25.7 The Eight Essential Monosaccharides

Monosaccharides are the building blocks of carbohydrates, including simple sugars, starches, and cellulose. These molecules come in various forms, like glucose, fructose, and ribose, each with unique structures and roles in biological processes.

Understanding monosaccharides is crucial for grasping how larger carbohydrates form and function. From providing energy to forming DNA, these simple sugars play vital roles in life. Their ability to link together creates complex molecules essential for numerous biological functions.

Essential Monosaccharides

Carbohydrates and Monosaccharides

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Carbohydrates are a diverse group of biomolecules that include sugars, starches, and cellulose. Monosaccharides, also known as simple sugars, are the basic building blocks of carbohydrates. They are the simplest form of carbohydrates and cannot be broken down into smaller sugar units. Monosaccharides can exist as stereoisomers, which are molecules with the same molecular formula but different spatial arrangements of atoms.

Essential monosaccharides and structures

• Glucose (Glc)
  • Most abundant monosaccharide found in nature exists in open-chain and cyclic forms (pyranose and furanose)
  • Classified as an aldohexose, a six-carbon sugar with an aldehyde group at one end and multiple hydroxyl groups along the chain
• Galactose (Gal)
  • Aldohexose that differs from glucose in the stereochemistry at the C-4 position (C-4 epimer)
  • Found as a component of lactose (milk sugar) and glycolipids in cell membranes
• Mannose (Man)
  • Aldohexose that differs from glucose in the stereochemistry at the C-2 position (C-2 epimer)
  • Commonly found as a component of glycoproteins and glycolipids in cells
• Fructose (Fru)
  • Ketohexose, a six-carbon sugar with a ketone group at the C-2 position and multiple hydroxyl groups
  • Sweetest naturally occurring sugar exists in open-chain and cyclic forms (furanose)
• Ribose (Rib)
  • Aldopentose, a five-carbon sugar with an aldehyde group at one end
  • Essential component of nucleotides, RNA, and some cofactors (ATP, NADH)
• Deoxyribose (dRib)
  • Aldopentose that lacks a hydroxyl group at the C-2 position compared to ribose
  • Key component of DNA, providing the backbone for the genetic material
• Xylose (Xyl)
  • Aldopentose commonly found as a component of hemicellulose in plant cell walls
  • Used in the production of biofuels and as a food additive
• N-Acetylneuraminic acid (Neu5Ac or NANA)
  • 9-carbon α-keto acid that is the most abundant member of the sialic acid family
  • Important component of glycoproteins and glycolipids on cell surfaces

Derivation from glucose

• Galactose
  • Formed by the epimerization of glucose at the C-4 position catalyzed by the enzyme UDP-galactose 4-epimerase
• Mannose
  • Formed by the epimerization of glucose at the C-2 position catalyzed by the enzyme phosphomannose isomerase
• Fructose
  • Formed by the isomerization of glucose from an aldose to a ketose catalyzed by the enzyme glucose isomerase
• Ribose and Deoxyribose
  • Derived from glucose via the pentose phosphate pathway involving oxidation, decarboxylation, and rearrangement reactions
• Xylose
  • Formed by the epimerization of ribose at the C-3 position catalyzed by the enzyme xylose isomerase
• N-Acetylneuraminic acid
  • Synthesized from UDP-N-acetylglucosamine (derived from glucose) and phosphoenolpyruvate
  • Synthesis involves condensation, epimerization, and phosphorylation reactions

N-acetylneuraminic acid in viral infections

• Cell surface recognition
  • Neu5Ac acts as a common terminal residue on glycoproteins and glycolipids
  • Serves as a receptor for various viruses (influenza, coronavirus, rotavirus) facilitating their attachment to host cells
• Viral attachment
  • Viruses bind to Neu5Ac using surface proteins like hemagglutinin in influenza
  • Binding allows the virus to enter the host cell and initiate infection
• Viral release
  • Some viruses (influenza) possess neuraminidase enzymes that cleave Neu5Ac from the host cell surface
  • Cleavage enables newly formed viruses to detach and spread to other cells, propagating the infection
• Antiviral drug targets
  • Neuraminidase inhibitors (oseltamivir, zanamivir) prevent viral release by blocking the enzyme's activity
  • Sialic acid analogs can compete with Neu5Ac for viral binding sites, preventing attachment and entry

Monosaccharide Linkages

• Monosaccharides can be joined together through glycosidic bonds to form larger carbohydrate molecules
• Glycosidic bonds are formed between the anomeric carbon of one monosaccharide and a hydroxyl group of another
• These linkages are crucial in the formation of disaccharides, oligosaccharides, and polysaccharides