Carbohydrates

• Common functional groups of biomolecules include nucleophilic groups like hydroxyl, sulfhydryl, amino, and imidazole groups, which are nucleophiles in their basic forms
• Major classes of biomolecules found in living organisms are:

  1. Carbohydrates

  2. Lipids

  3. Nucleic Acids

  4. Amino Acids (and proteins)

  • Their smallest components are called monomers
  • These monomers can polymerize to form more complex structures which comprise the various parts of cells and organelles
• Functions of carbohydrates include:

  • Sources of energy (glucose, trehalose, starch, glycogen)
  • Cellular structures (cellulose, chitin, peptidoglycan)
  • Backbone for important biologically molecules such as nucleotides (building blocks of DNA and RNA), ATP, reducing equivalents (NAD(H), FAD(H2), NADP(H))
• In the plasma membrane, carbohydrates are complexed with lipids and proteins to form glycolipids and glycoproteins. Glycoproteins function in cell adhesion and cell recognition. The extracellular matrix also contains glycoproteins. Glycolipids function as receptors in cell communication and signal transduction and also aid in cell adhesion
• Carbohydrates are classified into three main groups:

  • Monosaccharides
  • Disaccharides
  • Polysaccharides
  • Monosaccharides are the simplest carbohydrates and consist of a single sugar unit
  • Disaccharides are made up of two monosaccharides linked together
  • Polysaccharides are made up of many monosaccharides linked together
• Monosaccharides are the simplest carbohydrates where all the carbon atoms are joined together by direct carbon-carbon bonds. They are further classified according to the main functional group: aldehyde (aldo) or ketone (keto) and the number of carbons (5C - pentose, 6C – hexoses)
• Stereoisomers are molecules with the same molecular formula but atoms differ in spatial orientation. A molecule with n chiral centers can have 2n stereoisomers. Aldohexoses can have multiple stereoisomers
• Monosaccharides with backbones with 5C and more usually occur in solution as cyclic or ring structures. Ring formation is essentially the reaction between an alcohol and a carbonyl carbon (in the aldehyde or ketone of the sugar)
• Glycoside formation involves the reaction between an alcohol and a carbonyl carbon in the aldehyde or ketone of the sugar
• Representation of biological molecules can be done in various ways including ball and stick (3-D), perspective formulas (3-D), and Fischer Projection formulas (2-D)
• Glyceraldehyde is a reference compound used as a standard to name other compounds. Compounds with the same relative configuration of D-glyceraldehyde are given the D designation and those with its mirror image are given the L-designation
• Isomerism in sugars can result in different isomeric forms with the same empirical formula. For example, C6H12O6 represents several different sugars including glucose, mannose, and galactose – stereoisomers
• Enantiomers are isomers that are non-superimposable mirror images of each other. All physical and chemical properties of enantiomers are the same except for their interaction with polarized light and reactions with other chiral molecules
• Substances that rotate the plane of polarized light are optically active and may be either dextrorotatory (+) or levorotatory (-). Enantiomeric pairs rotate the plane of polarized light to the same extent but in opposite directions
• A mixture of equal amounts of each member of an enantiomeric pair is called a racemic mixture or a racemate, which shows no rotation of the plane of polarized light
• Epimers are stereoisomers that differ around one chiral carbon
• Monosaccharides are classified as aldose or ketose sugars based on the presence of an aldehyde or ketone functional group, and the number of carbons in the molecule
• Monosaccharides with a backbone of 5 carbons or more usually occur in solution as cyclic or ring structures

  
• Ring formation in monosaccharides is essentially the reaction between an alcohol and a carbonyl carbon (in the aldehyde or ketone of the sugar)
• Hemiacetals and hemiketals can react with a second alcohol to produce an acetal or ketal, which are stable and do not open to expose an aldehyde or ketone
• Mutarotation refers to the change in optical rotation via the linear conformation around the anomeric carbon (C1) in monosaccharides
• Anomers are the two isomers that differ around the anomeric carbon in monosaccharides
• Cyclic conformations of compounds with 6-membered rings can be represented as chair or boat conformations, with the chair conformation being more stable due to zero ring strain
• Cyclic monosaccharides are based on standard 6 and 5-membered ringed compounds called pyran and furan respectively
• Benedict’s and Tollen’s Reagents are common tests for reducing sugars, with Benedict’s reagent forming a red precipitate of Cu2O and Tollen’s reagent yielding a deposit of silver
• Mild oxidation of the carbonyl group in monosaccharides yields an aldonic acid, while oxidation of the primary alcohol group results in uronic acid
• Highly oxidizing environments can lead to the oxidation of both the carbonyl and primary alcoholic groups in monosaccharides, forming aldaric acid
• Esterification can occur between acids and alcohols to form esters, such as phosphate, sulfate, and carboxylate esters
• Glycosidic bond formation involves a covalent bond that joins the hemiacetal group of a saccharide and the hydroxyl group of another compound, with the orientation described using alpha (α) or beta (β) based on the anomeric carbon
• Maltose is a disaccharide important as a breakdown product of starch or glycogen, with a glycosidic bond that is α-1,4 and it is a reducing sugar
• Sucrose and trehalose are two widely occurring disaccharides in nature, with sucrose being a non-reducing sugar and trehalose being important as a water substitute for animals during desiccation
• Lactulose is a synthetic disaccharide used as a stool softener and colonic acidifier, and its hydrolysis results in the production of monosaccharides
• Raffinose is a trisaccharide found in plants and serves as a desiccation protectant in seeds, a transport sugar in phloem sap, and a plant storage sugar
• Stachyose is a naturally occurring tetrasaccharide that promotes the growth of beneficial bacteria in the human GI tract, found in peas and beans like soya and chickpeas
• Polysaccharides are polymers of high molecular weight that differ in the identity of recurring units, chain length, type of glycosidic bonds, and degree of branching
• Carbohydrates are classified into three main groups: monosaccharides, disaccharides, and polysaccharides

  
• Monosaccharides are the simplest carbohydrates and consist of a single sugar unit
• Disaccharides are made up of two monosaccharides linked together
• Polysaccharides are polymers of high molecular weight that differ in the identity of the recurring unit(s), the length of their chains, the type of glycosidic bonds linking the residues, and the degree of branching
• Polysaccharides you should know include Starch, Glycogen, Cellulose, Chitin, Peptidoglycan, and Glycosaminoglycans