Carbohydrates: Structure and Nomenclature

Carbohydrates

• Carbohydrates are produced from carbon dioxide and water through photosynthesis in plants.
• Carbohydrates serve many functions: as a source of energy, energy storage, structural component, and cell signaling molecules.
• Carbohydrates can covalently link to proteins and lipids.

Carbohydrate Nomenclature

• The basic nomenclature is based on the number of carbon atoms in the carbohydrate followed by "-ose."
• For example, a three-carbon carbohydrate is called a triose.
• All carbohydrates initially have a carbonyl functional group.
• Aldehydes are called aldoses, and ketones are called ketoses.

Monosaccharides

• Monosaccharides can be constitutional isomers: aldose is a carbohydrate with aldehyde functionality, ketose is a carbohydrate with ketone functionality
• Monosaccharides can be stereoisomers: epimers are stereoisomers differing at only one chiral center; epimers are not mirror images, therefore not enantiomers; epimers are diastereomers; diastereomers have different physical properties
• Monosaccharides can have straight and ring structures.

Disaccharides and Glycosidic Bond

• Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon.

Polysaccharides

• Polysaccharides are typically found in their polymer forms.
• Polysaccharides can be homopolysaccharides (one monomer unit), heteropolysaccharides (multiple monomer units), linear (one type of glycosidic bond), and branched (multiple types of glycosidic bonds).
• Polysaccharides don't have a defined molecular weight.
• Polysaccharides are often in a state of flux, with monomer units added and removed as needed by the organism.

Homopolymers of Glucose: Glycogen

• Glycogen is a branched homopolysaccharide of glucose.
• Glucose monomers form (α1 4) linked chains and branch points with (α1 6) linkers every 8–12 residues.
• Glycogen functions as the main storage polysaccharide in animals.

Homopolymers of Glucose: Starch

• Starch is a mixture of two glucose homopolysaccharides: amylose and amylopectin.
• Amylose is unbranched with (α1 4) linked residues.
• Amylopectin is branched like glycogen, but the (α1 6) linkers occur every 24–30 residues.
• Starch is the main storage polysaccharide in plants.

Dextrins

• Dextrins are low-molecular-weight carbohydrates produced by partial hydrolysis of starch or glycogen.
• They are polymers or D-glucose linked by α-(1→4) or α-(1→6) glycosidic bonds.
• Dextrins can be produced in the gut and during cooking.
• Dextrins can be used as water-soluble adhesives.

Homopolymers of Glucose: Cellulose

• Cellulose is a linear homopolysaccharide of glucose.
• Glucose monomers form (1 4) linked chains.
• Hydrogen bonds form between adjacent monomers and additional H-bonds between chains.
• Cellulose is tough, water insoluble, and the most abundant polysaccharide in nature.
• Cotton is nearly pure fibrous cellulose.

Chitin

• Chitin is a linear homopolysaccharide of N-acetylglucosamine.
• N-acetylglucosamine monomers form (1 4)-linked chains.
• Chitin forms extended fibers similar to those of cellulose.
• It is hard, insoluble, and cannot be digested by vertebrates.
• Chitin is found in cell walls of mushrooms and exoskeletons of insects, spiders, crabs, and other arthropods.

Inulin

• Inulin is a plant polysaccharide made from fructose units with a chain-terminating glucose.
• Inulin has a sweet taste and is present in roots for energy storage.
• Most plants that synthesize inulin do not store energy in the form of starch.
• Inulin is a good source of dietary fiber.

Agar and Agarose

• Agar is a branched heteropolysaccharide composed of agarose and agaropectin.
• Agar is a cell wall component in some seaweeds.
• Agar solutions form gels used in laboratories as a surface for growing bacteria.
• Agarose solutions form gels used in laboratories for DNA separation via electrophoresis.

Glycosaminoglycans

• Glycosaminoglycans are linear polymers of repeating disaccharide units.
• One monomer is either N-acetyl-glucosamine or N-acetyl-galactosamine.
• They are negatively charged due to uronic acids and sulfate esters.
• Glycosaminoglycans form an extended hydrated molecule which minimizes charge repulsion.
• They form a meshwork with fibrous proteins to form the extracellular matrix, connective tissue, and lubricate joints.

Heparin and Heparan Sulfate

• Heparin is a linear polymer of 3–40 kDa.
• Heparan sulfate is a heparin-like polysaccharide attached to proteins.
• They have the highest negative-charge density among biomolecules.
• Heparin prevents blood clotting by activating the protease inhibitor antithrombin.
• Heparin regulates development and blood vessel formation by binding to various cells.
• Heparin can bind to viruses and bacteria, decreasing their virulence.

This quiz covers the essential aspects of carbohydrates, including their production, functions, and nomenclature. It delves into monosaccharides, isomers, and their roles in biological systems. Test your understanding of these crucial biological molecules.