Comprehensive Study Notes on Carbohydrate Biochemistry: Structures, Reactions, and Biological functions

Carbohydrates: Fundamental Definitions and Nomenclature

• Conceptual Origin of the Name: The term "carbohydrate" or "hydrate of carbon" stems from the early observation of their general chemical formula. Historically, it was believed these molecules were literally carbon atoms bound to water molecules. While they are not actually hydrated in this manner, the name persists.
• General Chemical Formula: Initial naming was based on the formula $C_n(H_2O)_n$. In modern terms, the basic building block of a carbohydrate is the monosaccharide.
• Structural Categories by Size:
  • Monosaccharide: The basic building block or simple sugar. Its common general formula is $C(H_2O)_n$.
  • Oligosaccharide: Molecules consisting of 2 to 10 simple sugar residues (monosaccharides) connected to each other.
  • Polysaccharide: Carbohydrate structures containing more than 10 monosaccharide units.

Classification of Monosaccharides: Functional Groups and Chain Length

• Nomenclature Conventions: Monosaccharide names typically end with the suffix "-ose" (e.g., glucose, fructose, pentose).
• Classification by Functional Group:
  • Aldose: A monosaccharide containing an aldehyde functional group ($CHO$).
  • Ketose: A monosaccharide containing a ketone functional group ($C=O$).
• Classification by Carbon Count ($n$):
  • Triose: A 3-carbon sugar ($C_3$). Examples include aldophosphoglycerate or ketotriose.
  • Tetrose: A 4-carbon sugar ($C_4$).
  • Pentose: A 5-carbon sugar ($C_5$). Example: Ribose ($C_5H_{10}O_5$), which is an aldopentose.
  • Hexose: A 6-carbon sugar ($C_6$). Example: Glucose, which is an aldohexose.
  • Heptose: A 7-carbon sugar ($C_7$).
• Hybrid Classification: Specific sugars are named by combining functional group and carbon count, such as an "aldopentose" (5-carbon sugar with an aldehyde) or an "aldohexose" (6-carbon sugar with an aldehyde).

Stereochemistry and Isomerism in Sugars

• Enantiomers: Non-superimposable mirror images of each other. Sugars containing a stereocenter (a carbon atom connected to four different groups) exhibit this property.
• Smallest Chiral Sugar: Glyceraldehyde is an aldotriose and the simplest sugar capable of having a stereocenter. It exists as two enantiomers: $L$-glyceraldehyde and $D$-glyceraldehyde.
• $D$ and $L$ Designation (Fischer Projections):
  • To determine the $D$ or $L$ form, the molecule is arranged vertically with the aldehyde group ($CHO$) at the top and the $CH_2OH$ group at the bottom.
  • $L$-sugar: The hydroxyl ($OH$) group on the chiral carbon furthest from the aldehyde/ketone group is on the left.
  • $D$-sugar: The hydroxyl ($OH$) group on the chiral carbon furthest from the aldehyde/ketone group is on the right.
  • In a larger chain, the number is assigned prioritizing the functional group (aldehyde/ketone), and the stereocenter with the highest number determines the $D$ or $L$ configuration.
• Diastereomers: Stereoisomers that are not mirror images of one another.
• Epimers: A specific type of diastereomer where only one single stereocenter differs between the two molecules. Example: $D$-allose and $D$-glucose are epimers.

Intramolecular Reactions and Cyclization

• Aldehyde and Ketone Reactivity:
  • Hemiacetal Formation: Formed when an aldehyde reacts with an alcohol (nucleophilic attack by the alcohol on the electrophilic carbonyl carbon).
  • Acetal Formation: Formed when a hemiacetal reacts with a second alcohol molecule.
  • Hemiketal/Ketal: The analogous products formed when a ketone reacts with alcohols.
• Cyclization of Sugars: Because sugars contain both an aldehyde/ketone group and multiple hydroxyl ($OH$) groups, they can undergo intramolecular hemiacetal/hemiketal reactions to form ring structures.
• Ring Sizes:
  • Pyranose: A six-membered ring containing five carbons and one oxygen. This structure is named after the molecule pyran. Example: glucopyranose.
  • Furanose: A five-membered ring containing four carbons and one oxygen. This structure is named after furan. Example: ribofuranose or fructofuranose.
• The Anomeric Carbon: The carbon that was the carbonyl carbon (aldehyde or ketone) in the acyclic form becomes a new stereocenter upon cyclization. This carbon is known as the anomeric carbon and is the only carbon in the ring connected to two oxygen atoms.

Haworth Projections and Anomers

• Anomers ($\\alpha$ vs $\beta$): Upon cyclization, the hydroxyl group at the anomeric carbon can orient in two directions, creating two diastereomers called anomers:
  • $\\alpha$-anomer: The $OH$ group is pointing down (in a six-membered ring).
  • $\\beta$-anomer: The $OH$ group is pointing up (in a six-membered ring).
• Rules for Drawing Haworth Projections:
  1. Draw the heterocyclic ring skeleton (hexagon for pyranose) with the oxygen at the top right corner.
  2. For $D$-sugars, the $CH_2OH$ group is drawn on the top (above the ring).
  3. Groups on the left side of a Fischer projection point up in the Haworth projection.
  4. Groups on the right side of a Fischer projection point down in the Haworth projection.

Disaccharides and the Glycosidic Bond

• Formation: Disaccharides form when the anomeric hydroxyl group of one sugar reacts with the alcohol group of another sugar to form an acetal linkage, known as a glycosidic bond.
• Specific Examples and Connectivity:
  • Lactose: Composed of Galactose and Glucose. Connectivity is specified as Galactose $\beta-(1 \\rightarrow 4)$ Glucose (the $\beta$-anomeric carbon 1 of galactose binds to carbon 4 of glucose).
  • Maltose: Composed of two Glucose units. Connectivity is $\alpha-(1 \\rightarrow 4)$.
  • Sucrose: Composed of Glucose and Fructose. Connectivity is $\alpha-(1 \\rightarrow 2)$. Here, both anomeric carbons are involved in the bond ($C1$ of glucose and $C2$ of fructose).

Polysaccharides: Starch, Glycogen, and Cellulose

• Starch (Plant Storage): A homopolysaccharide made of glucose units. It consists of two forms:
  • Amylose: A linear, unbranched polymer of glucose connected by $\alpha-(1 \\rightarrow 4)$ linkages. It adopts a helical structure.
  • Amylopectin: A branched polymer. It features $\alpha-(1 \\rightarrow 4)$ linear linkages with $\alpha-(1 \\rightarrow 6)$ branching points every 12 to 30 glucose residues.
• Iodine Test: Amylose's helical structure allows iodine ($I_2$) to bind inside the helix, producing a characteristic blue-purple color. This is used in crime scene investigations (analyzing stomach contents) or kitchen chemistry (testing boiled potatoes).
• Glycogen (Animal Storage): Stored in the liver. It is highly branched, with $\alpha-(1 \\rightarrow 6)$ branches every 8 to 12 residues. Its structure is more globular and gives a red-violet color with iodine.
• Cellulose (Plant Structural): A linear polymer of glucose units connected by $\beta-(1 \\rightarrow 4)$ linkages.
  • Structural Difference: Unlike the helical alpha-linked starch, beta-linkages result in elongated, flat strands.
  • Hydrogen Bonding: Extensive intermolecular hydrogen bonding between strands forms sheets, and these sheets stack to provide high structural integrity for plant cell walls.
  • Digestibility: Humans lack the enzymes to hydrolyze $\beta-(1 \\rightarrow 4)$ linkages, making cellulose undigestible "fiber" for us, though herbivores (like cows) can process it.
• Bacterial Cell Walls: Contain peptidoglycan, which are sugar chains (typically NAM and NAG) cross-linked by amino acids like L-Alanine and D-Glutamate.

Reducing Sugars and Chemical Tests

• Definition: A reducing sugar is any sugar that has a free aldehyde group or a hemiacetal group (which can revert to an open-chain aldehyde form). Aldehydes act as reducing agents because they can be oxidized into carboxylic acids.
• Reducing Ends: In disaccharides like maltose or lactose, one part of the molecule may be an acetal (non-reducing), but the other end may still be a hemiacetal (reducing end).
• Non-Reducing Sugars: Sucrose is a non-reducing sugar because the glycosidic bond involves both anomeric carbons, locking the molecule in the acetal/ketal form and preventing it from opening into an aldehyde or ketone.
• Benedict's Solution Test:
  • Contains copper ($Cu^{2+}$).
  • When heated with a reducing sugar, $Cu^{2+}$ (blue) is reduced to $Cu^+$ (forming a brick-red precipitate of $Cu_2O$).
  • Historical Application: Before modern test strips, this was a common at-home diagnostic test for diabetes. Urine was mixed with Benedict's solution and heated; a red or orange color indicated high glucose content.

Questions & Discussion

• Student Question: "So what does it name? Does it name, like, some of them familiar? So carbohydrates hydrates. What is that?"
• Instructor Response: The instructor explains that "hydrate" refers to water and that "carbohydrate" literally means "hydrate of carbon." While not chemically accurate in the sense of water molecules being stuck to carbon, the name was kept because the general formula $C_n(H_2O)_n$ matched the empirical observation at the time.
• Student Question: "When we talk about epimers and diastereomers, is that where epimers only has, like, one change with the stereocenter and diastereomers have one or more than one?"
• Instructor Response: Yes, epimers are a specific case where only one center is different. Diastereomers can have one or more centers different, provided they are not mirror images. In enantiomers, every single center must be inverted/opposite.
• Discussion on High Fructose Corn Syrup: The instructor notes that fructose is processed in the liver, and excessive intake (such as through high fructose corn syrup) can lead to health issues like elevated triglycerides and fatty liver disease.
• Instructional Anecdote: The instructor recounts performing Benedict's tests for his grandfather, who had long-term diabetes, to determine if his diet was working or if he could eat specific foods like bread or rice based on the urine glucose levels.