Study sheet: Carbohydrates: Oses and Osides

📋 Course Outline

1. Chemical Reduction of Oses
2. Esterification and Amino Substitution
3. Glycation and Maillard Reaction
4. Glycosidic Bonds and Nomenclature
5. Oside Analysis Methods
6. Major Disaccharides
7. Reserve and Structural Polysaccharides
8. Heterosides and Glycosylation

📖 1. Chemical Reduction of Oses

★ Must-know

📌 In alkaline solution, the free pseudoaldehydic or pseudoketonic functions of oses reduce copper salts in Fehling’s solution, converting blue Cu2+ into brick-red Cu2O.

• Reduction of an ose carbonyl group converts its aldehyde or ketone function into an alcohol and produces a polyol.

Further detail

• Fehling’s solution is used to assay reducing sugars.

💡 Memory Hook

Carbonyl reduction → polyol formation

📖 2. Esterification and Amino Substitution

🔑 Key Concepts & Definitions

• Osamine : formed when the hydroxyl group at C2 of an ose is replaced by an NH2 group, as in glucosamine

★ Must-know

📌 The hydroxyl functions of oses react with acids to form ester or phosphoester functions.

• During the first step of glycolysis, hexokinase or glucokinase phosphorylates D-glucose using ATP to form D-glucose 6-phosphate and ADP.

Further detail

• The NH2 group of osamines is often acetylated to form N-acetyl-osamines, such as N-acetylglucosamine.

💡 Memory Hook

OH esterification → NH2 substitution → N-acetylation

📖 3. Glycation and Maillard Reaction

🔑 Key Concepts & Definitions

• Glycation : the association of a reducing ose with a primary amine from an amino acid or protein to form an imine through the Maillard reaction

📝 Essential Points

• Glycation of hemoglobin produces HbA1c, which is used to distinguish healthy and diabetic conditions in the course example.

💡 Memory Hook

Reducing ose + primary amine → imine formation

📖 4. Glycosidic Bonds and Nomenclature

🔑 Key Concepts & Definitions

• Oside : a substance whose hydrolysis releases one or more oses
• O-glycosidic bond : forms by condensation between hydroxyl functions of two oses with elimination of one water molecule

★ Must-know

📌 An ose–oside bond involves the hemiacetal function of one ose and a non-hemiacetal hydroxyl function of another, leaving one reducing function free; an oside–oside bond involves both hemiacetal functions and leaves no reducing function free.

Further detail

📌 A residue whose hemiacetal function participates in a glycosidic bond takes the suffix -osyl when internal and -oside when terminal, whereas a residue whose hemiacetal function does not participate takes the suffix -ose.

💡 Memory Hook

Ose–oside retains a reducing end; oside–oside does not

📖 5. Oside Analysis Methods

🔑 Key Concepts & Definitions

• Osidase : an enzyme that hydrolyzes glycosidic bonds specifically according to the anomeric conformation of a given ose and only when that ose’s hemiacetal function participates in the bond
• Thin-layer chromatography : separates molecules according to their different affinities for the stationary phase and the mobile phase

★ Must-know

• Acid hydrolysis with concentrated HCl breaks the glycosidic bonds of osides and releases their constituent oses.

• The released oses can be analyzed by:

  • thin-layer chromatography
  • gas chromatography
  • high-performance liquid chromatography

📌 A positive Fehling test indicates an ose–oside linkage with a free reducing function, whereas a negative test indicates an oside–oside linkage without a free reducing function.

📌 Haworth methylation transfers methyl groups to the free hydroxyl functions of osides, so hydroxyl groups that are not methylated identify positions involved in glycosidic bonding.

Further detail

📌 In thin-layer chromatography, silica gel is the polar stationary phase and an organic solvent is the nonpolar mobile phase.

💡 Memory Hook

Hydrolysis → separation → linkage testing → methylation → enzymatic identification

📖 6. Major Disaccharides

📝 Essential Points

• Sucrose is α-D-glucopyranosyl (1-2) β-D-fructofuranoside, contains an oside–oside bond, is non-reducing, and can be hydrolyzed by α-glucosidase and β-fructosidase.

• Maltose is α-D-glucopyranosyl (1-4) D-glucopyranose, contains an ose–oside bond, is reducing, and is hydrolyzed by α-glucosidase.

• Lactose is β-D-galactopyranosyl (1-4) D-glucopyranose, contains an ose–oside bond, is reducing, and is hydrolyzed by β-galactosidase.

💡 Memory Hook

Sucrose is non-reducing, whereas maltose and lactose are reducing

📖 7. Reserve and Structural Polysaccharides

🔑 Key Concepts & Definitions

• Polyoside : a polymeric molecule made exclusively of oses

★ Must-know

📌 Homopolyosides contain one type of ose, whereas heteropolyosides contain several types of ose; homopolyosides made only of glucose are glucosanes.

• Starch contains 20–30% amylose, a linear α(1-4) glucan of 1000–4000 D-glucopyranoses, and 70–80% amylopectin, an α(1-4) glucan branched by α(1-6) bonds every 25–30 glucose units.

• Salivary and pancreatic α-amylase cleave starch α(1-4) bonds, and oligosaccharidases then release D-glucose from the digestion products for assimilation.

• Glycogen has a structure analogous to amylopectin but is more highly branched and serves as a carbohydrate energy reserve in animal liver and muscle.

📌 Glycogenesis is activated when blood glucose is high, whereas glycogenolysis is activated when blood glucose is low or during physical effort.

• Cellulose is a linear chain of D-glucopyranoses linked by β(1-4) bonds and is not assimilated by animals because their digestive tract lacks specific osidases.

• Hyaluronic acid is a glycosaminoglycan made of glucuronic acid and N-acetylglucosamine linked by β(1-3) bonds, with disaccharide motifs linked by β(1-4) bonds.

Further detail

• Starch is the carbohydrate energy reserve of plants, is poorly soluble in water, and its α(1-4) chains form helices through hydrogen bonds.

• Inulin is a linear chain of approximately 30 D-fructofuranoses linked by β(2-1) bonds, associated at one end with α-D-glucopyranose through an α(1-2) bond, and is found in roots and tubers.

• Chitin is a linear chain of N-acetylglucosamines linked by β(1-4) bonds, organized into sheets that form fibrils in arthropods.

• Pectins are linear chains of D-galacturonic acid linked by α(1-4) bonds with a small amount of α-L-rhamnose, and they form water-retaining gels in plant cell walls.

• Hyaluronic acid is a major component of extracellular matrices in connective, epithelial, and nervous tissues and has important viscoelastic properties because it binds many water molecules.

💡 Memory Hook

Reserve polysaccharides store energy; structural polysaccharides build tissues and matrices

📖 8. Heterosides and Glycosylation

🔑 Key Concepts & Definitions

• Heteroside : a polymeric molecule containing a carbohydrate part called the glycone and a non-carbohydrate part called the aglycone, linked by an N-glycosidic or O-glycosidic bond
• N-glycosidic bond : links the hemiacetal function of an ose to a primary amine function of a non-carbohydrate molecule
• Glycosylphosphatidylinositol : an O-glycosidic glycolipid made from a carbohydrate chain linked to phosphatidylinositol and used to anchor proteins lacking a hydrophobic domain in the membrane

★ Must-know

• N-glycosylation occurs at the endoplasmic-reticulum membrane during protein maturation, involves N-acetylglucosamine linked to an asparagine in an Asn–X–Ser/Thr sequence, and can be modified in the Golgi apparatus.

📌 In the Asn–X–Ser/Thr motif of N-glycosylation, X can be any amino acid except proline, and N-glycosylation is especially associated with membrane or secreted proteins and protein addressing.

• O-glycosylation occurs mainly in the Golgi apparatus and links N-acetylglucosamine to serine or threonine residues of a protein, although it can occur in the cytoplasm for some proteins.

Further detail

• In ATP, adenine is linked to β-D-ribofuranose by an N-glycosidic bond, while phosphate is linked by a phosphoester bond.

• Proteoglycans are O-glycosylated glycoproteins found in extracellular or membrane forms that participate in cell signaling, cell–cell adhesion, growth-factor presentation, extracellular-matrix organization, and cell–matrix adhesion.

• Glycosphingolipids consist of oses or osides linked to ceramides and are components of plasma membranes of nervous cells with roles in cell growth, proliferation, and signaling.

💡 Memory Hook

N-glycosylation targets Asn; O-glycosylation targets Ser or Thr

📊 Synthesis Tables

Major disaccharides

Disaccharide	Glycosidic linkage	Reducing property	Specific osidase
Sucrose	α-D-glucopyranosyl (1-2) β-D-fructofuranoside	Non-reducing	α-glucosidase and β-fructosidase
Maltose	α-D-glucopyranosyl (1-4) D-glucopyranose	Reducing	α-glucosidase
Lactose	β-D-galactopyranosyl (1-4) D-glucopyranose	Reducing	β-galactosidase