Carbohydrates & Glycoconjugates

Comprehensive vocabulary flashcards covering the basics of carbohydrates, glycosides, disaccharides, polysaccharides, glycoconjugates, extracellular matrix components, bacterial cell walls, glycoproteins, and dietary fiber.

Carbohydrates

Components of supportive structures in plants (cellulose), crustacean shells (chitin), and animal connective tissues (acidic polysaccharides) that are classified by the number of simple sugars they contain.

Monosaccharides

Carbohydrates that cannot be hydrolyzed to simpler carbohydrates, characterized as sweet, colorless crystalline solids that are very soluble in water and slightly soluble in ethanol.

Oligosaccharides

Carbohydrates composed of a few simple sugar units.

Polysaccharides

Polymers containing many monosaccharide units.

Anomeric carbon

The carbon atom of a hemiacetal that forms the new bond to the –OR group during acetal formation.

Glycoside

A cyclic acetal derived from a monosaccharide that cannot undergo mutarotation because its ring cannot open.

Glycosidic bond

The bond formed from the anomeric carbon of a hemiacetal to the –OR group.

Sorbitol

An alditol found in berries, cherries, plums, pears, apples, seaweed, and algae; it is about $60\%$ as sweet as sucrose and used in candies and as a sugar substitute for diabetics.

Sucrose

The most abundant disaccharide in the biological world; a non-reducing sugar made of $\alpha\text{-D-glucopyranose}$ and $\beta\text{-D-fructofuranose}$ joined by an ($\alpha 1 \rightarrow \beta 2$) linkage.

Lactose

The principal sugar in milk ($5\text{--}8\%$ of human milk, $4\text{--}6\%$ of cow's milk), consisting of D-galactose bonded to carbon 4 of D-glucose by a $\beta(1 \rightarrow 4)$ glycosidic bond.

Maltose

A disaccharide present in malt (juice of sprouted barley and cereal grains) consisting of two glucose units linked by an $\alpha\text{-1,4-glycosidic}$ bond.

Amylose

An unbranched starch component composed of chains of up to $4000$ D-glucose units joined by $\alpha\text{-1,4}$ bonds that adopt a helical structure.

Amylopectin

A branched starch component composed of chains of up to $10{,}000$ D-glucose units joined by $\alpha\text{-1,4-glycosidic}$ bonds, with branch chains of $24\text{--}30$ units attached by $\alpha\text{-1,6}$ linkages.

Glycogen

The energy-reserve carbohydrate of animals consisting of a branched polysaccharide of approximately $10^6$ glucose units joined by $\alpha\text{-1,4}$ and $\alpha\text{-1,6}$ bonds, with about $350\,\text{g}$ stored in liver and muscle.

Cellulose

An insoluble structural chain of D-glucose units joined by $\beta\text{-1,4-glycosidic}$ bonds (averaging approximately $2200$ units per molecule) whose parallel bundles cannot be digested by humans and most animals except ruminants.

Glycobiology

The study of the structure and function of glycoconjugates.

Glycoconjugates

Biologically active molecules made up of informational carbohydrates covalently joined to a protein or a lipid.

Proteoglycans

Macromolecules of the cell surface or extracellular matrix consisting of approximately $95\%$ carbohydrate, in which the glycosaminoglycan moiety dominates the structure and functions as the main site of biological activity.

Glycoproteins

Proteins containing one or several covalently attached oligosaccharides that serve as highly specific recognition sites on cell membranes, in blood, and in organelle membranes.

Glycolipids

Membrane lipids whose hydrophilic head groups are oligosaccharides that provide specific recognition sites for carbohydrate-binding proteins.

Extracellular matrix (ground substance)

A gel-like material filling extracellular space in animal tissues composed of an interlocking meshwork of heteropolysaccharides and fibrous proteins that holds cells together and provides a diffusion pathway.

Glycosaminoglycans (GAGs)

Unbranched polysaccharides composed of alternating hexosamine and hexuronic acid residues bearing sulfate groups, usually linked to proteins as proteoglycans.

Hyaluronan

A ubiquitous, non-sulfated GAG composed of repeating disaccharides of glucuronic acid and N-acetylglucosamine, forming chains of up to $25{,}000$ sugars (about $50{,}000$ disaccharides).

Trisaccharide linker

The sequence $\text{GlcA} \rightarrow \text{Gal} \rightarrow \text{Gal} \rightarrow \text{Xyl} \rightarrow \text{Ser}$ that covalently connects a glycosaminoglycan to a serine residue of a core protein.

Proteoglycan aggregate

A supramolecular assembly consisting of many aggrecan core proteins bound to a single molecule of hyaluronate along with keratan sulfate, chondroitin sulfate, and link proteins.

Aggrecan

A cartilage proteoglycan core protein ($\text{MW } 210{,}000$) carrying approximately $130$ chondroitin sulfate and keratan sulfate chains that provides mechanical support and interacts with collagen.

Decorin

A proteoglycan with a core protein of $\text{MW } 40{,}000$ carrying one chondroitin/dermatan sulfate chain, found widely in connective tissues where it binds type I collagen fibrils and TGF-$\beta$.

Perlecan

A basal lamina proteoglycan with a core protein of $\text{MW } 600{,}000$ bearing $2\text{--}15$ heparan sulfate chains that provides structural and filtering functions.

Syndecan

An epithelial cell surface proteoglycan ($\text{MW } 32{,}000$) with $1\text{--}3$ chondroitin and heparan sulfate chains that participates in cell adhesion and growth factor binding.

Dally

A Drosophila cell surface proteoglycan ($\text{MW } 60{,}000$) bearing $1\text{--}3$ heparan sulfate chains that acts as a co-receptor for Wingless and Decapentaplegic signaling proteins.

Fibronectin

An extracellular fibrous matrix protein that contains separate binding domains for fibrin, heparan sulfates, collagen, and integrins.

Integrins

Membrane proteins possessing binding sites for extracellular macromolecules that mediate signaling between the cell interior and the extracellular matrix.

Peptidoglycan

A bag-like encasing molecule in bacterial cell walls made of alternating NAG and NAM units joined by $\beta(1 \rightarrow 4)$ linkages, with tetrapeptides and pentaglycine cross-links.

Lysozyme

An enzyme found in tears and produced by certain bacterial viruses that hydrolyzes the glycosidic bonds between MurNAc and GlcNAc in peptidoglycan.

O-linked oligosaccharide

An oligosaccharide attached to a protein through a glycosidic bond to the hydroxyl (–OH) group of a serine or threonine residue.

N-linked oligosaccharide

An oligosaccharide attached to a protein through a glycosidic bond to the amide nitrogen atom of an asparagine residue.

Glycoforms

Variant species of an N-linked glycoprotein that differ in the sequence, location, and number of covalently attached oligosaccharide chains.

Mucins

Large O-linked glycoproteins in mucus containing sulfated, mutually repelling carbohydrate chains that adopt random-coil conformations and create protective viscoelastic gels.

Lectins

Carbohydrate-binding proteins present in all organisms that bind specific sugars with high affinity to facilitate cell–cell recognition and adhesion.

Gangliosides

Membrane glycolipids composed of sialic acid, galactose, N-acetylgalactosamine, and glucose attached to a ceramide base.

Lipopolysaccharides (LPS)

The dominant surface molecules on the outer membrane of gram-negative bacteria, composed of Lipid A, a core oligosaccharide, and an O-antigen.

Crude fiber

The residue of plant food that remains after chemical extraction with acid and alkali.

Dietary fiber

The total residue of plant food resistant to human digestive enzymes ($1\,\text{g}$ crude fiber = $2\text{--}4\,\text{g}$ dietary fiber), encompassing all plant polysaccharides and lignins.

Resistant starch

Raw or retrograded starch granules that resist small intestinal hydrolysis because of their physical structure and are degraded by bacteria in the large intestine.

Lignins

The primary non-carbohydrate component of dietary fiber, consisting of inert three-dimensional polymer networks of phenylpropane units in woody plant parts.

Soluble fiber

Dietary fiber components (such as pectins, gums, and mucilages) that dissolve and thicken in water to produce a viscous gel.

Insoluble fiber

Dietary fiber roughage (including cellulose, lignin, and some hemicelluloses) from structural plant walls that does not dissolve in water.