Chapter 20: Carbohydrates

• Carbohydrates are a large class of naturally occurring polyhydroxy aldehydes and ketones.

• Monosaccharides(simple sugars), are the simplest carbohydrates. They have from three to seven carbon atoms, and ^^each contains one aldehyde or one ketone functional group^^.

• If the sugar has an aldehyde group, it is classified as an aldose.

• If it has a ketone group, the sugar is classified as a ketose. The aldehyde group is always at the end of the carbon chain, and the ketone group is always on the second carbon of the chain.

• Disaccharide is a carbohydrate composed of two monosaccharides and polysaccharide (complex carbohydrate) is a carbohydrate that is a polymer of monosaccharides.

• Monosaccharides are chiral molecules. They exist mainly in cyclic forms rather than the straight-chain forms.

• Fischer projection is a structure that represents chiral carbon atoms as the intersections of two lines, with the horizontal lines representing bonds pointing out of the page and the vertical lines representing bonds pointing behind the page. For sugars, the aldehyde or ketone is at the top.

  • D Sugar is a monosaccharide with the ¬OH group on the chiral carbon atom farthest from the carbonyl group pointing to the right in a Fischer projection.
  • L Sugar is a monosaccharide with the ¬OH group on the chiral carbon atom farthest from the carbonyl group pointing to the left in a Fischer projection.

• d-Glucose, also called dextrose or blood sugar, is the most abundant of all monosaccharides and has the most important function. In nearly all organisms, d-glucose serves as a source of energy to fuel biochemical reactions. It is stored as starch in plants and glycogen in animals

• Cyclic monosaccharides that differ only in the positions of substituents at carbon 1 are known as anomers, and carbon 1 is said to be an anomeric carbon atom.

  • It is the carbonyl carbon atom (C1 in an aldose and C2 in a ketose) that is now bonded to two O atoms.

• Mutarotation is the change in rotation of plane-polarized light resulting from the equilibrium between cyclic anomers and the open-chain form of a sugar.

• In glucose, the hemiacetal carbon (the anomeric carbon) is chiral, and a and b anomers differ in the orientation of the ¬OH groups on this carbon.

  • The a anomer has the ¬OH on the opposite side of the ring from the ¬CH2OH, and the b anomer has the ¬OH on the same side of the ring as the ¬CH2OH.

• Monosaccharides can form ^^multiple hydrogen bond^^s through their hydroxyl groups and are generally high-melting, white, crystalline solids that are soluble in water and insoluble in nonpolar solvents.

• Most monosaccharides and disaccharides are sweet-tasting, digestible, and nontoxic.

• Reactions of monosaccharides:

  • Oxidation of a monosaccharide can result in a carboxyl group on the first carbon atom (C1 in the Fischer projection). Ketoses, as well as aldoses, are reducing sugars because the ketose is in equilibrium with an aldose form that can be oxidized.
  • Reaction of a hemiacetal with an alcohol produces an acetal. For a cyclic monosaccharide, reaction with an alcohol converts the ¬OH group on the anomeric carbon to an ¬OH group. The bond to the ¬OR group, known as a glycosidic bond, is alpha or beta to the ring as was the ¬OH group. Disaccharides result from glycosidic bond formation between two monosaccharide.
  • Hydrolysis reactions of polysaccharides produce the monomeric units that formed the polysaccharide. For example, hydrolysis of starch yields glucose.

• ^^Hyaluronate, chondroitin-6-sulfate, heparin, and glycoproteins have different modified glucose subunits paired^^ (dimers) as repeating units in the polymer chains.

  • Joints and intracellular spaces are lubricated by polysaccharides like hyaluronate and chondroitin 6-sulfate.
  • Heparin binds to a clotting factor in the blood and thus acts as an anticoagulant. Glycoproteins function as receptors at cell surfaces.

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