Carbohydrates in Human Biology

Carbohydrate Chemistry

• Carbohydrates are chemically defined as aldehyde or ketone derivatives of higher polyhydric alcohols, or compounds that yield these derivatives on hydrolysis.
• Key components include aldehyde groups and ketone groups, with many hydroxyl groups.

Biomedical Importance of Carbohydrates

• Carbohydrates are the primary source of energy.
• They are components of complex lipids and proteins.
• Degradation products act as catalysts.
• Some carbohydrate derivatives are used as drugs (e.g., cardiac glycosides and antibiotics).
• Lactose is the main sugar in milk produced by lactating mammary glands.
• Degradation products are used to create other substances such as fatty acids, cholesterol, and amino acids.
• Carbohydrates are constituents of mucopolysaccharides, which form the fundamental substance of mesenchymal tissues.
• Inherited deficiencies in carbohydrate metabolic pathways can cause diseases (e.g., galactosemia, glycogen storage diseases, lactose intolerance).
• Glucose metabolism abnormalities are observed in diabetes mellitus.

Carbohydrate Classification

• Carbohydrates are categorized into four main groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
• Monosaccharides have one sugar unit.
• Disaccharides have two sugar units.
• Oligosaccharides have 3-10 sugar units.
• Polysaccharides have more than 10 sugar units.
• The ratio between carbon and water is typically 1: 2 in carbohydrates.

1. Monosaccharides

• Also known as simple sugars or glycoses.
• Cannot be further hydrolyzed.
• Subclassified based on carbon atoms (trioses, tetroses, pentoses, hexoses, etc.).
• Further distinguished as aldoses or ketoses depending on the presence of an aldehyde or ketone group, respectively.
• Trioses include Glyceraldehyde and Dihydroxyacetone.
• Tetroses include Erythrose and Erythrulose.
• Pentoses include Ribose, Arabinose, Lyxose, and Xylose. (Ribose, arabinose, xylose, and lyxose are aldopentoses; ribulose and xylulose are ketopentoses).
• Hexoses include Glucose, Mannose, Galactose, and Fructose.
  • Glucose is the chief carbohydrate, the primary metabolic fuel for mammals and a universal fuel for the fetus.
  • Fructose is found in fruits, honey, and semen. It's converted to glucose in the liver, and is utilized by seminal fluid and sperm for energy production.
  • Galactose is synthesized in mammary glands to produce lactose (milk sugar). It can be converted to glucose in the liver and found in glycosaminoglycans, glycolipids, and agar.
  • Mannose is not found freely in nature, but found as constituent of many glycoproteins.

1. Monosaccharides (continued)

• Monosaccharides exist in cyclic forms due to the reaction between the carbonyl group (C=O) and a hydroxyl (alcohol) group.
• Cyclic forms are either pyranose (5-membered ring) or furanose (4-membered ring).
• Different ring forms of a monosaccharide are called anomers. Alpha (α) and Beta (β) forms differ by the position of a hydroxyl group on the anomeric carbon (C1 in aldoses, C2 in ketoses).
• Asymmetric Carbon atoms result in optical activity.
• A substance with an asymmetric carbon atom can exist in different optical isomers.
  • Glucose is dextrorotatory, hence called dextrose.
  • Fructose is levorotatory, hence called Levulose.

Sugar Derivatives

• Deoxy sugars are formed by replacing hydroxyl groups with hydrogen atoms. Deoxyribose is an example of a deoxy sugar.
• Amino sugars contain an amino group (-NH2). Glucosamine, galactosamine, and mannosamine are important examples. Some antibiotics also contain amino sugars.
• Neuraminic acid is an unstable amino sugar acid found naturally in acylated derivative form called sialic acids. It is a vital part of peptidoglycan composition.
• Glycosides are compounds composed of a carbohydrate and a non-carbohydrate molecule (aglycone). The carbohydrate moiety is attached to the non-carbohydrate residue via a carbon-1 acetal linkage. Cardiac glycosides, such as digitalis, are examples of this type of compound.

2. Disaccharides

• Disaccharides are formed from two monosaccharides linked together.
• Common disaccharides: maltose, isomaltose, trehalose, lactose, and sucrose.
• Maltose contains two glucose monomers.
• Isomaltose also contains two glucose monomers.
• Trehalose also contains two glucose monomers.
• Lactose is made up of glucose and galactose.
• Sucrose is made up of glucose and fructose.

3. Polysaccharides

• Polymers made up of many monosaccharides.

• Homopolysaccharides are composed of a single type of monosaccharide. Examples include starch, glycogen, cellulose, and inulin.

• Heteropolysaccharides are made up of more than one type of monosaccharide and may also contain other groups such as hexuronic acid. Mucopolysaccharides are an example of heteropolysaccharides.

• Starch is composed of amylose and amylopectin. Amylose is linear, while amylopectin is branched.

• Glycogen is the primary storage polysaccharide in animals, similar in structure to amylopectin but more extensively branched.

• Dextrins are products of starch partial hydrolysis.

• Cellulose is a linear polysaccharide composed of glucose units joined by β(1→4) glycosidic linkages.

• Agar is a sulphated polysaccharide derived from seaweed.

• Inulin is a storage polysaccharide composed of fructose units, found in roots and tubers.