Module 3 Lecture 2 - Biochemistry: Carbohydrate Structure and Function

Transition to Module Three and the Four Biological Macromolecules

• Module Two Summary: The previous module focused extensively on proteins, covering their structures, fundamental units (amino acids), and classification based on structural properties and specific characteristics.

• Module Three Focus: This current module shifts focus toward other macromolecules, particularly carbohydrates and lipids. Carbohydrates are prioritized as they are the primary source of energy and energy builders in the body.

• The Four Core Macromolecules: In biochemistry, there are four primary macromolecules:   * Proteins.   * Carbohydrates.   * Lipids.   * Nucleic Acids.

• Previous Modules Context:   * Module One: Focused on nucleic acids, exploring the transfer of information from the cell nucleus to the ribosome and the process of protein synthesis.   * Module Two: Focused on proteins.

• Practical Context: The current module's lecture follows a previously shared recording (due to the lecturer attending a ceremony), which introduced metabolic pathways and structural importance.

Introduction to Carbohydrates: Composition and Formulas

• Etymology: The term "carbohydrate" is derived from "carbon," "hydrogen," and "oxygen." The "hydrate" portion refers to hydrogen and oxygen in a ratio similar to water.

• Chemical Composition: Carbohydrates are organic molecules composed of Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$).

• General Chemical Formula: The general formula for a carbohydrate is expressed as $(CH_2O)_n$. The specific type of carbohydrate depends on the number of carbon atoms present.

• Alternative Naming: Carbohydrates are also commonly referred to as "saccharides," which forms the basis for terms like monosaccharides, disaccharides, and polysaccharides.

• Abundance: Carbohydrates are the most abundant biomolecules on Earth. For example, through photosynthesis, more than $100,000,000,000\text{ metric tons}$ of carbon dioxide ($CO_2$) and water ($H_2O$) are converted into cellulose and other plant products annually.

Biological Functions of Carbohydrates

• Energy Source: They serve as the primary fuel for cell survival. For example, Glucose is converted into $ATP$ (the universal currency for energy) via metabolic pathways including:   * Glycolysis.   * The Citric Acid Cycle ($TCA$ Cycle).   * The Electron Transport Chain ($ETC$).

• Energy Storage: Excess glucose is not wasted but converted into storage forms.   * In animals (humans), it is stored as glycogen via hormonal processes.   * In plants, it is stored as starch.

• Structural Support:   * Cellulose: Present in the cell walls of almost all plants; it is a structural carbohydrate providing rigidity.   * Chitin: Found in the cell walls of fungi and the exoskeletons of certain organisms.

• Cell Signaling and Information:   * Carbohydrates can covalently link with proteins to form glycoproteins.   * Carbohydrates can link with lipids to form glycolipids.   * These molecules act as informational units, cell recognition molecules, and signaling molecules.   * Cytokines, specifically those produced by helper $T$ cells, are often glycoproteins. These cytokines coordinate immune responses by signaling $B$ cells to produce antibodies after encountering an antigen.

Chemical Classification and Nomenclature

• Nomenclature System: The naming of monosaccharides is based on the number of carbon atoms followed by the suffix "-ose":   * $3\text{ carbons}$: Triose (Example: Glyceraldehyde).   * $4\text{ carbons}$: Tetrose.   * $5\text{ carbons}$: Pentose (Examples: Ribose, Deoxyribose).   * $6\text{ carbons}$: Hexose (Examples: Glucose, Fructose, Galactose, Mannose).

• Functional Groups:   * Aldose: A carbohydrate with a carbonyl functional group (aldehyde) at the end of the carbon chain.   * Ketose: A carbohydrate with a carbonyl functional group (ketone) at any other position (middle of the chain).

• Complexity-Based Classification:   * Monosaccharides: Single sugar units; the basic structural and functional units (analogous to amino acids in proteins).   * Disaccharides: Two monosaccharides joined by a glycosidic bond (Examples: Maltose, Lactose, Sucrose).   * Oligosaccharides: Short chains containing approximately $3$ to $15$ or $20$ monosaccharides (Examples: Raffinose, Verbenose).   * Polysaccharides: Long chains, often containing more than $15$ to $20$ monosaccharides (Examples: Starch, Glycogen, Cellulose, Chitin).

Properties and Isomerism of Monosaccharides

• Physical Properties:   * Colorless, crystalline solids at room temperature.   * Highly soluble in water due to their simple structure.   * Mostly insoluble in non-polar solvents.   * Generally possess a sweet taste.

• Structural Behavior: Because they are simple, they rarely stay in a flat linear configuration, preferring a cyclic structure to maintain conformation.

• Complexity of Stereochemistry:   * Chiral Carbons: Monosaccharides contain one or more asymmetric centers (chiral carbons).   * Isomers: Compounds with the same chemical formula but different structures (e.g., Fructose and Glucose). Fructose forms a cyclic structure typically involving $5$ carbons in the ring, whereas Glucose involves $5$ carbons in its ring plus an oxygen, but their overall carbon count is $6$.   * Epimers: Isomers that differ in configuration around only one specific carbon atom. Example: $D$-Mannose and $D$-Glucose differ only at carbon number $2$ ($C2$). $D$-Glucose and $D$-Galactose are epimers at carbon number $4$ ($C4$). Epimers possess different physical properties like water solubility and melting temperature.   * Enantiomers: Mirror-image structures that cannot be superimposed (e.g., $D$-Glucose and $L$-Glucose). In humans, most hexoses are $D$-stereoisomers (though $L$-Arabinose exists as an exception).   * Anomers: Structures that differ only in stereochemistry around the hemiacetal (anomeric) carbon.

• Biological Importance: The specific orientation/isomerism is vital for enzyme specificity. Enzymes possess specific active sites; if the substrate configuration is wrong, the enzyme cannot bind, affecting digestion and metabolism.

Cyclization and the Anomeric Carbon

• Formation: Cyclization occurs when an aldehyde group (at $C1$) or ketone group reacts with a hydroxyl group (at $C5$ for hexoses).   * Aldehyde + Alcohol $\rightarrow$ Hemiacetal linkage.   * Ketone + Alcohol $\rightarrow$ Hemiketal linkage.

• Anomeric Carbon: In a linear sugar, this is the carbon that was part of the carbonyl group. In the cyclic form, it becomes a neochiral center bonded to two oxygens.

• Anomer Types ($\alpha$ vs $\beta$):   * $\alpha$ Configuration: The hydroxyl group ($OH$) at the anomeric carbon is on the opposite side (trans) of the ring relative to the $CH_2OH$ moiety (carbon $6$).   * $\beta$ Configuration: The hydroxyl group ($OH$) is on the same side (cis) as the $CH_2OH$ moiety.

Key Monosaccharides and Their Roles

• Glucose:   * Most important metabolic fuel.   * Primary energy source for the brain and red blood cells.   * The brain represents $2\%$ of body mass but consumes over $70\%$ of available glucose.   * Precursor for fatty acids, amino acids, and nucleotides.   * Hypoglycemia (low blood glucose) leads to poor concentration, confusion, hallucinations, coma, and death.

• Fructose: Found in fruits and is a component of sucrose.

• Galactose: Found in milk.

• Pentoses (Ribose and Deoxyribose): Essential components of nucleic acids ($DNA/RNA$).

• Trioses (Glyceraldehyde): Intermediates in the glycolysis pathway.

Reducing Sugars and Glycosidic Bonds

• Reducing Sugars: Carbs that can donate electrons to mild oxidizing agents (like cupric iron).   * Requires a free anomeric carbon (free aldehyde or ketone group).   * Examples: Glucose, Fructose, Lactose, Maltose.

• Non-reducing Sugars: These cannot donate electrons because their anomeric carbons are locked in glycosidic bonds.   * Example: Sucrose.

• Glycosidic Linkage: Formed via a condensation reaction (release of water) between sugar units.   * Named based on the carbon atoms involved ($\text{1-4}$ or $\text{1-6}$ linkage) and the configuration of the anomeric carbon ($\alpha$ or $\beta$).   * If two chiral carbons are involved (specified as $\alpha, \beta\text{…}$), the sugar is non-reducing.

Disaccharides and Oligosaccharides

• Maltose: Two glucose units ($Glu + Glu$) with an $\alpha-1,4$ bond.

• Lactose: Galactose and Glucose ($Gal + Glu$) linked by a $\beta-1,4$ glycosidic bond. Found in milk. It is a reducing sugar because it has one free anomeric carbon.

• Sucrose: Glucose and Fructose ($Glu + Fru$) linked by an $\alpha-1, \beta-2$ bond. Common table sugar; non-reducing as both anomeric carbons are involved in the bond.

• Raffinose: An oligosaccharide found in peas and beans. It is indigestible by human enzymes but broken down by gut microbiota, releasing hydrogen, oxygen, methane, and hydrogen carbonate. It acts as a prebiotic.

Polysaccharides (Glycans)

• General: Large polymers of monosaccharides linked by glycosidic bonds.

• Starch: Storage polysaccharide in plants.   * Composed of $\alpha D$-glucose.   * Two forms: Amylose ($20-30\%$) and Amylopectin.   * Iodine Test: Iodine interacts with amylose to form a deep blue to black color.

• Glycogen: Storage polysaccharide in animals (liver and muscles).   * Highly branched $\alpha D$-glucose.   * Links: $\alpha-1,4$ in linear chains, $\alpha-1,6$ at branch points.

• Cellulose: Structural component of plant cell walls.   * Most abundant polysaccharide.   * Composed of $\beta D$-glucose with $\beta-1,4$ linkages.   * Indigestible by humans (lacking cellulase); broken down by fungi, bacteria, and ruminants.

• Chitin: Structural component of fungal cell walls and exoskeletons.   * Polymer of $N$-acetyl-D-glucosamine (transcript uses "enestyle D glucosamine").   * Contains an acetylated amino group at $C2$ instead of a hydroxyl group.   * Linkage: $\beta-1,4$ bond (transcript also mentions "V1 and V glycosidic bond").

• Complex Carbohydrates (Non-starch Polysaccharides): Include fiber and resistant starch. They provide sustained glucose release and influence gut health (e.g., oats rich in beta-glucan; inulin as a prebiotic).

Questions & Discussion

• Student Question on Macromolecules: "Carbohydrates, genetic material, proteins, and I think it's libids."

• Lecturer Clarification: The four are proteins, carbohydrates, lipids, and nucleic acids.

• Student Question on Chitin: "It's structural… could be cell walls?"

• Lecturer Response: Yes, specifically the cell walls of fungus and exoskeletons.

• Quiz/Exam Prep: Students should be able to define carbohydrates, list compositions/functions, classify them, define enantiomers, and name specific aldotrioses or ketotrioses.

• Logistics: Tutorial from 10:00 to 11:00 is pre-recorded and on the module site. A specific staff member (likely the module coordinator or lab manager) is available in the fourth room on the right until 14:30.