CH 8. Polysaccharides and on PDF

**Polysaccharides** - Polysaccharides (glycans) are monosaccharides linked together by glycosidic bonds. - Homopolysaccharides have one type of monosaccharide. - Heteropolysaccharides have more than one type of monosaccharide. - Polysaccharides form branched as well as linear polymer...

**Polysaccharides** - Polysaccharides (glycans) are monosaccharides linked together by glycosidic bonds. - Homopolysaccharides have one type of monosaccharide. - Heteropolysaccharides have more than one type of monosaccharide. - Polysaccharides form branched as well as linear polymers - Specific exo-glycosidases and endoglycosidases, hydrolyze monosaccharide units from polysaccharide. **Lactose: A Reducing Disaccharide** - Oligosaccharide containing three or more residues are relatively rare, accruing almost entirely in plants. - Disaccharides, the simplest polysaccharides, are more common. A reducing disaccharide Lactose (because it has a free anomeric carbon on glucose) occur naturally only in milk. It is made up of galactose and glucose called as O-β-D-galactopyranosyl-(1→4)-D-glucopyranose. - C1→4O symbolizes glycosidic bond linking C1 of the β anomer of galactose to O4 of glucose. **Sucrose: A Non-Reducing Disaccharide** - The most abundant disaccharide is sucrose. The major form in which carbohydrates are transported in plants. It is similar to common table sugar. - Made of glucose and fructose and Name is O-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside, indicating that the anomeric carbon of each sugar (C1 in glucose and C2 in fructose) participates in glycosidic bond , hence sucrose is a non-reducing sugar. **Artificial Sweeteners** - Artificial sweeteners (added to processed foods and beverages to impart a sweet taste without adding calories) like the ones listed here, have sweetness relative to sucrose. - Saccharin is one of the oldest sweeteners. Found to be cancerous in laboratory rats. Is not metabolized by human body. - Aspartame is currently the market leader. Aspartame I s broken down into aspartate (green), phenylalanine (red) and methanol (blue). The Asp and Phe can be metabolized, so aspartate is not calorie free and Methanol in large amount is toxic. Individuals with phenylketonuria (genetic disease) are unable to metabolize Phe and are advised to avoid ingesting excess aspartame. Drawback is, it is unstable to heating and not suitable for baking. Acesulfame is sometimes used in combination with aspartame to get synergistic greater combined sweetness. **Cellulose Fibers** - Plants have rigid cell walls. Cellulose the primary structural component of cell walls accounts for over half of the carbon in biosphere (zone of life on earth). - Figure shows the electron micrograph of cellulose fibers. Cellulose fibers in this sample of cell wall from alga *Chaetomorpha* are arranged in layers. **Cellulose: β(1→4)-Linked d-Glucose** - Cellulose is a linear polymer of up to 15, 000 D-glucose residues linked by β(1→4) glycosidic bonds. **Cellulose: Tightly Packed, Fully Extended Conformation** - Cellulose fibers consist of \~40 parallel, extended glycan chains. Each of the β(1→4) linked glucose units in a chain is rotated 180 ◦ with respect to its neighboring residues. - This permits the C3-OH group of each glucose residue to form a hydrogen bond with the ring oxygen (O5) of the next residue. - [Parallel cellulose chains form sheets] with interchain hydrogen bonds (dashed lines), including O2-H^.....^ O6 and O6-H^.....^O3 bonds. Stack of these sheets are held together by hydrogen bonds and Van der Waals interactions. - This highly cohesive structure gives cellulose fibers exceptional strength and make them water insoluble despite hydrophilicity. - Cellulose is linked with other polysaccharides and [lignin] which makes it hard to convert to biofuels due to difficulty in removing other substances from cellulose. - [Digestive tract of herbivores and termites have enzyme **cellulases** to hydolyze cellulose]. **Chitin:** **β(1→4)-Linked N-Acetyl-d-Glucosamine** - **Chitin** is the structural component of the exoskeletons of invertebrates such as crustaceans, insects and spiders and also present in cell walls of most fungi and many algae. It is second most abundant biomolecule after cellulose. - It is a homopolymer of β(1→4) linked N-acetyl-d-glucosamine residues. It differs from cellulose only in that each C2-OH group is replaced by an acetamido group. X-ray analysis indicate that cellulose and chitin have similar structures. **α-Amylose: α(1→4)-Linked d-Glucose** - **Starch** is a mixture of glycans that plant synthesize as their principal energy reserve. It is deposited in the chloroplasts of plants as insoluble granules composed of α-amylose and amylopectin. - **α-amylose** is a linear polymer of several thousand glucose residues linked by α (1→4) bonds. - **α-Amylose**'s α-glycosidic bonds cause it to adopt an irregularly aggregating helical coiled (left-handed helix) conformation. - (unlike cellulose's β-glycosidic linkages cause it to assume a tightly packed fully extended conformation) **Amylopectin: α(1→6)-Branches** - Amylopectin consist mainly of α(1→4) linked glucose residues but is a branched molecule with α(1→6) branch points every 24 to 30 glucose residues on average. Amylopectin molecules contains up to 10^6^ glucose residues, making them some of the largest molecules in nature. - Starch is a reducing sugar, although it has only one residue called reducing end that lacks a glycosidic bond. - Starch is the main carbohydrate source in the human diet. The digestion of starch begins in mouth. Saliva contains [amylase], which hydrolyses α (1→4) glycosidic bonds of starch. - In small intestine [pancreatic amylase] degrades starch to mixture of small oligosaccharides. Further hydrolysis occur by [α-glucosidase] which removes one glucose residue at a time and by a [debranching enzyme] which hydrolyses α (1→6) and α (1→4) bonds, producing monosaccharides that are absorbed by the intestine and transported to the blood stream. **Glycogen Granules In Liver Cell** - Glycogen, the storage polysaccharide of animals is present in all cells but is most prevalent in skeletal muscles and liver, where it occur as cytoplasmic granules (in photomicrograph, glycogen granules are pink, mitochondrion are greenish, and fat globules are yellow). - The glycogen content of liver may reach 10% of its net weight. The 1◦ structure of glycogen resemble of amylopectin but more [branched], at [every 8 to 14 glucose residues]. - In cells glycogen is degraded for metabolic use by [glycogen phosphorylase] cleaving α(1→4) bond. Glycogen's highly branched structure having many nonreducing ends permits rapid mobilization of glucose in times of metabolic needs. - The α(1→6) branches of glycogen are cleaved by [glycogen debranching] enzyme. -