Monosaccharides: Structure and Isomers

Monosaccharides

• Conformational isomers are distinguished from configurational isomers in that the former can interconvert without breaking and re-forming bonds.
• Epimers are stereoisomers differing in their configuration about only one asymmetric carbon atom.

Derivatives of Monosaccharides

• Sugar phosphates are important intermediates in metabolism, functioning as activated compounds in syntheses.
• Oxidation of monosaccharides can proceed in several ways, depending upon the oxidizing agent used, producing aldonic acids.
• Enzyme-catalyzed oxidation of monosaccharides gives other products, including uronic acids such as glucuronic acid.
• Uronic acids are important constituents of certain natural polysaccharides.
• Free aldonic acids, such as gluconic acid, are in equilibrium in solution with lactones, which are cyclic esters.
• Reduction of the carbonyl group on a sugar gives rise to the class of polyhydroxy compounds called alditols.
• Important naturally occurring alditols include erythritol, D-mannitol, and D-glucitol (also known as sorbitol).
• D-glucitol is part of bacterial cell wall, which is built from alternating units of GlcNAc and N-acetylmuramic acid, forming a layered structure called peptidoglycan.

Oligosaccharides

• Elimination of water between the anomeric hydroxyl of a cyclic monosaccharide and the hydroxyl group of another compound yields an O-glycoside.
• The acetal bond formed is referred to as a glycosidic bond.
• Two naturally occurring glycosides are Ouabain (cardiac glycoside, Na+/K+ ATPase inhibitor) and amygdalin (cyanide!), which are highly toxic glycosides produced by plants.
• Distinguishing features of different disaccharides include:
  • The two specific sugar monomers involved, and their stereoconfigurations.
  • The carbons involved in the linkage.
  • The order of the two monomer units, if they are different kinds.
  • The configuration of the anomeric hydroxyl group of each residue.
• Note the convention used to draw glycosidic bonds, where the “bent bonds” allow the Haworth projections of the monomers to be drawn in parallel.

Polysaccharides

• Hyaluronic acid is distributed widely throughout connective, epithelial, and neural tissues, and is unique among glycosaminoglycans in that it is nonsulfated, forms in the plasma membrane instead of the Golgi apparatus, and can be very large.
• Hyaluronic acid is one of the chief components of the extracellular matrix, and contributes significantly to cell proliferation and migration.
• Heparin is a highly sulfated glycosaminoglycan, appears to be a natural anticoagulant, and is found in many body tissues.
• Heparin binds strongly to a blood protein, antiprothrombin III, and the complex inhibits enzymes of the blood clotting process.
• The cell wall of a Gram-positive bacterium, S. aureus, consists of a thick peptidoglycan layer made up of polysaccharide chains and short peptides.
• The cell wall of a Gram-negative bacterium, E. coli, has a thin peptidoglycan layer and an outer lipid membrane.

Glycoconjugates of Interest

• The structure of a lipotechoic acid consists of D-Alanyl and NAG groups arranged irregularly on the chain, which is anchored in the membrane by lipid.
• The cross-linking reaction in peptidoglycan synthesis involves the action of a transpeptidase enzyme, which is inhibited by penicillin.
• Penicillin inhibits the completion of the synthesis of peptidoglycans by binding to proteins with the β-lactam ring, causing the cell wall to weaken due to fewer cross-links, and ultimately leading to cell lysis.