Introduction to Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these compounds on hydrolysis.
• The term carbohydrate comes from the French "hydrate de carbone."
• Carbohydrates have the general formula (CH2O)n and contain only C, H, and O atoms.
• Carbohydrates have C=O and –OH functional groups.
• The term saccharide comes from the Greek word sakcharon, meaning "sugar."
• Carbohydrates are the most abundant compounds in nature and are products of photosynthesis.
• During photosynthesis, energy from the Sun combines CO2 and H20 to produce glucose (C6H1206) and 02.
• During respiration, carbohydrates are oxidized to CO2 and H20, releasing energy.
• Carbohydrates function as energy sources (glucose) and for energy storage (glycogen, starch)
• Carbohydrates act as Carbon sources and are used for structure/protection (chitin, cellulose, connective tissue)
• Carbohydrates also play Recognition/Signaling roles (Antibodies used for immune system recognition)
• Carbohydrates can be attached to other macromolecules, forming glycoproteins and glycolipids.
• Monosaccharides contain a single sugar unit.
• Disaccharides are made of two monosaccharide units linked together.
• Polysaccharides consist of many monosaccharide units linked together.
• Honey contains the monosaccharides fructose and glucose.
• Milk contains the disaccharide lactose.
• Cotton consists of the polysaccharide cellulose.

Monosaccharides

• Polyhydroxy aldehydes or ketones are known as monosaccharides.
• The basic formula for monosaccharides is (CH2O)n, where n is an integer from 3 to 7.
• Classification can be based on functional group (aldehyde or ketone) and the number of carbon atoms.

Functional Group Classification

• Ketone groups contain a carbonyl group (C=O) bonded to two other carbon atoms.
• Aldehyde groups contain a carbonyl group (C=O) bonded to at least one hydrogen atom.

Number of Carbon Atoms Classification

• Trioses have 3 carbon atoms.
• Tetroses have 4 carbon atoms.
• Pentoses have 5 carbon atoms.
• Hexoses have 6 carbon atoms.

Properties of Monosaccharides

• Usually has a sweet taste
• They are colorless, crystalline solids at room temperature.
• Have low melting points.
• They are soluble in water and insoluble in nonpolar solvents.
• They are digestible and non-toxic.
• Monosaccharides contain chiral carbon atoms and therefore can form enantiomers.

Stereochemistry of Carbohydrates

• Glyceraldehyde has a chiral carbon atom and thus exists as two enantiomers.
• Emil Fischer made arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde in 1891.
• The letters D and L distinguish between members of a pair of enantiomers and are based on the chiral carbon furthest from the carbonyl carbon.

D and L Sugars

• D-Glucose, D-Galactose, and D-Fructose are examples of biologically important monosaccharides.
• The "n rule" for monosaccharides: for a monosaccharide with n chiral carbons, there are 2n stereoisomers.
• Glucose has 4 chiral carbons and 24 = 16 stereoisomers, grouped into 16/2 = 8 pairs of enantiomers.

Trioses

• The simplest aldose is -glyceraldehyde.
• The simplest ketose is dihydroxyacetone.

Biologically Important Monosaccharides

• D-Glucose, or dextrose (C6H12O6), is an aldohexose found in fruits, vegetables, corn syrup, and honey.
• Glucose is a building block of sucrose, lactose, maltose, amylose, cellulose, and glycogen.
• D-Galactose is an aldohexose with the formula C6H12O6 found in milk products.
• It has a similar structure to -glucose except for the -OH on C4 and is important in cell membranes of the brain and nervous system.
• D-fructose (C6H12O6) is a ketohexose also known as levulose or fruit sugar, found in fruit juices
• 40% fructose is found in honey
• It is the sweetest of the carbohydrates around twice as sweet as sucrose
• High-fructose corn syrup (HFCS) used as a sweetener that converts glucose in corn syrup to fructose.
• HFCS with 42% fructose is used in bakery goods, while 55% fructose is used in soft drinks.
• D-ribose and D-2-deoxyribose are found in DNA, RNA and various coenzymes.

Epimers

• Epimers are sugars that differ only in the configuration around one carbon atom.
• This term is used when comparing sugars with multiple chiral centers and only one center differs

HaWroth Structures

• The carbonyl group of an aldehyde or ketone reacts with the hydroxyl group (-OH group) of an alcohol, forming a hemiacetal or hemiketal.
• Anomers are stereoisomers that differ in the arrangement of groups at the carbon of an acetal, ketal, hemiacetal, or hemiketal group.
• The -OH group can be above or below the ring forms generating two forms, identified using the a and ẞ.

Fructose

• Forms Haworth structures when the -OH on C5 bonds to the C=O on C2.
• The five groups of rings & C6 is up

Rings types

• Furanose rings contain a five-membered
• A pyranose rings conatin a six-membered sugar ring system containing an oxygen atom.

Properties of Haworth Structures

• The following represent the percent compositions of a-D-glucose (36%), D-glucose (0.02%) , and B-D-glucose (64%)
• When a carbon with an adjacent hydroxyl combines it can be oxidized to a carboxylic aid by an oxidizing agent, sugars acids result as a result
• The sugar acids produced by the oxidizing have names thay end in -onic acid

Monosacharide and carbonyl groups

• Reduction of the carbonyl group in monosaccharides produces sugar alcohols, also called alditols.
• This results in replacement of -ose ending of the monosaccharide with -itol.
• Sugar alcohols like d-sorbitol, d-xylitol from d-xylose, and d-mannitol from d-mannose are used as sweeteners

Disacharides

• Disaccharides are composed of two monosaccharides linked together, with formula 2C6H12O6 -> C12H22O11 + H₂O
• Key formula: glucose + glucose turns to maltose + H2O , glucose + galactose turns to lactose + H2O and glucose + fructose turns to sucrose + H2O

Sucrose

• A disaccharide with two anomeric carbons: C-1 (glucose) and C-2 (fructose)
• Because of the anomeric carbons, it's a non-reducing sugar
• Hydrolyzed by enzymes to form a mixture of glucose and fructose - “invert sugar"

Lactose

• Consists of Galactose
• Is found in Milk

Maltose

• Is composed of two D-glucose monomers connected by an alpha linakge betwee C-1 and C-4
• It is an ingredient in syrups, and is used in cereals, candies, and brewing, and is found in alpha and beta forms

Polysacharides (Glycans)

• Polysaccharides contain over ten monosaccharide units and are bonded by glycosidic linkages.
• Homopolysaccharides consist of one type of monosaccharide (e.g., starch, glycogen, cellulose polymers of glucose and Inulin which is polymer of fructose)
• Heteropolysaccharides contain more than one type of monosaccharide unit (e.g., Mucopolysaccharides, Agar, Pectins etc.)

Properties of Mono, Di and Polysaccharides

• Molecular weight: low (mono and disacharides) very high (polysacharides)
• Taste: Sweet (mono and disacharides) Tasteless (polysacharides)
• Solubility in water: Soluble (mono and disacharides) Insoluble or form colloidal dispersions (polysacharides)
• Size of particles: Pass through a membrane (mono and disacharides) Do not pass through a membrane (polysacharides)
• Test with Cu2+ for reducing sugars: Positive (except for sucrose) (mono and disacharides) Negative (polysacharides)

Biologically Important polysacharides

• Starches are made of a-D-glucose molecules, amylose, and amylopectin.
• Glycogen (animal starch in muscle) is made of a-D-glucose.
• Cellulose (plants and wood) is made of β-D-glucose.
• Starch is a plant storage-homopolysaccharide with a-D-glucose molecules as the monomeric unit.
• Amylose and amylopectin are structural components and provide a main dietary source of glucose for human beings.
• Dextrins form during the hydrolysis (digestion) and contain glucose molecules linked by a-1, 4 glycosidic linkages and a- 1,6 glycosidic. The linkages yield glucose Amylose
• Amylose is a polymer of a-D-glucose molecules linked by a-1,4 glycosidic bonds to form a continuous unbranched chain with 60-600 glucose molecules.

Amylopectin

• Is composed of a branched chain polymer of alpha D-Glucose, which contains alpha 1,4-glycosidic bonds between glucose units, and alpha-1,6 bonds to branches

Glycogen

• An animal storage-homopolysaccharide. Structure: Glycogen has a-D-glucose molecules as the monomeric unit.
• Glycogen forms a branched structure, structurally similar to amylopectin of starch and contains 11-18 glucose residues
• Glycogen has a-1, 4 glycosidic linkages in the straight chain and a-1, 6 linkages at branch points, it's a storage as Glycogen in liver and muscle
• Liver glycogen provides maintains blood glucose concentration. Muscle glycogen is used by muscles during prolonged contraction.
• Starch has two structual components whilst glycogen only has one
• 24-30 glucose molecules between any two branch points in Amylopectin in conrast to 11-18 in glycogen

Cellulose

• It is the main cell wall polysaccharide of plants with a linear non-branched structure of -D-glucose molecules
• The -D-glucose molecules are monomeric units linked by 1,4 glycosidic linkage and and is only found in plants
• Cellulose is the main dietary fiber and because humans lack enzyme that breaks down -1,4-glycosidic bonds, it's tough to digest

Glycosaminoglycans (GAGs)

• A a group of unbranched carbohydrates with alternating amino sugar and glucuronate units, a gel-like matrix that is a lubricant & key components in connective tissue and joints.
• Hyaluronate has is extracellular fluid that provides lubrication for joints
• Chondroitin a component of cartilage and tendons
• Heparin is stored in the mast cells of the liver prevents blood clotting

Glycolipids and Glycoproteins: Cell Recognition

• Glycolipids are lipid molecules with one or more carbohydrate units covalently bonded.
• Glycoproteins are protein molecules with one or more carbohydrate units covalently bonded.