Biochemistry: Monosaccharides, Disaccharides, and Polysaccharides

Biochemistry: Monosaccharides, Disaccharides, and Polysaccharides

# Monosaccharides (MS)

• Structure: MS consists of a carbon chain containing a reducing functional group (aldehyde or ketone) and hydroxyl groups.

• Classification:

• Chemical Isomers:

• Ketose: MS with a ketone functional group.

• Aldose: MS with an aldehyde functional group.

• Stereoisomers:

• D-isomer: The OH group on the last asymmetric carbon is on the right.

• L-isomer: The OH group on the last asymmetric carbon is on the left.

• Optical Isomers:

• (+) Isomer: Rotates the plane of polarized light to the right.

• (-) Isomer: Rotates the plane of polarized light to the left.

• Cyclic Form: MS can cyclize into a 6-membered ring (pyranose) or a 5-membered ring (furanose) according to Haworth’s formula.

• Haworth’s Cyclization Rule:

• D: CH2OH is above.

• L: CH2OH is below.

• Right OH: Below.

• Left OH: Above.

• alpha: Hemiacetal OH is below.

• beta: Hemiacetal OH is above.

• Reactions:

• Reduction: MS reacts with heavy metal salts in a hot alkaline environment.

• Fehling: A reduction reaction used for qualitative and quantitative determination of MS.

• Oxidation: MS is oxidized into polyalcohols like sorbitol.

• With organic acids: Forms phosphate esters like Glucose-6-phosphate (G6P).

• With strong inorganic acids: Forms furfural and its derivatives, used for MS identification.

• Typical Reactions: Molish reaction, Sellwanoff reaction.

• Glycosidic Linkages:

• O-glycosidic: OH of MS links to OH of another compound.

• N-glycosidic: OH of MS links to NH2 of another compound.

• Derivatives:

• Uronic acids: Play important biological roles, conjugate bilirubin in the liver, components of complex PS.

• Phosphate esters: Intermediate metabolic products, activate substrates in glucid metabolism.

• Osamines: Participate in the structure of many complex PS.

• Sialic acids: Components of animal cell membrane PS.

• Common triose derivatives: D-glyceraldehyde and dihydroxyacetone.

• Common pentose derivatives: Ribose (structure of RNA), deoxyribose (structure of DNA).

• Common hexose derivatives: D-glucose (structure of many important glucids), fructose, galactose.

# Disaccharides (DS)

• Classification: Maltose, lactose, saccharose.

• Structure: Two monosaccharide units linked together through glycosidic bonds.

• Lactose: 1 beta-D-glucosepyranose linked to 1 alpha-D-galactopyranose (1,4-alpha-galactosidic linkage).

• Maltose: 2 alpha-D-glucopyranose molecules (1,4-alpha-glycosidic linkage).

• Saccharose: 1 alpha-D-glucopyranose molecule linked to 1 beta-D-fructofuranose molecule (1,2-alpha,beta-glycosidic linkage).

• Reducing properties: Lactose and maltose have reducing properties due to the presence of a free aldehyde group.

# Polysaccharides (PS)

• Classification:

• Homopolysaccharide: Composed of the same monosaccharide units.

• Heteropolysaccharide: Composed of different monosaccharide units or linked to other molecules like proteins.

• Homopolysaccharide:

• Starch:

• Structure: 20% amylose and 80% amylopectin.

• Amylose: Alpha-D-glucose molecules linked together (alpha-1,4-glycosidic linkage), straight chain, soluble in water.

• Amylopectin: Alpha-D-glucose molecules linked together (alpha-1,4-glycosidic and alpha-1,6-glycosidic linkages), branched chain.

• Glycogen: Alpha-D-glucose molecules linked together (alpha-1,4-glycosidic and alpha-1,6-glycosidic linkages), branched chain, is the storage glucid in the liver and muscles.

• Dextrin: Intermediate hydrolysis product of starch.

• Cellulose: Beta-D-glucose molecules linked together (beta-1,4-glycosidic linkage), straight chain.

• Heteropolysaccharide:

• Mucopolysaccharide: Contains amines (osamines) and uronic acids.

• Hyaluronic acid: Abundant in synovial fluid, highly viscous.

• Chondroitin sulfate: Abundant in cartilage, connective tissue, protective tissue, supporting tissue.

• Heparin: Abundant in blood, produced in the liver, used as an anticoagulant.

• Mucoprotein: Proteins with attached saccharide chains.

• Function: Found in fluids, tissues, cell membranes, many hormones are glycoproteins like LH, FSH, TSH.

# Conclusion

Monosaccharides, disaccharides, and polysaccharides are important glucid molecules in living organisms, participating in many biological activities like energy supply, cell structure, and biological activity regulation. Understanding the structure, properties, and functions of glucid molecules will help us better understand the functioning of living organisms and lead to effective applications in medicine and biotechnology.