Monosaccharides and Their Configurations

Carbohydrates Overview

• Carbohydrates are the most abundant bioorganic molecules on Earth.
• They constitute about 75% of dry plant materials, produced through photosynthesis (reactants: carbon dioxide, water; energy source: sunlight).
• Major carbohydrate sources for humans and animals come from dietary plant materials, ideally two-thirds of the average diet by mass.

Classification of Carbohydrates

• Carbohydrates are classified based on molecular size into:
  • Monosaccharides: Single sugar units (e.g., glucose and fructose).
  • Disaccharides: Two sugar units linked (e.g., sucrose).
  • Oligosaccharides: Short chains of sugar units.
  • Polysaccharides: Long chains of sugar units (e.g., starch, cellulose).

Molecular Structure and Chirality

• Carbohydrates can be polyhydroxy aldehydes or polyhydroxy ketones.
• Chirality refers to "handedness" in molecules; chiral molecules have non-superimposable mirror images.
• The presence of a chiral center (a carbon atom bonded to four different groups) determines chirality.
• Chiral molecules exhibit different biological responses; the body may respond differently to left-handed versus right-handed forms.

Importance of Chiral Molecules

• Right-handed and left-handed forms can elicit different responses in biological systems.
• Example: Human body reacts 20 times more effectively to the right-handed form of epinephrine compared to its left-handed counterpart.

Stereoisomerism

• Stereoisomers share the same molecular formula but differ spatially:
  • Enantiomers: Non-superimposable mirror images (e.g., right and left-handed forms).
  • Diastereomers: Stereoisomers that are not mirror images (e.g., cis-trans isomers).

Designating Handedness Using Fischer Projections

• Enantiomers can rotate polarized light (dextrorotatory = clockwise, levorotatory = counterclockwise).
• D,L system helps determine configuration based on the highest-numbered chiral center's hydroxyl group:
  • If -OH is right, it is a D-isomer.
  • If -OH is left, it is an L-isomer.
• Fischer projections visually represent spatial arrangement around chiral centers.

Summary

• Understanding carbohydrates involves reviewing molecular structures, classifications, and the importance of chirality in their biological functions.
• Applications of carbohydrates extend to their roles in energy storage, structural roles in plants, and their implications in health, notably in carbohydrate-related diseases.### Structures and Classification of Monosaccharides
• Monosaccharides typically contain 3 to 7 carbon atoms.
• Three-carbon monosaccharides are called trioses (e.g., glyceraldehyde).
• Four-carbon monosaccharides are tetroses (e.g., erythrose).
• Five-carbon monosaccharides are pentoses (e.g., ribose, arabinose).
• Six-carbon monosaccharides are hexoses (e.g., glucose, fructose).
• Aldoses contain an aldehyde functional group; ketoses contain a ketone functional group.
• Classification can be based on both the number of carbon atoms and functional groups (e.g., aldohexose, ketopentose).
• "Sugar" refers to both monosaccharides and certain disaccharides, often associated with sweetness.

Optical Isomers and Chiral Centers

• The number of possible optical isomers is determined by the number of chiral centers in the monosaccharide.
• Each chiral carbon can result in varying configurations, affecting the sugar's stereoisomers.
• Aldohexose (6C) can produce 16 optical isomers; aldohexose has 4 chiral centers.
• Ketoses have fewer optical isomers due to fewer chiral centers.

Epimers

• Epimers are forms of carbohydrates that differ at only one chiral center (e.g., D-glucose and D-galactose differ at carbon 4).
• Unlike enantiomers, epimers are not mirror images of each other.

Cyclic Monosaccharides: Haworth Projection

• Monosaccharides can exist in cyclic forms, represented by Haworth projection formulas.
• Cyclic structures result from the reaction of carbonyl groups with hydroxyl groups.
• Pyranoses (six-atom rings) and furanoses (five-atom rings) are the two cyclic forms.
• The formation of cyclic monosaccharides usually leads to two stereoisomers known as anomers (alpha and beta forms).
• The anomeric carbon is the hemiacetal carbon; its configuration defines the type of anomer.

Biochemically Important Monosaccharides

• D-Ribose: Essential component of nucleic acids and ATP; involved in various metabolic pathways.
• D-Ribulose: An intermediate in the pentose phosphate pathway.
• D-Arabinose: Present in various plant gums; constituent of glycoproteins.
• D-Glucose: Known as the "sugar" of the body; normal concentration is 70-100 mg/dL; key energy source.
• D-Fructose: Found in fruit juices; used by the liver and considered the sweetest sugar.
• D-Galactose: Derived from lactose; crucial for brain function and appears in glycolipids.
• D-Mannose: Found in plant mannans; a constituent of glycoproteins used for prevention of certain diseases.### Urinary Tract Infections (UTIs)
• UTIs are common infections affecting the urinary system, often treated with antibiotics.
• Understanding carbohydrate metabolism can help manage genetic disorders like carbohydrate-deficient glycoprotein syndrome.

Reactions of Monosaccharides

• Five major reactions include oxidation to acidic sugars, reduction to sugar alcohols, glycoside formation, phosphate ester formation, and amino sugar formation.

Oxidation to Acidic Sugars

• Involves redox reactions of monosaccharides, transforming them into acidic forms:
  • Aldonic Acid: Acid group at terminal carbon.
  • Alduronic Acid: Acid group at the terminal carbon opposite the aldehyde.
  • Aldaric Acid: Acid groups at both ends of the molecule.

Reduction to Sugar Alcohols

• Carbonyl group can be reduced to a hydroxyl group.
• The product is a sugar alcohol (e.g., D-glucose reduced to D-glucitol or D-sorbitol) used in cosmetics and as a sweetener.

Glycoside Formation

• Hemiacetal forms of monosaccharides react with alcohols to create acetals, termed glycosides.
• Glycosides can exist in alpha and beta forms, defined by the orientation of the -OH group.

Phosphate Ester Formation

• Hydroxyl groups of monosaccharides may react with oxyacids to form phosphates.
• Forming glucose 1-phosphate and glucose 6-phosphate is important for carbohydrate metabolism.

Amino Sugar Formation

• A hydroxyl group replaced by an amino group leads to amino sugar formation (common examples: glucosamine and galactosamine).
• Amino sugars are crucial in constructing polysaccharides like chitin and hyaluronic acid.

Disaccharides

• Formed from the glycosidic linkage between two monosaccharides functioning as a hemiacetal and an alcohol.
• Glycosidic linkages consist of carbon–oxygen–carbon bonds.

Features of Disaccharides

• Maltose: Comprised of two D-glucose units, linked by α(1-4) linkage.
• Cellobiose: Comprised of β(1-4) glucose and galactose units.
• Lactose: Composed of D-galactose and D-glucose with β(1-4) linkage.
• Sucrose: Comprising glucose and fructose via α,β(1-2) linkage.

Oligosaccharides

• Contain 3 to 10 monosaccharides linked via glycosidic bonds.
• Example trisaccharide: Raffinose (α-D-galactose, α-D-glucose, β-D-fructose).
• Example tetrasaccharide: Stachyose (two α-D-galactose, one α-D-glucose, one β-D-fructose).

Importance of Oligosaccharides

• Blood types (O, A, B, AB) are defined by specific oligosaccharides on red blood cells.

Polysaccharides

• Polymers of numerous monosaccharide units, often non-sweet and limited in solubility.
• Used as thickeners in cooking due to the hydrophilic nature of -OH groups.

Types of Polysaccharides

• Storage Polysaccharides:

  • Starch: Energy storage in plants, includes amylose (linear) and amylopectin (branched).
  • Glycogen: More branched than amylopectin, serves as energy storage in animals.

• Structural Polysaccharides:

  • Cellulose: Main component of plant cell walls, indigestible by humans.
  • Chitin: Structural element in exoskeletons of arthropods, similar structure to cellulose.

• Acidic Polysaccharides:

  • Composed of disaccharide repeating units with amino sugars and negative charges (e.g., hyaluronic acid and heparin).

Glycosaminoglycans (GAGs)

• Also known as mucopolysaccharides, consist of repeating disaccharide units.
• Often contain one amino sugar, playing essential roles in biological processes such as joint lubrication and cellular hydration.