Understanding Carbohydrates: Chemistry and Importance

Carbohydrates

• A carbohydrate constitutes either a polyhydroxyaldehyde, a polyhydroxyketone, or a substance yielding these compounds upon hydrolysis.
• A monosaccharide is a carbohydrate that remains unhydrolyzed into simpler carbohydrates.
• Monosaccharides are generally represented by the formula CnH2nOn, where n ranges from 3 to 8.
• An aldose is a monosaccharide incorporating an aldehyde group.
• A ketose is a monosaccharide possessing a ketone group.

Why Carbohydrates Are Essential

• Carbohydrates provide energy by producing 3,961 calories of heat per gram.
• Certain carbohydrate metabolism products participate as catalysts in biological oxidation.
• Carbohydrates convert into fat.
• Carbohydrates spare protein.

Classification and Nomenclature of Carbohydrates

• Carbohydrates are categorized into monosaccharides, oligosaccharides, and polysaccharides.
• Functional groups of monosaccharides include aldoses (like glucose) and ketoses (like fructose).
• Monosaccharides are also classified by the number of carbon atoms they contain, such as trioses, tetroses, pentoses, hexoses, and heptoses.
• Oligosaccharides are composed of di- (e.g., maltose, lactose), tri- (e.g., raffinose), and tetra-saccharides (e.g., stachyose).
• Polysaccharides are classified as homopolysaccharides (e.g., starch, dextrin, glycogen, cellulose, inulin) and heteropolysaccharides (e.g., hyaluronic acid, heparin, chondroitin sulfate, dermatan sulfate, keratan sulfate).

Isomers

• Isomers are categorized as constitutional (structural) isomers and stereoisomers (spatial isomers).
• Stereoisomers include diastereomers (such as cis-trans isomers and conformers), enantiomers, and rotamers.

Constitutional Isomerism

• Constitutional isomers are compounds sharing the same molecular formula but having distinct structural formulas.

Enantiomers

• Enantiomers are mirror images of each other

Glucose Anomers

• Glucose can exist in two anomeric forms: alpha-D-glucose and beta-D-glucose.
• The difference lies in the orientation of the hydroxyl group on carbon-1.

Epimers

• D-glucose and D-galactose are epimeric at carbon-4.
• Epimers are different at only one chiral center.

Monosaccharides

• The suffix "-ose" signifies a carbohydrate molecule.
• Prefixes such as "tri-," "tetra-," and "penta-" denote the number of carbon atoms in a 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).
• Aldo- and keto- may be omitted, with compounds referred to as trioses.
• The term "triose" indicates the number of carbons but does not specify the carbonyl group.

Fischer Projection

• Fischer projection is a 2-dimensional representation of the tetrahedral stereocenters.
• Horizontal lines indicate bonds projecting forward from the stereocenter.
• Vertical lines indicate bonds projecting to the rear.
• The stereocenter is in the plane.

D- and L- Monosaccharides

• In 1891, Emil Fischer arbitrarily assigned "D-" and "L-" 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 its penultimate carbon is on the left in a Fischer projection.

Common D-Tetroses and D-Pentoses

• The common D-tetroses and D-pentoses include D-Erythrose, D-Threose, D-Ribose, and 2-Deoxy-D-ribose.

Common D-Hexoses

• The common D-hexoses include D-Glucose, D-Galactose, and D-Glucosamine.

Amino Sugars

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

Cyclic Structures

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

Haworth Projections

• D-Glucose can form two cyclic hemiacetals.

Representing Cyclic Hemiacetals

• A five- or six-membered cyclic hemiacetal is represented as a planar ring, roughly perpendicular to the paper plane.
• The groups bonded to the ring carbons are either above or below the plane of the ring.
• The new carbon stereocenter in the cyclic structure is the anomeric carbon.

Anomers

• Stereoisomers that differ in configuration only at the anomeric carbon are anomers.
• The anomeric carbon is C-1 in aldoses and C-2 in common ketoses.

Carbohydrate Chemistry Terminology

• β signifies that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH.
• α signifies that the -OH on the anomeric carbon is on the opposite side of the ring from the terminal -CH2OH.
• A six-membered hemiacetal ring is called a pyranose.
• A five-membered hemiacetal ring is called a furanose.

Furanoses

• Aldopentoses also form cyclic hemiacetals.
• prevalent forms of D-ribose and other pentoses are furanoses.
• The prefix "deoxy" means "without oxygen."

Chair Conformations

• For pyranoses, the six-membered ring is accurately represented in a strain-free chair conformation.
• For β-D-glucopyranose, the orientations on carbons 1-5 are up, down, up, down, and up and all are equatorial, in both Haworth projections and chair conformations.

Mutarotation

• Mutarotation is the change in specific rotation that accompanies the equilibration of α- and β-anomers in an aqueous solution.
• When α-D-glucose or β-D-glucose dissolves in water, the specific rotation changes to an equilibrium value of +52.7°, corresponding to 64% beta and 36% alpha forms.

Disaccharides

• Sucrose is made of a fructose and a glucose unit connected by a,β-(12)-glycosidic (head-to-head) linkage.
• Lactose is a disaccharide with β-(1,4) glycosidic bond.
• Maltose is a disaccharide with α-1,4 Glycosidic bond.

Identifying Glycosidic Linkages

• Identify the two glycosidic linkages.
• The glycosidic linkage between sugars 1 and 2 is β-(14) because the bond is directed up from the anomeric carbon.
• The glycosidic linkage between sugars 2 and 3 is α-(14) because the bond is directed down from the anomeric carbon.

Polysaccharides

• Structural: used to provide protective walls or lubricative coatings to cells.
• Storage: store energy.

Structural Polysaccharides

• Examples include cellulose
• Cellulose is composed of glucose molecules

Storage Polysaccharides

• Examples include starch and glycogen.

Properties of Carbohydrates

• Most monosaccharides are sweet, with fructose being the sweetest, 73% sweeter than sucrose.
• Sucrose's sweetness is the standard.
• Lactose (milk sugar) is slightly sweet.
• Monosaccharides are water-loving due to -OH groups, solid at room temperature, and crystalline.
• Most polysaccharides are tasteless and insoluble in water, forming colloidal suspensions and are amorphous.
• Monosaccharides and disaccharides yield positive results in the iodine test (blue color/ppt).
• Polysaccharides generally have negative results in the iodine test, except for starch.

Nutritive Sweeteners

• Nutritive sweeteners (e.g., sucrose, fructose) are generally recognized as safe by the FDA, but there are health concerns about increased sweetener intake.
• Dietary quality is compromised at intakes above 25% of total energy.
• Nutritive sweeteners increase the risk of dental caries, and high-fructose intakes may cause hypertriglyceridemia and gastrointestinal symptoms.
• Minimal amounts of sugar and nonnutritive sweeteners are recommended for optimal health.
• Emphasize fresh, local, and natural foods for health and sustainability.
• Balanced diets should incorporate whole grains, vegetables, fruits, legumes, nuts/seeds, low-fat dairy, and lean meats while minimizing processed foods/additives.
• Saccharin is 200-700 times sweeter than sucrose, with no glycemic response.
• Aspartame is 160-220 times sweeter than sucrose with a limited glycemic response.
• Acesulfame-K is 200 times sweeter than sucrose, also with no glycemic response.
• Sucralose is 600 times sweeter than sucrose and causes no glycemic response.
• Sorbitol, mannitol, xylitol, erythritol, isomalt, lactitol, maltitol, and HSH are examples of novel sugar sweeteners.

Osazone Formation

• Hexoses, lactose, and maltose react with phenylhydrazine (C6H5-NH-NH2) at boiling point.
• Osazone formation identifies distinct sugars based on crystal structure.

Chemical Properties of Carbohydrates

• Monosaccharides (glucose, fructose, galactose) undergo fermentation with yeast.
• Monosaccharides (glucose, fructose, galactose) form osazones.
• Monosaccharides (glucose, fructose, galactose) reduce.
• Monosaccharides (glucose, fructose, galactose) are already the simplest form.
• Disaccharides (Sucrose) perform fermentation, do not form osazones, do not reduce, and generate Glucose & fructose when hydrolyzed.
• Disaccharides (Maltose) perform fermentation, form osazones, reduce, and generate two molecules of glucose when hydrolyzed.
• Disaccharides (Lactose) do not perform fermentation, form osazones, reduce, and generate Glucose & galactose when hydrolyzed.
• Polysaccharides (Starch) do not perform fermentation, do not form osazones, do not reduce, and generate glucose when hydrolyzed..
• Polysaccharides (Dextrin, glycogen, cellulose) do not perform fermentation, do not form osazones, do not reduce, and generate glucose when hydrolyzed.

Optical Activity of Carbohydrates

• A general property of carbohydrates is the ability to rotate polarized light.
• Levortatory (L) rotates light to the left.
• Dextrorotatory (D) rotates light to the right.

Fermentation with Yeast

• Decomposes substances with enzymes secreted by bacteria, yeasts, or molds.
• Monosaccharides like glucose are converted into ethyl alcohol and carbon dioxide by zymase.
• The product of simple alcoholic fermentation in yeast.
• If yeasts are present, sucrose and maltose ferment solutions, sucrose and maltose ferment solutions as the enzymes sucrase and maltase in yeast hydrolyzed sugars to simple sugars.
• Lactose solution and ethyl alcohol cannot be formed without yeasts hydrolytic enzyme lactase.

Tests for Reducing Sugars

• Fehling's test: Reducing sugars yield a brick red precipitate of cuprous oxide with Fehling's A and B solutions.
• Benedict's Test: The test features the use of reducing sugars which yields a brick-red precipitate.
• Tollen's test: The formation of a silver mirror when reacted to an ammoniacal silver nitrate solution.
• Nylander's test: With bismuth nitrate, KOH, and Rochelle salt, produces a black precipitate of metallic bismuth.
• Picric acid test: Reducing sugars with picric acid and sodium carbonate produce a mahogany red color from sodium picramate formation.
• Moore's test: Sugars with a free aldehyde or ketone group boiled with NaOH turn brown with a caramel-like odor.

Barfoed's Test

• Monosaccharides reduce Barfoed's reagents, which results in a brick red precipitate while disaccharides do not reduce.

Tests to Distinguish Ketoses From Aldoses

• Seliwanoff's Test: The reagents consists of resorcinol and concentrated HCl, resulting in the production of a red color in the presence of ketoses.

General Test for Carbohydrates

• Molisch's Test: When a-naphthol in alcohol is combined with any carbohydrate produces a violet ring forming at the interface.

Testing for Galactose

• Mucic's Test: Boil Galactose with a concentrated nitric acid with form white sandy precipitate of mucic acid.

Digestion and absorption of carbohydrates

• In the mouth, the enzyme salivary amylase begins breaking down starch into shorter polysaccharides.
• In the stomach, salivary amylase gets inactivated and no further carbohydrate digestion occurs.
• In the small intestine, most digestion happens as the enzyme pancreatic amylase breaks down starch.
• The rest of carbohydrate digestion is completed by enzymes that are attached to the brush border of the small intestine's that break down di and oligosaccharides into monosaccharides.
• In the large intestine, fiber and other indigestible carbohydrates are partially broken down by bacteria to form short chain fatty acids and gas.

Glycolysis

• Glucose gets broken down into water, CO2, and energy in the form of ATP.

Krebs Cycle

• Complex cycle that occurs after Glycolysis

Electron Transport Chain

• Multiple complexes that transfer hydrogen ions across a membrane to control ATP production.

The Cori Cycle

• It is a metabolic pathway done by our muscles and liver.
• Glucose --> 2x Lactate --> 2x Pyruvate

Glycogenesis

• The process by which glycogen is synthesized from glucose: glucose --> glycogen

Glycogenolysis

• The process by which glycogen is broken down to glucose
• It is the reverse process to Glycogenesis

Mexico's Obesity Epidemic

• Mexico ranks first in the world in childhood obesity.
• In 2018, 75% of Mexicans aged 20+ were obese, with 35.6% of children ages 5-11 rated as obese.

Metabolic Syndrome

• Associated with hypertension, cancer
• Associated with PCOS and heart disease
• Associated with insulin resistance, lipid problems
• Associated with dementia, NAFLD, and type 2 diabetes