Carbohydrates and Their Functions

Carbohydrate Chemistry

• Carbohydrates are defined chemically as aldehyde or ketone derivatives of higher polyhydric alcohols, or compounds that yield these derivatives during hydrolysis.
• They are the chief source of energy.
• They are constituents of compound lipids and conjugated proteins.
• Degradation products act as catalysts.
• Certain carbohydrate derivatives are used as drugs (e.g., cardiac glycosides/antibiotics).
• Lactose is the principal sugar in milk produced by lactating mammary glands.
• Degradation products are used in the synthesis of other substances (e.g., fatty acids, cholesterol, amino acids).
• They are constituents of mucopolysaccharides that form the ground substance of mesenchymal tissues.
• Inherited deficiencies in carbohydrate metabolic pathways can lead to diseases (e.g., galactosemia, glycogen storage diseases (GSDs), lactose intolerance).
• Disruptions in glucose metabolism are associated with diabetes mellitus.

Classification of Carbohydrates

• Carbohydrates are divided into four major groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides contain one sugar unit (cannot be hydrolyzed).
• Disaccharides contain two sugar units.
• Oligosaccharides contain 3-10 sugar units.
• Polysaccharides contain more than 10 sugar units.
• Ratio of carbon to H₂O is 1:2.

Monosaccharides

• Also known as simple sugars or glucoses
• Cannot be hydrolyzed further
• Can be subdivided further based on:
  • Number of carbon atoms (trioses, tetroses, pentoses, hexoses)
  • Presence of aldehyde or ketone groups (aldoses or ketoses)
• Examples of trioses: Glyceraldehyde, Dihydroxyacetone.
• Examples of tetroses: Erythrose.
• Examples of pentoses: Ribose, arabinose, xylose, lyxose.
• Examples of pentoses: Ribulose, xylulose.
• Examples of hexoses: Glucose, mannose, galactose, fructose.

Importance of Monosaccharides

• Ribose is a component of nucleic acids (RNA and DNA), ATP, GTP, and coenzymes NAD and FAD.
• Arabinose is found in some fruits and can cause alimentary pentosuria when ingested in high amounts.
• Fruit sugar (Fructose) is present in fruits and semen, and converted to glucose in the liver.
• Seminal fluid is rich in fructose, used by sperm for energy.
• Galactose is synthesized in the mammary gland, converted to glucose in the liver, and forms components for agar, glycosaminoglycan (GAG), and glycolipid.
• Mannose is a component of many glycoproteins; doesn't occur freely in nature.
• Glucose is a chief carbohydrate; the major metabolic fuel for mammals and a universal fuel for the fetus. Used by all tissues as energy. It is converted to glycogen and galactose in the liver and other tissues.
• Fructose (fruit sugar), is also present in honey and semen; can be converted into glucose in the liver.
• Seminal fluid is rich in fructose, used by sperm for energy.
• Fructose is formed in seminiferous tubular epithelial cells from glucose.

Cyclic Structures of Monosaccharides

• Cyclic form is due to the reaction between the carbonyl group and an alcoholic hydroxyl group.
  • Furanose- 4 carbon ring
  • Pyranose- 5 carbon ring
• α- and β-sugars forms exist depending on the position of the hydroxyl group on the anomeric carbon.
• Examples of sugars exist in both α and β forms: glucose and fructose.

Optical Activity and Isomers

• Asymmetric carbon atoms are those with four different groups attached.
• Optical activity refers to the ability of substances to rotate plane-polarized light.
• Dextro- rotatory sugars rotate light to the right (+).
• Levo- rotatory sugars rotate light to the left (-). Glucose is dextro- rotatory.
• Optical isomerism refers to the ability of substances to exist in more than one form that are mirror images of each other (enantiomers).

Anomeric Carbons and Anomers

• Anomeric carbon is obtained from the carbonyl group(C1 in aldoses and C2 in ketoses).
• Anomers are isomers that differ only in the configuration of the hydroxyl group around the anomeric carbon.

Aldose-Keto Isomers

• Aldose-keto isomers have the same molecular formula but different functional groups (e.g., glucose and fructose).

Oligosaccharides

• A carbohydrate containing 3-10 monosaccharides.

Epimers

• Epimers are diastereomers that differ in the configuration around one carbon atom

Disaccharides

• Maltose. Is formed during the digestion of starch and glycogen.
• Isomaltose. Is formed during the digestion of starch and glycogen.
• Trehalose.
• Lactose. Found in milk.
• Sucrose. Common table sugar.

Sucrose Hydrolysis

• Hydrolysis of sucrose using the sucrase (invertase) enzyme produces glucose and fructose. This process is known as inversion.
• Honey primarily consists of invert sugars.

Polysaccharides

• Starch, glycogen, cellulose, heteropolysaccharides e.g. mucopolysaccharides, agar, inulin.
• These are polymers of monosaccharides.
• Some are homopolysaccharides (one kind of monosaccharide).
• Some are heteropolysaccharides (more than one kind of monosaccharide).

Starch

• Starch is a polymer of glucose found in plants as storage.
• Compositionally consists of amylose and amylopectin.
• Amylose is unbranched and soluble in water, amylopectin is highly branched and insoluble in water.
• Partial hydrolysis creates dextrins.

Glycogen

• Glycogen is the animal starch.
• A branched polymer of glucose.
• Deposited in the liver and muscles for immediate energy.
• Glycogenesis is the formation of glycogen.
• Glycogenolysis is the breakdown of glycogen to glucose.
• Highly branched, similar to amylopectin.

Dextrins

• Created by partial hydrolysis of starch.
• A group of lower molecular weight products.
• Types are classified by their cleavage patterns. i.e.: amylo-, erythrod-, and achrodextrins.

Cellulose

• Long chains of linked glucose molecules.
• Cannot be digested by humans but increases the bulk of stool.

Agar

• A homopolysaccharide obtained from seaweed.
• Used as a laxative and in microbiology for culturing bacteria.

Inulin

• Fructose linked together with β1-2 bonds.
• Present in Jerusalem artichokes, onion and garlic.
• Used to measure glomerular filtration rate (GFR).

Sugar Derivatives of Biomedical Importance

• Deoxy Sugars
  • Replacement of hydroxyl groups by hydrogen. Example: Deoxyribose.
• Amino Sugars (hexosamines)
  • Contain an amino group (-NH2). Example: Glucosamine, galactosamine, mannosamine, and others
• Neuraminic acid
  • Unstable. Found form of acylated derivatives known as sialic acids (N-acetyl neuraminate).
• Muramic acid
  • Component of peptidoglycan, forming cell walls of most bacteria.
• Glycosides
  • A carbohydrate molecule covalently attached to a non-carbohydrate molecule.
  • The non-carbohydrate component is called an aglycone. Example: cardiac glycosides like digitalis.