Week 3 BC L2

Introduction to Carbohydrates

  • Carbohydrates play a crucial role in energy storage and structural components in nature.

  • Examples of carbohydrates include sugars and complex materials like wood.

  • Learning outcomes for this lecture: Understanding types of carbohydrates (monosaccharides, disaccharides, polysaccharides), DNL and alpha/beta configurations, writing sugar structures in Fischer, Haworth projections, and identifying glycosidic bonds.

Definitions and Structure of Carbohydrates

  • Carbohydrates: Molecules that consist of carbon, hydrogen, and oxygen, often with the formula Cn(H2O)n.

    • Complex structures incorporate additional functional groups.

  • Monosaccharides: Simple sugars, with a basic structure that contains one sugar unit. Type of carbohydrate that cannot be hydrolized to a simpler carbohydrate. Cn(H2O)n

  • Can be classified as either aldoses (with aldehyde groups) or ketoses (with ketone groups).

  • Naming: Cn(H2O)n

    • Exist typically with 3 to 8 carbon atoms (C3 - C8).

    • Common classifications based on the number of carbons: trioses, tetroses, pentoses, hexoses, heptoses, octoses.

    • Example: Glyceraldehyde is a monosaccharide; it can exist as both R and S forms based on chirality.

  • Monosaccharides:

    • Simple sugars have aldose verson or ketose version

    • Altose: Always find aldehyde on top, the next one will have an OH (chiral) and the last one will have a COH (not chiral)

    • Ketose: Always find ketone group on the second carbon. Ch2OH on first and Oh on last

    • Know that you have an altose if you do the mirror trick and get silver

Chirality and Stereochemistry

  • Stereocenters: Most carbohydrates have one or more stereogenic centers, leading to enantiomers.

  • Number of possible stereoisomers increases with the number of stereocenters; each new carbon adds a factor of 2 for possible arrangements (2^n).

  • D and L configurations: Based on the orientation of the hydroxyl group on the last chiral carbon in Fischer projections.

    • D-sugars have the hydroxyl group on the right; L-sugars have it on the left.

  • Natural sugars are predominantly D-form in structural biology.

Projections for Carbohydrates

  • Fischer Projection: A 2D representation of sugars showing the arrangement of functional groups. Just drawn with lines representing the bonds between carbon atoms, with vertical lines indicating bonds that extend behind the plane and horizontal lines showing those that come forward.

  • D-monosaccharide: -OH on its penultimate carbon on the right

  • L-monosaccharide: -OH on its penultimate carbon on the left

  • Determine by looking at the second last group (group on top of the CH2OH) to see if the OH above it is on the left or right

    • The carbon chain is vertical, with the highest oxidation state at the top.

    • Stereochemistry is identified by the positioning of hydroxyl and other groups on the left or right.

    • Nature only makes D sugers.

  • Should know: D-glucose, D-galactose, D-mannose, D-xylose, D-ribose, D-arabinose

  • Hayorth Projection: A cyclic representation that depicts sugars' ring structures.

    • Fischer projections can be converted to Hayorth by adjusting the molecule's orientation and forming rings from the linear structure.

  • Aldehydes and Ketones react with alcohols to form Hemiacetals

  • Monosaccharides have hydroxyl and carbonyl groups in the same molecule existing almost exclusively as 5- and 6- (most stable as tetrahedral) membered cyclic hemiacetals

  • Turning the fisher structure 90º to the right, conecting the second last OH with the top carbon gives you the heyworth projections

  • This forms to different products:

    • An OH group at the top (beta) and one with OH group at bottom (alpha)

    • Commonly drawn with anomeric carbon on right and hemiacetal oxygen on back right.

    • beta-anomer - OH on annomeric carbon is cis to terminal -CH2OH

    • alpha- anomer - the OH on the anomeric carbon is trans to the terminal -CH2OH

  • Anomeric carbon: New seterocentre resulting from cyclic hemiacetal formation

  • Anomer: Carbohydrates that differ in configueration at their anomeric carbons

  • Any 6 membered ring is a pyranose (more accuratly represented in chair conformation) structure while a 5 membered ring is a furanose structure

Glucose

  • beta-D-glucose is formed in more abundance than alpha because it is more stable due to the equatorial positioning of the hydroxyl group at the anomeric carbon, minimizing steric hindrance.

  • In contrast, alpha-D-glucose has the hydroxyl group in an axial position, leading to increased steric strain and reduced stability.

Equatorial bond (line straight down or straight up) is a bond that minimizes steric hindrance by allowing substituents to be positioned away from the ring, thus enhancing the overall stability of the molecule.

This type of bond allows for greater spatial separation between bulky groups, ultimately contributing to the overall stability of the molecule.

This can change:
Ring closer of sugars is eqilibrium reaction; so always get an interchange between alpha and beta anomers

process leading to that is mutarotation (change from alpha to beta anomer or vise versa)

Complexity of Sugars

  • Sugars can take various forms including linear, cyclic, anomeric, and different ring sizes (five and six membered).

  • Keto functions form rings as well, adding to the complexity.

Hemiacetals and Acetals

  • Hemiacetal: Can react with alcohol to form an acetal.

  • Formation of Acetals: Example given with beta-D-glucose reacting with methanol leading to a new C-O-C bond.

  • Once an acetal is formed, the structure is locked (no reverting to the linear form) so don’t have mutarotation anymore and stuck in either alpha or beta form.

  • Forms a glycosidic bond, indicating a connection between sugar units.

Glycosides and Polymer Formation

  • Sugars are polyols (contain multiple hydroxyl -OH groups); they can react to form polymers.

  • Monosaccharides: Single sugar molecules (linear or cyclic).

  • Disaccharides: Formed when two monosaccharides bond.

    • Example: Maltose formed from two glucose molecules (alpha 1,4 glycosidic bond) as anemeric carbon is always number 1, and then count on the second anomeric carbon to the join point whihc is 4

    • Enzymes in the body can break down these bonds to release glucose.

Types of Glycosidic Bonds

  • Notation of glycosidic bonds: e.g., alpha 1,4 indicates the positions of the carbons in the sugars involved.

  • Alpha vs Beta Forms: Notation remains consistent with up (beta) and down (alpha) orientations.

    • Example: Different glycosidic bonds can form between sugars leading to various disaccharides.

Nucleosides and Nucleotides

  • Amines can react to form bonds similar to glycosides, important in nucleic acid structure.

  • Ribofuranose sugar connects with nucleobases to form nucleotides for DNA/RNA.

Sucrose: A Special Case

  • Sucrose is formed from glucose and fructose via a glycosidic bond.

  • Fructose being a ketose means it forms different cyclic structures, contributing to the uniqueness of sucrose.

    • Example: Involved bonds could be referred to from different perspectives (alpha 1,2 vs beta 2,1).

Polysaccharides

  • Polysaccharides are long chains of monosaccharides.

  • Cellulose: Usually formed by beta 1,4 links; very stable due to hydrogen bonding that allows for stacking.

    • Important in plant structure, high tensile strength due to its stacking ability.

  • Starch: Comprises alpha 1,4 links, digestible forms of carbohydrate found in plants.

    • Amylose: Linear chains of glucose giving helical structures making up 20% of starch. Not linear, starts to form helical structures

    • Amylopectin: Contains both alpha 1,4 and alpha 1,6 links (about every 25 units along chain), resulting in a branched structure (80% of starch) called hierarchical structure. Not soluble.

Glycogen: Energy Storage

  • Glycogen: Similar to amylopectin but with more random branching.

    • Defined as a hyperbranched polymer, allowing for rapid glucose release.

  • Glycogen can consist of up to 1,000,000 glucose units, significantly larger than amylopectin.

Summary of Key Concepts

  • Reviewed key points including terminology: monosaccharides, disaccharides, polysaccharides, anomeric Carbon, hemiacetals, acetals, and glycosidic bonds.

  • Importance of understanding structural forms (Fischer, Haworth, chair conformations).

  • Comprehensive understanding of sugars and their chemistry, integral for further studies in biochemistry.