Carbohydrates

Carbohydrates “Hydrates of carbon” (C, H, & O) Polyhydroxy aldehydes (ALDOSES) or ketones (KETOSES) Usually Cx(H2O)y “Sugars” Single unit: Monosaccharide Two units: Disaccharide Three units: Trisaccharide, etc. Many units: Polysaccharide

Carbohydrates Functions Energy stores, fuels, and metabolic intermediates Ribose and deoxyribose serve as structural framework to RNA and DNA Structural elements in the cell walls of bacteria and plants Linked to lipids and proteins Mediates interactions between cells and other elements in the cellular environment

Monosaccharides The simplest carbohydrates; aldehydes or ketones that have two or more hydroxyl groups

Glucose Cyclizes

Fructose Cyclizes

Ribose and Deoxyribose

Glycosidic Bonds

Modified Monosaccharides Addition of substituents other than alcohols; often found on cell surfaces

Disaccharides

Polysaccharides Energy storage and structural roles Are homopolymers if all of the monosaccharides are the same, heteropolymers if not.

Some Common Polysaccharides Starch: glucose polymer (alpha) Potato, rice, wheat, corn Glycogen: branched glucose polymer Animal storage Cellulose: glucose polymer (beta) Plant structures, paper, cotton, wood Chitin: Modified glucose (N-Acetyl Glucose) Fungi cell wall, insect exoskeleton

Polysaccharides

Starch A plant energy storage polysaccharide Two forms: amylose and amylopectin Most starch is 10-30% amylose and 70-90% amylopectin Branches in amylopectin every 12-30 residues Amylose has alpha(1,4) links, one reducing end

Starch Amylose-unbranched

Starch Amylopectin-branched

Starch A plant storage polysaccharide Amylose is poorly soluble in water, but forms micellar suspensions In these suspensions, amylose is helical iodine fits into the helices to produce a blue color

Why branching in Starch? Consider the phosphorylase reaction... Phosphorylase releases glucose-1-P products from the amylose or amylopectin chains The more branches, the more sites for phosphorylase attack Branches provide a mechanism for quickly releasing (or storing) glucose units for (or from) metabolism

The glucose storage device in animals Glycogen The glucose storage device in animals Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass Glycogen is stored energy for the organism Only difference from starch: number of branches Alpha(1,6) branches every 8-12 residues Like amylopectin, glycogen gives a red-violet color with iodine

Dextrans A small but significant difference from starch and glycogen If you change the main linkages between glucose from alpha(1,4) to alpha(1,6), you get a new family of polysaccharides - dextrans Branches can be (1,2), (1,3), or (1,4)

Dextrans A small but significant difference from starch and glycogen Dextrans formed by bacteria are components of dental plaque Cross-linked dextrans are used as "Sephadex" gels in column chromatography These gels are up to 98% water!

Structural Polysaccharides Composition similar to storage polysaccharides, but small structural differences greatly influence properties Cellulose is the most abundant natural polymer on earth Cellulose is the principal strength and support of trees and plants Cellulose can also be soft and fuzzy - in cotton

Cellulose

Structural Polysaccharides Composition similar to storage polysaccharides, but small structural differences greatly influence properties Beta(1,4) linkages make all the difference! Strands of cellulose form extended ribbons

Other Structural Polysaccharides Chitin - exoskeletons of crustaceans, insects and spiders, and cell walls of fungi similar to cellulose, but C-2s are N-acetyl cellulose strands are parallel, chitins can be parallell or antiparallel

Other Structural Polysaccharides Alginates - Ca-binding polymers in algae Eg. CARRAGEENAN alternating 3-linked-α-D-galactopyranose and 4-linked-β-D-galactopyranose units Sulfonated, uses cations to gel; Common types  Agarose and agaropectin - galactose polymers

Glycosaminoglycans Made of disaccharide repeating units containing a derivative of an amino sugar, either glucosamine or galactosamine

Glycosaminoglycans Present in the animal cell surface or the extracellular matrix Usually attached to proteins to form proteoglycans Proteoglycans resemble polysaccharides more than proteins Function as lubricants and structural components in connective tissue, mediate adhesion of cells to the extracellular matrix, and bind factors that stimulate cell proliferation

Glycoproteins Carbohydrates attached to proteins Carbohydrates are a much smaller percentage of the weight of glycoproteins than of proteoglycans Present in cell membranes Cell adhesion and the binding of sperm to eggs Present in soluble proteins

CARBOHYDRATES Extra carbohydrates is stored in the liver and muscle tissues as glycogen Carbohydrates supply 4 kcal of energy per gram “Good” and “Bad” carbohydrates Glycemic index

On the “Sugarfree” Phenomenon Saccharin Oldest artificial sweetener Aspartame and Acesulfame-K Not carbohydrates Sucralose “Splenda”