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 among cells • 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
Monosaccharides • Fischer projections of monosaccharides • D,L designation refers to the configuration of the highest-numbered asymmetric center • D,L only refers the stereocenter of interest back to D- and L-glyceraldehyde! • D,L do not specify the sign of rotation of plane-polarized light!
Modified Monosaccharides • Addition of substituents other than alcohols; often found on cell surfaces
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
StarchA plant 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
StarchA 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
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
DextransA 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)
DextransA 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
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. CARAGEENAN • 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 - repeating disaccharides with amino sugars and negative charges
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”