Carbohydrates: Structure, Importance, and Classification

Carbohydrates

• A carbohydrate is a polyhydroxyaldehyde or polyhydroxyketone, or a substance yielding these upon hydrolysis.
• A monosaccharide is a carbohydrate that cannot be hydrolyzed into simpler forms.
• Monosaccharides share the general formula CnH2nOn, where n varies from 3 to 8.
• An aldose is a monosaccharide that has an aldehyde group.
• A ketose is a monosaccharide containing a ketone group.

Why Carbohydrates are Essential

• Carbohydrates provide energy; one gram yields 3,961 calories of heat.
• Certain products from carbohydrate metabolism catalyze biological oxidation.
• Carbohydrates can be converted into fat.
• They also act as protein-sparers.

Classification and Nomenclature of Carbohydrates

• Carbohydrates are classified into monosaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides are grouped by functional group (aldoses or ketoses) and number of carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses).
• Examples of monosaccharides include glucose and fructose.
• Disaccharides include maltose and lactose.
• Trisaccharides include raffinose.
• Tetrasaccharides include stachyose.
• Homopolysaccharides include starch, dextrin, glycogen, cellulose, and inulin.
• Heteropolysaccharides include hyaluronic acid, heparin, chondroitin sulfate, dermatan sulfate, and keratan sulfate.

Isomers

• Isomers are molecules with the same molecular formula but different structures.
• Constitutional isomers (structural isomers) have different connectivity.
• Stereoisomers (spatial isomers) have the same connectivity but different arrangements in space.
• Types of stereoisomers include diastereomers, enantiomers, conformers, and rotamers.

Constitutional Isomerism

• Constitutional isomers are compounds sharing a molecular formula but differing in structural formulas.

Enantiomers

• Enantiomers are molecules that are mirror images of each other and are non-superimposable.

Glucose Anomers

• Glucose can exist in different anomeric forms (alpha and beta) due to the configuration at the anomeric carbon.

Epimers

• D-glucose and D-galactose are epimeric at carbon-4; they differ in configuration at this carbon.

Aldohexose

• Examples of aldohexoses include D-Allose, D-Altrose, D-Glucose, D-Mannose, D-Gulose, D-Idose, D-Galactose and D-Talose, all with molecular formula C6H12O6.

Ketohexose

• Examples ketohexoses include D-Psicose, D-Fructose, D-Sorbose and D-Tagatose.

Monosaccharides

• The suffix "-ose" indicates a carbohydrate.
• Prefixes like "tri-," "tetra-," "penta-" indicate the number of carbon atoms in the chain.
• Monosaccharides with an aldehyde group are classified as aldoses.
• Monosaccharides with a ketone group are classified as ketoses.
• There are only two trioses: Glyceraldehyde (an aldotriose) and Dihydroxyacetone (a ketotriose).
• "Triose" indicates the number of carbons, but not the nature of the carbonyl group.

Fischer Projection

• A Fischer projection is a two-dimensional representation of a tetrahedral stereocenter.
• Horizontal lines in a Fischer projection represent bonds projecting forward.
• Vertical lines represent bonds projecting to the rear.
• The stereocenter is in the plane.

D- and L- Monosaccharides

• In 1891, Emil Fischer assigned D- and L- configurations to glyceraldehyde enantiomers.
• In a D-monosaccharide, the -OH group on the penultimate carbon is on the right in a Fischer projection.
• In an L-monosaccharide, the -OH group on the penultimate carbon is on the left.

Common D-Tetroses and D-Pentoses

• Common D-tetroses and D-pentoses include D-Erythrose, D-Threose, D-Ribose, and 2-Deoxy-D-ribose

Common D-Hexoses

• Common D-hexoses include D-Glucose, D-Galactose, and D-Glucosamine.

Amino Sugars

• Amino sugars have an -NH2 group in place of an -OH.
• Common amino sugars are D-glucosamine, D-mannosamine, and D-galactosamine.
• N-acetyl-D-glucosamine is an acetylated derivative of D-glucosamine.

Cyclic Structures and Hemiacetals

• Aldehydes and ketones react with alcohols to form hemiacetals.
• Cyclic hemiacetals form when hydroxyl and carbonyl groups are part of the same molecule, creating a five- or six-membered ring.

Haworth Projections and Cyclic Hemiacetals

• D-Glucose forms two cyclic hemiacetals.

Representing Cyclic Hemiacetals

• A five- or six-membered cyclic hemiacetal can be represented as a planar ring, roughly perpendicular to the plane of the paper.
• Groups bonded to the carbons of the ring lie above or below the plane.
• The anomeric carbon is the new stereocenter from cyclic structure formation.
• Anomers are stereoisomers differing only in configuration at the anomeric carbon.
• The anomeric carbon in an aldose is C-1; in common ketoses, it is C-2.

Carbohydrate Terminology

• β means that the -OH on the anomeric carbon is on the same side as the terminal -CH2OH.
• α means that the -OH on the anomeric carbon is on the opposite side from the terminal -CH2OH.
• A six-membered hemiacetal ring is a pyranose.
• A five-membered hemiacetal ring is a furanose.
• These ring sizes correspond to heterocyclic compounds furan and pyran.

Aldopentoses and D-Ribose

• Aldopentoses also form cyclic hemiacetals.
• The most prevalent form of D-ribose and other pentoses in the biological world are furanoses.
• The prefix "deoxy" means "without oxygen.”

Chair Conformations and Pyranoses

• The six-membered ring in pyranoses is more accurately represented as a strain-free chair conformation.

Haworth Projections vs Chair Conformations

• In both Haworth projections and chair conformations, the orientations of groups on carbons 1-5 of β-D-glucopyranose are up, down, up, down, and up, and all are equatorial.

Mutarotation

• Mutarotation is the change in specific rotation accompanying the equilibration of α- and β-anomers in aqueous solution.
• For example, when either α-D-glucose or β-D-glucose is dissolved in water, the specific rotation of the solution changes to +52.7°, comprising 64% beta and 36% alpha forms.

Sucrose

• Sucrose is made of fructose and a glucose unit connected by α,β-(12)-glycosidic (head-to-head) linkage.

Lactose and Maltose

• Lactose has β-(1,4) Glycosidic bond
• Maltose has α-1,4 Glycosidic bond

Polysaccharides

• Structural polysaccharides provide protective walls or lubricative coatings to cells, e.g., cellulose.
• Storage polysaccharides store energy, e.g., starch and glycogen.

Amylose, Amylopectin

• Amylose has α-(1→4)-glycosidic linkages
• Amylopectin has α-(1→6)-glycosidic linkages

Properties of Carbohydrates

• Most monosaccharides taste sweet, with fructose being the sweetest, 73% sweeter than sucrose.
• Monosaccharides are water-loving due to -OH groups and are solids at room temperature in crystalline form.
• Polysaccharides are tasteless and insoluble, forming colloids in water, and are amorphous.
• Monosaccharides and disaccharides give a positive iodine test (blue color), while polysaccharides give negative results, except for starch.

Nutritive Sweeteners

• Nutritive sweeteners like sucrose and fructose are generally recognized as safe, but high intake poses health concerns.
• Consumption above 25% of total energy can reduce dietary quality.
• High-fructose intakes may cause hypertriglyceridemia in susceptible individuals.

Nonnutritive Sweeteners

• Saccharin is 200-700 times sweeter than sucrose with 0 kcal/g and doesn't produce a glycemic response.
• Aspartame is 160-220 times sweeter than sucrose with 4 kcal/g and produces only a limited glycemic response.
• Acesulfame-K is 200 times sweeter than sucrose with 0 kcal/g and doesn't produce a glycemic response.
• Sucralose is 600 times sweeter than sucrose with 0 kcal/g and doesn't produce a glycemic response.

Sugar Sweeteners

• Sorbitol has 2.6 kcal/g and is 50-70% as sweet as sucrose with a laxative effect in some.
• Mannitol has 1.6 kcal/g and is 50-70% as sweet as sucrose with a laxative effect in some.
• Xylitol has 2.4 kcal/g and is as sweet as sucrose.
• Erythritol has 0.2 kcal/g and is 60-80% as sweet as sucrose with flavor enhancement and texturizing properties.
• Isomalt has 2 kcal/g and is 45-65% as sweet as sucrose, used as a bulking agent.
• Lactitol has 2 kcal/g and is 30-40% as sweet as sucrose, used as a bulking agent.
• Maltitol has 2.1 kcal/g and is 90% as sweet as sucrose, used as a bulking agent.
• HSH has 3 kcal/g and is 25-50% as sweet as sucrose, also known as hydrogenated starch hydrolysates.

Osazone Formation

• Hexoses, lactose, and maltose react with phenylhydrazine C6H5-NH-NH2 at boiling point.
• Osazone formation serves to identify different sugars through unique crystal structures.

Chemial Properties of Carbohydrates

• Glucose, Fructose, Galactose are all ready at their simplest form and test positive with Fermentation, Osazone and Reducing action
• Sucrose tests postive by Fermentation but none with Osazone and Reducing action, end product will be Glucose & fructose
• Maltose tests postive by Fermentation, Osazone and Reducing action, end product will be two molecules of glucose
• Lactose tests positive only with Osazone and Reducing action, end product will be Glucose & galactose
• Starch, Dextrin, Glycogen, Cellulose (all Polysaccharides) all test positive with After hydrolysis and gives Glucose as end product

Optical Activity

• Carbohydrates can rotate the plane of polarized light.
• Enantiomers rotating light to the left are levorotatory (L or -)
• Enantiomer's rotating light to the right are dextrorotatory (D or +).
• Glucose is named dextrose because it rotates light to the right.
• Galactose, sucrose, maltose, and lactose are dextrorotatory.

Fermentation with Yeast

• This process involves the decomposition of a substance by enzymes secreted by bacteria, yeasts, or molds.
• Monosaccharides like glucose are converted into ethyl alcohol and carbon dioxide by the zymase enzyme.
• C6H12O6 > 2 C2H5OH + 2 CO2
• Sucrase and maltase in yeast hydrolyze sucrose and maltose into simpler sugars.
• Lactose does not undergo fermentation due to the absence of lactase in yeast.

Testing for Reducing Sugars

• Fehling's test results in a brick red precipitate with reducing sugars.
• Benedict's test is a positive result shows a brick-red precipitate.
• Tollens' test results in the formation of a silver mirror with ammoniacal silver nitrate.
• Nylander's test uses bismuth nitrate, KOH, and Rochelle salt to produce a black precipitate.
• Picric acid test: Reducing sugars with picric acid and sodium carbonate produce a mahogany red color as a result of sodium picramate formation.
• Moore's test: Sugars with free aldehyde or ketone group boiled with NaOH will give brown color with a caramel-like odor.

More Distinguishing Tests

• Nylander's reagent, containing bismuth nitrate, potassium, and sodium tartrate, causes a black precipitate of metallic bismuth in the presence of reducing sugars.
• Moore's test: Sugars with a free aldehyde or ketone group boiled with NaOH will result in a brown color with a caramel-like odor; sucrose, however, is an exception because it is nonreducing.

Barfoed's Test

• Barfoed's test: Monosaccharides reduce Barfoed's reagents forming a cuprous oxide brick-red precipitate, while disaccharides do not.

Seliwanoff's Test

• In Seliwanoff's test, the reagents are resorcinol and concentrated HCl. A red color forms with ketose sugars.

Testing for Carbohydrates

• Molischs test results in forms a violet ring at the interface

Mucic Test for Galactose

• Galactose boiled with concentrated nitric acid: forms a white sandy precipitate of mucic acid.

Digestion of Carbohydrates

• Mouth: Salivary amylase begins starch breakdown.
• Stomach: Salivary amylase is inactivated; no further action occurs.
• Small intestine: Pancreatic amylase breaks down starch; digestion of disaccharides occurs.
• Small intestine villi: Enzymes complete carbohydrate digestion, breaking down disaccharides and oligosaccharides into monosaccharides.
• Large intestine: Bacteria partially break down indigestible carbohydrates and fiber to short-chain fatty acids and gas; undigested fiber is excreted.

Glycolysis

• Glycolysis is a sequence of reactions extracting energy from glucose.

Krebs Cycle

• Krebs cycle, also know as Citric Cycle provides the next stage of breakdown

Cori Cycle

• Cori Cycle a Lactate Cycle done by the action between our skeletal muscles and liver cells
• Glucose <> (2x) Pyruvate
• Liver cells convert the 2x Pyruvate into (2x) Lactate to take it to the Muscle Cells
• Muscle cells turn the (2x) Lactate to (2x) Pyruvate again to create Muscle Cells

Glycogenesis

• Glycogenesis is the process of converting Glucose to Glycogen

Glycogenolysis

• Glycogenolysis Is the process of converting Glycogen to Glucose