Carbohydrates and Their Functions

Biochemistry of Carbohydrates

• Carbohydrates are substances containing carbon, hydrogen, and oxygen.
• They are aldehyde (CHO) or ketone (C=O) derivatives of polyhydric alcohols (with more than one OH group).
• Carbohydrates are the chief source of energy for the body.
• They are important structural components in animal and plant cells.
• They are a part of nucleic acids, nucleotides, coenzymes, hormones, receptors, and enzymes.
• Carbohydrates function as antigens, such as blood group substances.
• Carbohydrates have a 1:2:1 ratio of carbon, hydrogen, and oxygen (Cn(H₂O)n).

Definition of Carbohydrates

• Carbohydrates are organic compounds containing carbon, hydrogen, and oxygen atoms.
• They are essentially aldehydes or ketones with multiple hydroxyl groups.
• They can be broadly classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Importance of Carbohydrates

• Carbohydrates are the primary source of energy in the body.
• They serve as structural components of animal and plant cells.
• They are essential parts of nucleic acids, nucleotides, and coenzymes.
• Carbohydrates form important antigens, like blood group determinants.
• They participate in hormone and enzyme structures.

Classification of Carbohydrates

• Monosaccharides: Simple sugars, containing one sugar unit. Cannot be hydrolyzed.
  • Named by the number of carbon atoms (e.g., triose, pentose, hexose) and the functional group (aldose, ketose).
  • Examples: glucose, fructose, galactose, ribose.
• Disaccharides: Two monosaccharide units linked together through glycosidic bonds.
  • Can be hydrolyzed into monosaccharides.
  • Examples: maltose, sucrose, lactose.
• Oligosaccharides: Short chains of 3-10 monosaccharide units linked together.
• Polysaccharides: Long chains of monosaccharides joined by glycosidic bonds (more than 10 units).
  • Examples: starch, glycogen, cellulose.

Monosaccharides

• Definition: Simple sugars – the basic units of carbohydrates.
• General Formula: Cn(H₂O)n
• Naming:
  • Named according to the functional group: aldoses (aldehyde group) and ketoses (ketone group).
  • Classified based on the number of carbon atoms: trioses, tetroses, pentoses, hexoses.
• Examples: Trioses (glyceraldehyde, dihydroxyacetone), Tetroses (erythrose, threose), Pentoses (ribose, deoxyribose, xylose), and Hexoses (glucose, fructose, galactose, mannose).
  • Specific functions of the individual pentoses and hexoses will be discussed later.

Classification of Monosaccharides

• Trioses: 3 carbon atoms. -Aldotriose: Glyceraldehyde (glycerose)
  • Ketotriose: Dihydroxyacetone.
• Tetroses: 4 carbon atoms.
  • Aldotetrose: Erythrose
  • Ketotetrose: Erythrulose
• Pentoses: 5 carbon atoms.
  • Aldopentoses: Ribose, arabinose, xylose, and lyxose
  • Ketopentoses: Ribulose and xylulose
    • Ribose is crucial in the structure of RNA and DNA. Ribose is also found in ATP, GTP and coenzymes NAD, NADP, and FAD.
• Hexoses: 6 carbon atoms.
  • Aldohexoses: Glucose, mannose, and galactose
  • Ketohexose: Fructose

Importance (Functions) of Hexoses

• Glucose: The primary energy source in the body, also known as "grape sugar"
  • Produces other carbohydrates, e.g., glycogen, galactose
  • Formed by hydrolysis of starch, dextrins, glycogen, sucrose, maltose, and lactose.
  • Found in urine in diabetes mellitus.
• Galactose: Converted to glucose in the liver.
  • Part of lactose (milk sugar) synthesis.
  • Component of glycoproteins, glycolipids, and mucopolysaccharides.
• Fructose: "Fruit sugar" or "semen sugar".
  • Sweetest sugar, levorotatory (levulose).
  • Part of inulin and sucrose hydrolysis.
  • Converted to glucose in the liver.
• Mannose: Found in glycoproteins and linked to sialic acid.

Ring (cyclic) Structure of Sugars

• Cyclic structures of glucose and fructose result from the reaction between the carbonyl carbon and a hydroxyl group.
• Furanose: 4-membered ring structure
• Pyranose: 5-membered ring structure

Asymmetric Carbon Atom

• Asymmetric carbon atoms are carbons that have four different groups attached to them.
• Glyceraldehyde is the reference sugar due to its single asymmetric carbon atom.
• Substances containing asymmetric carbon atoms exhibit optical activity and optical isomerism.

Optical Activity

• The ability of a substance to rotate plane-polarized light.
• Dextrorotatory (+) rotates light to the right.
• Levorotatory (-) rotates light to the left.

Optical Isomerism

• Isomers that are mirror images of each other but not superimposable.
• D and L isomers are a type of optical isomerism.
• Key sugars in the body are D-forms.

Structural Isomerism

• Isomers with the same chemical formula but different structural arrangement of the atoms.

Stereoisomerism

• Types of stereoisomerism:
  • D and L isomerism (enantiomers)
  • Anomers,
  • Epimers,
  • Geometric isomers

Examples of stereoisomers (Anomers & Epimers):

• Anomers: alpha and beta forms of the same sugar.
• Epimers: Isomers that differ in the configuration around only one carbon atom. Examples: Glucose and Mannose are epimers at C-2, and Glucose and Galactose are epimers at C-4.

Sugar Derivatives

• Sugar acids: Oxidation of carbohydrates. Examples: gluconic, glucuronic, and glucaric (saccharic) acids. L-ascorbic acid (Vitamin C) is an important sugar acid.
• Sugar alcohols: Reduction of the carbonyl group. Examples include sorbitol, mannitol, ribitol, and dulcitol.
• Deoxy sugars: Loss of the oxygen from a hydroxyl group. Examples include deoxyribose and fucose.
• Amino sugars: Hydroxyl group replaced by an amino group. Examples: glucosamine, galactosamine, and mannosamine. Amino sugars are often found in glycoproteins.

Disaccharides

• Reducing disaccharides: Contain a free anomeric carbon. Example: Maltose, isomaltose, cellobiose, and lactose
• Non-reducing disaccharides: Both anomeric carbons are involved in the glycosidic linkage. Example: Sucrose and trehalose

Examples of Disaccharides and their properties

• Maltose: Two glucose units linked by an α(1→4) glycosidic bond. Reducing disaccharide, produced by starch digestion.
• Isomaltose: Two glucose units linked by an α(1→6) glycosidic bond. Reducing disaccharide
• Cellobiose: Two glucose units linked by a β(1→4) glycosidic bond. Reducing disaccharide, building block of cellulose.
• Lactose: Glucose and galactose linked by a β(1→4) glycosidic bond. Reducing disaccharide, milk sugar. High in pregnant/nursing females.
• Sucrose (Table sugar): Glucose and fructose linked by an α(1→2) glycosidic bond. Non-reducing disaccharide. Sucrose is the most abundant disaccharide.
• Trehalose: Two glucose units linked by an α(1→1) glycosidic bond. Non-reducing disaccharide.

Test your knowledge on carbohydrates with this quiz that covers definitions, classifications, and examples of various sugar types and their roles in living organisms. Explore important concepts like anomers, epimers, and the significance of sugars such as trehalose and fructose.