Carbohydrates I - MEDCHEM.21

General Overview of Carbohydrates

• Carbohydrates are polyhydroxylated aldehydes or ketones commonly referred to as sugars.

• They are synthesized by green plants through photosynthesis.

• The name carbohydrate is derived from glucose, which was the first simple carbohydrate isolated.

• Molecular formula of glucose: C6H{12}O6, initially thought to be a hydrate of carbon (C6(H2O)6).

• Glucose polymers constitute approximately 50% of the dry weight of earth's biomass.

Classification of Carbohydrates

• Simple Carbohydrates:

  • Also known as monosaccharides, cannot be hydrolyzed into simpler sugars, examples include glucose and fructose.

• Complex Carbohydrates:

  • Composed of two or more monosaccharides linked by glycosidic bonds.

  • Examples include sucrose (disaccharide: glucose + fructose) and cellulose (polysaccharide: multiple glucose units).

Fischer Projections

• The Fischer projection is a two-dimensional representation of a carbohydrate molecule.

• It projects a tetrahedral carbon onto a flat surface:

  • Horizontal lines come out of the plane (toward you).

  • Vertical lines go behind the plane (away from you).

Monosaccharides: Aldose and Ketose

• Aldose: Contains an aldehyde group, indicated by the prefix "aldo-".

• Ketose: Contains a ketone group, indicated by the prefix "keto-".

• Number of carbons is denoted by the prefixes: tri-, tetr-, pent-, hex-.

D and L Sugars

• D-sugars have the same stereochemical configuration at the chiral center furthest from the carbonyl as D-glyceraldehyde, with the hydroxyl group on the right in Fischer projections.

• L-sugars are the mirror image of D-sugars, with the hydroxyl group on the left.

Examples of Common Sugars

• D-Glucose:

  • Configuration in Fischer projection:

  • Hydroxyl on C5 is on the right.

  • Hydroxyl on C4 is on the right.

  • Hydroxyl on C3 is on the left.

  • Hydroxyl on C2 is on the right.

• Epimers:

  • D-Mannose (C2 epimer) and D-Galactose (C4 epimer) are diastereomers of D-glucose, differing at C2 and C4, respectively.

Conversion Between Fischer and Haworth Projections

• Cyclic forms can be depicted using Haworth projections.

• In this conversion:

  • The aldehyde becomes a hemiacetal.

  • Hydroxyl groups on the right in Fischer appear below the ring, while those on the left appear above.

• For D-sugars, the -CH_2OH group is above the ring.

Formation of Hemiacetals and Cyclic Structures

• Monosaccharides can undergo intramolecular nucleophilic addition to form cyclic hemiacetals.

• Glucose primarily exists in aqueous solutions as a pyranose (six-membered ring).

  • This is formed by the attack of the hydroxyl group at C5 on the carbonyl at C1.

• The two anomers of D-Glucose, eta-D-Glucose and eta-D-Glucose, differ at C1 (the anomeric carbon).

Key Visual Representations

• Haworth Projections: Represent cyclic sugars but are less commonly used than chair structures now.

• Chair Structures: Preferred for depicting the cyclic form of aldohexose sugars such as glucose, mannose, and galactose.

Anomers of D-Fructose

• D-Fructose, a ketose, can also form five-membered (furanose) and six-membered cyclic structures.

• Both α- and β-anomers of D-Fructose are formed during cyclization, based on the orientation of the hydroxyl group.

Summary of Important Terms and Definitions

• Monosaccharide: A simple sugar that cannot be hydrolyzed.

• Disaccharide: Composed of two monosaccharides.

• Polysaccharide: Composed of several monosaccharides linked together.

• Epimer: Stereoisomers that differ at one specific carbon atom.

• Anomer: Stereoisomers differing at the anomeric carbon (C1 for aldoses).

• Hemiacetal: Product of the reaction between an aldehyde and an alcohol.

Carbohydrates are polyhydroxylated aldehydes or ketones commonly referred to as sugars. They are one of the four primary macromolecules essential to life, alongside proteins, lipids, and nucleic acids. Carbohydrates serve several critical functions, including providing energy, serving as structural components, and participating in cell-cell recognition processes. They are synthesized by green plants through the process of photosynthesis, where carbon dioxide and water are converted into glucose and oxygen using sunlight.

The name carbohydrate is derived from glucose, which was the first simple carbohydrate isolated. The molecular formula of glucose is C6H{12}O6, which was initially misinterpreted as a hydrate of carbon represented by the formula C6(H2O)6. Glucose is not only an essential energy source for living organisms but also plays a key role in various metabolic pathways. It is estimated that glucose polymers constitute approximately 50% of the dry weight of Earth's biomass, highlighting the significant role carbohydrates play in ecological systems and food webs.

1. Understanding Carbohydrate Structures: Students will be able to identify and explain the structural classifications of carbohydrates, distinguishing between simple carbohydrates (monosaccharides) and complex carbohydrates (disaccharides and polysaccharides). They will learn how monosaccharides like glucose and fructose differ in structure and function, including their roles in energy provision and cellular metabolism.

2. Analyzing Fischer and Haworth Projections: Learners will gain proficiency in drawing and interpreting Fischer projections of carbohydrates, understanding how these representations relate to three-dimensional molecular geometry. They will also become adept at converting Fischer projections to Haworth projections to illustrate cyclic forms and comprehend how the configuration of hydroxyl groups affects molecular behavior.

3. Identifying D and L Stereoisomers: Students will understand the concepts of D and L sugars, including how to determine the configuration of sugars based on the position of hydroxyl groups in Fischer projections. They will learn the significance of stereoisomerism in carbohydrate chemistry, particularly in the context of biochemical pathways and the recognition of carbohydrates by biological systems.

4. Understanding the Formation of Hemiacetals and Cyclic Structures: The course will cover how monosaccharides can undergo intramolecular nucleophilic addition to form cyclic hemiacetals. Students will learn about the stability of different anomers in solution, the transition from open-chain forms to cyclic structures, and how this affects the properties of carbohydrates.

5. Recognizing the Importance of Carbohydrates in Biology: Learners will explore the critical functions of carbohydrates beyond energy storage, including their roles as structural components in cell walls (like cellulose), their involvement in biochemical recognition and signaling, and their contribution to molecular diversity in biological systems.

6. Applying Knowledge to Real-world Scenarios: Students will apply their understanding of carbohydrates in various contexts, such as nutrition, health, and biochemistry. They will analyze how different carbohydrates affect metabolic processes, the implications of glycemic index in dietary choices, and the role of carbohydrates in the human body and ecosystems.