![]() Multiple glycogen chains are arranged in a spherical fashion to form a granule. Another enzyme known as branching enzyme hydrolyzes one of these linkages and starts a new branch using the oligosaccharide and attaching it via an α-1,6 glycosidic linkage. Starting with a peptide core, an enzyme known as glycogen synthase linearly arranges the glucose molecules via α-1,4 glycosidic links. (For a further discussion on solvation energy, refer to our guide on proteins.) Glycogen is a polysaccharide that is highly branched and compacted-to maximize storage capacity and minimize the solvation energy needed to store this molecule. As a result, our bodies store carbohydrates in the form of a polysaccharide known as glycogen. While monosaccharides and disaccharides are the most usable form of energy, they can be highly inefficient to store. However, this sugar has four cyclic isomers, with two being pyranoses and the other two being furanoses. Galactose, like glucose, is another aldose sugar. Note that it’s possible for a sugar to be both a hexose and a furanose-the sugar would simply have five atoms within the ring and at least one carbon atom outside of the ring.) (You may also see certain sugars referred to as hexoses and pentoses, meaning they have six and five carbon atoms, respectively. This monosaccharide is a ketose sugar, and in its cyclic form, it is classified as a furanose, meaning it is a five-membered ring. This is an aldose sugar, and in its cyclic form, it is classified as a pyranose, meaning it is a six-membered ring.Īnother common sugar you may see is fructose. One of the most common monosaccharides you will encounter is glucose. ![]() Some of these structures are discussed below. The MCAT will test your knowledge of how these sugars behave in reactions and also on the structure of particular monosaccharides and disaccharides. This composition varies slightly in disaccharides and polysaccharides but holds true for the composition of all monosaccharides. Sucrose, a disaccharide we will discuss below, is a common example of a non-reducing sugar.Īll monosaccharides, regardless of form, share a common chemical composition: Multiply monomers that are linked together form a polysaccharide.Īll monosaccharides are considered reducing sugars. Two monosaccharides can be attached via a glycosidic linkage to form a disaccharide. ![]() The two reagents react with reducing functional groups in unique ways: Benedict’s reagent reacts with aldoses to form a red copper precipitate, while Tollen’s reagent reacts with aldehydes to form a silver, mirrorlike precipitate.Ī sugar monomer is referred to as a monosaccharide. ANOMERIC CARBON FREEConversely, non-reducing sugars lack a free anomeric carbon.īenedict’s reagent and Tollen’s reagent are two reagents that are commonly used to detect the presence of reducing sugars. Reducing sugars can be identified through the presence of a free anomeric carbon, meaning it is not in a glycosidic bond and has a free hydroxyl group. Sugars can also be described as being “non-reducing” or “reducing.” A reducing sugar is one that can act as a reducing agent. (The final ratios of each of these forms are usually affected by stereochemistry unique to each molecule.) This tendency to undergo spontaneous cyclization is known as mutarotation. As a result, many sugars can be found in various combinations of D-, L-, and linear forms. Note that as we convert the structure of the sugar from its two-dimensional to three-dimensional structure, the functional group that is pointing to the right of the Fischer projection will end up pointing downward in the Haworth projection.įor most sugars that are dissolved in a solution, this cyclization occurs spontaneously. ![]() A sugar is in its β-anomer form when the anomeric carbon’s substituent is on the same side of the plane as the highest numbered chiral center’s substituent. A sugar is in its α-anomer form when the anomeric carbon’s substituent is on the opposite side of the plane as the highest numbered chiral center’s substituent. In a cyclic sugar, the anomeric carbon is the carbon that has two bonds to oxygen.īe aware that a cyclic sugar can exist in two possible anomers: an ⍺-anomer and a β-anomer. While Fischer projections represent the straight-chain form of carbohydrates, you may also see sugars represented in their cyclic form as Haworth projections. If the hydroxyl group of the highest-numbered chiral carbon is on the left, it is in the L-configuration. If the hydroxyl group of the highest-numbered chiral carbon is on the right, the sugar is in the D-configuration. For sugars, the absolute configuration is designated using D- and L- nomenclature instead of the R and S system used in organic chemistry. ![]()
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