOCHEMISTRY 2 Hemiacetals and Hemiketals May take place either intermolecularly or intramolecularly as in the case of sugars, provided there are sufficient number of carbons between the aldehyde or ketone and the alcohol group to permit a stable ring form Five- or six-membered hemiacetal rings are stable. FUNDAMENTALS OF BIOCHEMISTRY 3 Hemiacetals and Hemiketals FUNDAMENTALS OF BIOCHEMISTRY 4 What will happen if sugar forms a CYCLIC molecule? Cyclization of sugars takes place due to interaction between functional groups on distant carbons, C1 to C5, to make a cyclic hemiacetal Cyclization using C2 to C5 results in hemiketal formation. In both cases, the carbonyl carbon is new chiral center and becomes an anomeric carbon FUNDAMENTALS OF BIOCHEMISTRY 5 Cyclic Hemiacetals Forms of D-Glucose In the cyclic hemiacetals of glucose, C1*, is now a chiral center (an anomeric carbon) – Two anomers of D-glucose: α-D-glucose & β-D-glucose FUNDAMENTALS OF BIOCHEMISTRY 6 Cyclic Hemiacetals Forms of D-Glucose The cyclic hemiacetals are readily interconvertible in aqueous solution – This interconversion of α-and β-anomers in solution is accompanied by a change in specific rotation called MUTAROTATION. – Only sugars that form hemiacetal or hemiketal structure mutarotate. FUNDAMENTALS OF BIOCHEMISTRY 7  ANOMERS Anomers: Cyclic monosaccharides that differ only in the position of the substituents on the anomeric carbon atom. 2 Anomeric forms of D-glucose: – Alpha-form: -OH of C1 and CH2OH of C5 are on opposite sides – Beta-form: -OH of C1 and CH2OH of C5 are on same sides FUNDAMENTALS OF BIOCHEMISTRY 8 Fischer → Haworth -OH group that is written to the right of the carbon in a Fischer projection has a downward direction in a Haworth projection. -OH group that is written to the left in a Fischer projection has an upward direction in a Haworth projection. FUNDAMENTALS OF BIOCHEMISTRY 9 Cyclic Forms of Monosaccharides All aldoses with five or more carbon atoms establish similar equilibria but with different percentages of the alpha, beta, and open-chain forms FUNDAMENTALS OF BIOCHEMISTRY 10 Practice Which of the monosaccharides glucose, fructose, galactose, and ribose has each of the following structural characteristics? (There may be more than one correct answer for a given characteristic) a. It is a pentose. b. It is a ketose. c. Its cyclic form has a 6-membered ring. d. Its cyclic form has two carbon atoms outside the ring. FUNDAMENTALS OF BIOCHEMISTRY 11 REACTIONS OF MONOSACCHARIDES Five important reactions of monosaccharides: – Oxidation to acidic sugars – Reduction to sugar alcohols – Phosphate ester formation – Amino sugar formation – Glycoside formation These reactions will be considered with respect to glucose; other aldoses, as well as ketoses, undergo similar reactions. FUNDAMENTALS OF BIOCHEMISTRY 12  Oxidation Reactions Oxidation to Uronic Acids Enzyme-catalyzed oxidation of the primary alcohol at C-6 of a hexose yields a uronic acid. – Enzyme-catalyzed oxidation of D-glucose, for example, yields D-glucuronic acid. In biochemical systems, enzymes can oxidize the primary alcohol end of an aldose such as glucose, without oxidation of the aldehyde group, to produce an alduronic acid FUNDAMENTALS OF BIOCHEMISTRY 13  Oxidation Reactions Oxidation to Uronic Acids – D-Glucuronic acid is widely distributed in the plant and animal world. – In humans, it is an important component of the acidic polysaccharides of connective tissues. – It is used by the body to detoxify foreign phenols and alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine. FUNDAMENTALS OF BIOCHEMISTRY 14  Oxidation Reactions Oxidation to Aldonic Acids Weak oxidizing agents like Tollens and Benedict’s solutions oxidize the aldehyde end to give an aldonic acid. FUNDAMENTALS OF BIOCHEMISTRY 15  Reduction Reactions Reduction to Alditol The carbonyl group in a monosaccharide (either an aldose or a ketose) is reduced to a hydroxyl group using hydrogen as the reducing agent, such as H2 or NaBH4 Reduction of the C=O group of a monosaccharide gives a polyhydroxy alcohol compound called an alditol, a sugar alcohol FUNDAMENTALS OF BIOCHEMISTRY 16  Reduction Reactions Reduction to Alditol FUNDAMENTALS OF BIOCHEMISTRY 17  Reduction Reactions Reduction to Alditol – Sorbitol (or glucitol) is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae. – It is used as a moisturizing agents in foods and cosmetics and as a sweetening agent in chewing gum – It is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics. FUNDAMENTALS OF BIOCHEMISTRY 18  Reduction Reactions Reduction to Alditol FUNDAMENTALS OF BIOCHEMISTRY 19 REDUCING SUGARS Under prescribed conditions, some sugars reduce silver ions to free silver and copper(II) ions to copper(I) ions. Such sugars are called reducing sugars. A reducing sugar will have one of the following groups. an aldehyde group (as in glyceraldehyde) a hydroxyketone (as in fructose) a cyclic hemiacetal group (as in glucose and maltose) FUNDAMENTALS OF BIOCHEMISTRY 20 REDUCING SUGARS The Benedict, Barfoed, and Fehling tests are based on the formation of a brick red copper(I) oxide precipitate as a positive result while the Tollens test is based on the formation of a silver mirror. FUNDAMENTALS OF BIOCHEMISTRY 21 REDUCING SUGARS The Barfoed test is more sensitive in that it can distinguish a reducing monosaccharide from a reducing disaccharide. The sugars are oxidized to carboxylic acids and the metal ions are reduced FUNDAMENTALS OF BIOCHEMISTRY 22 FUNDAMENTALS OF BIOCHEMISTRY 23  REDUCING SUGARS Sugars with the hemiacetal structure can be reducing sugars under alkaline conditions because the ring opens forming an aldehyde group. FUNDAMENTALS OF BIOCHEMISTRY 24 REDUCING SUGARS FUNDAMENTALS OF BIOCHEMISTRY 25  PHOSPHATE ESTER FORMATION The hydroxyl groups of a monosaccharide can react with inorganic oxyacids to form inorganic esters. Phosphate esters of various monosaccharides are stable in aqueous solution and play important roles in the metabolism of carbohydrates. FUNDAMENTALS OF BIOCHEMISTRY 26  AMINO SUGAR FORMATION One of the hydroxyl groups of a monosaccharide is replaced with an amino group In naturally occurring amino sugars the carbon 2 hydroxyl group is replaced by an amino group Amino sugars and their N-acetyl derivatives are important building blocks of polysaccharides such as chitin and hyaluronic acid FUNDAMENTALS OF BIOCHEMISTRY 27  GLYCOSIDE FORMATION The cyclic forms of monosaccharides, the hemiacetals, react with alcohols to form acetals(also called glycosides) A glycoside is an acetal formed from a cyclic monosaccharide by the replacement of the hemiacetal carbon —OH group with an—OR group to form a double ether A glycoside produced from: glucose -glucoside galactose –galactoside FUNDAMENTALS OF BIOCHEMISTRY 28 GLYCOSIDE FORMATION FUNDAMENTALS OF BIOCHEMISTRY 29 Glycoproteins These materials contain carbohydrate residues on protein chains. Very important examples of these materials are antibodies-chemicals that bind to antigens and immobilize them. The carbohydrate part of the glycoprotein plays a role in determining the part of the antigen molecule to which the antibody binds. FUNDAMENTALS OF BIOCHEMISTRY 30  Antigens used in the ABO blood group classification The human blood groups A, B, AB, and O depend on the oligosaccharide part of the glycoprotein on the surface of erythrocyte cells. The terminal monosaccharide of the glycoprotein at the nonreducing end determines the blood group. FUNDAMENTALS OF BIOCHEMISTRY 31  Antigens used in the ABO blood group classification FUNDAMENTALS OF BIOCHEMISTRY 32  Antigens used in the ABO blood group classification FUNDAMENTALS OF BIOCHEMISTRY 33 FUNDAMENTALS OF BIOCHEMISTRY 34