Carbohydrates: Disaccharides and Polysaccharides Disaccharides Linkage of the anomeric carbon of one monosaccharide to the OH of another monosaccharide via a condensation reaction. The bond is termed a glycosidic bond. The end of the disaccharide that contains the anomeric carbon is referred to as the reducing end because it is capable of reducing various metal ions.

Formation of a glycosidic bond between two glucose molecules.

Nomenclature: To describe disaccharides you need to specify the following: 1. The names of the two monosaccharides. 2. How they are linked together (one anomeric is always used). 3. The configuration of the anomeric carbons on both monosaccharides. The Six Simple Rules for Naming Disaccharides are as follows:

1. The non-reducing end defines the first sugar. 2. Configuration of the anomeric carbon of the 1st sugar (α,β). 3. Name of 1st monosaccharide, root name followed by pyranosyl (6- ring) or furanosyl (5-ring). 4. Atoms which are linked together, 1st sugar then 2nd sugar. 5. Configuration of the anomeric carbon of the second sugar (α,β) (often omitted if the anomeric carbon is free since α and β forms are in equilibrium.) 6. Name of 2nd monosaccharide, root name followed by pyranose (6- ring) or furanose (5-ring) (If both anomeric carbons are involved, then the name ends in ‘oside”, not ‘ose’) Example: Sucrose (table sugar): The anomeric carbon of glucose forms a bridge to the anomeric carbon of fructose. Since there is no reducing end, either sugar can be used to begin the name.

Sucrose, drawn in two different ways. Note that both anomeric carbons are involved in the glycosidic linkage, thus both conformations have to be specified.

Example: Lactose (sugar in mammalian milk): Galactose and glucose are linked together. Note the alternative way to indicate the β linkage in the right-hand drawing. Since the reducing end is free, and can interconvert between α and β forms, it is not necessary to specify the configuration of the anomeric carbon on the glucose.

Lactose, drawn in two different ways. The anomeric carbon on galactose is involved in the β glycosidic linkage; consequently its configuration has to be specified. The anomeric carbon on the reducing end (glucose) can be in either configuration; the α configuration is represented here. Polysaccharides Polysaccharides are many monosaccharides linked by glycosidic bonds. Most poly-saccharides are polymers of either glucose, or modified glucose. They play important roles in the structure of plants (cellulose) as well as energy storage in both plants (starch) and animals (glycogen).

Nomenclature: : In the case of homo-polymers, the short-hand notation is to simply describe the linkage between the glucose units: both the conformation of the anomeric carbon and the carbons participating in the glycosidic bond, i.e. "β (1-4) glucose" instead of "β- glucopyranosyl (1-4) β glucopyranose."

Release of glucose from glycogen/amylopectin: Glucose units are released from non-reducing end by the enzyme glycogen phosphorylase, producing glucose-1-phosphate (G-1-P). G-1-P is used to generate energy by entering glycolysis.

Release of glucose-1-phosphate from glycogen. The glucose is released from the non-reducing end.

Structural Polysaccharides

1. Cellulose: Structural polysaccharide of plants. 1. Mammalian enzymes can’t digest β 1-4 glucose. 2. Similar in structure to β-sheets in proteins: forms flat sheets with multiple hydrogen bonds between strands. 3. Digested by symbiotic microorganisms (such as those in termites).

Cellulose. The left and right diagrams are both of cellulose; they differ in how the β linkage is drawn.

Hydrogen bonded networks in cellulose forms a stable sheet-like structure.

2. Bacterial Cell Walls: 1. Polysaccharide chains of alternating N-acetylglucose amine (NAG) and N-acetylmuraminc acid (NAM) 2. Muramic acid on NAM linked to a small peptide (L-Ala D-Gln L-Lys D-Ala). One of the few places in biochemistry where D-amino acids are found. 3. NAM peptide chains are crosslinked with pentaglycine bridges that extend off of terminal Ala and join to L-Lys sidechain on adjacent chain, forming a tough crosslinked cell wall. 4. A structure that has covalent interactions between the carbohydrate chains and the protein chains is called a peptido-glycan 5. Synthesis of bacterial cell walls is inhibited by penicillin.

Bacterial Cell Wall. This peptido-glycan consists of linear polysaccharides, which consist of alternating NAM and NAG residues, covalently crosslinked by peptide chains. Left image is the molecular structure; the right image shows how a number of carbohydrate strands are linked together.