Comprehensive Study Notes on Carbohydrates: Structure, Function, and Classification

Definition and Chemical Composition of Carbohydrates

Carbohydrates are essential biomolecules consisting of carbon ($C$), hydrogen ($H$), and oxygen ($O$) atoms. Conventionally, they are defined as polyhydroxyaldehydes or polyhydroxyketones. The ratio of hydrogen to oxygen atoms in these molecules is typically $2:1$, mirrored in the chemical structure of water ($H_2O$). This proportionality leads to the general empirical formula $C_n(H_2O)_n$ or $C_m(H_2O)_n$, where the number of carbon atoms ($m$) may differ from the number of water units ($n$).

Despite this standard stoichiometric definition, there are notable exceptions. For instance, uronic acids and deoxy sugars such as fucose do not adhere strictly to the $2:1$ ratio. Conversely, certain chemical compounds satisfy the empirical formula but are not classified as carbohydrates; primary examples include formaldehyde ($CH_2O$) and acetic acid ($C_2H_4O_2$). At their core, carbohydrates are polymers constructed from monomeric units known as monosaccharides, which include common sugars like glucose, fructose, and galactose.

Classification and Nomenclature

Carbohydrates are categorized into three primary classes based on the number of monomeric units they contain: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are simple sugars that cannot be hydrolyzed into smaller units. They are further classified by their functional group into aldoses (containing an aldehyde group, such as glucose) or ketoses (containing a ketone group, such as fructose), and by the number of carbon atoms they contain: trioses ($3$ carbons), tetroses ($4$ carbons), pentoses ($5$ carbons), hexoses ($6$ carbons), and heptoses ($7$ carbons).

Oligosaccharides consist of short chains of monosaccharide units, typically ranging from $2$ to $20$ monomers. This group includes disaccharides (two units), trisaccharides (three units), and tetrasaccharides (four units). In cellular environments, oligosaccharides with three or more units are rarely found in a free state; instead, they usually exist as components of glycoconjugates such as glycoproteins and glycolipids. Polysaccharides comprise more than $20$ monosaccharide units, with some structures reaching hundreds or even thousands of monomers. These are divided into homopolysaccharides, which contain only one type of monosaccharide, and heteropolysaccharides, which contain two or more different types.

Biological Functions of Carbohydrates

Carbohydrates serve several critical roles within biological systems. They act as a primary source and storage form of energy; for example, starch provides energy storage in plants, while glycogen serves this function in animals. Structurally, they contribute to the integrity of organisms; cellulose is a vital component of plant cell walls, and various heteropolysaccharides form the extracellular matrix. Additionally, carbohydrates provide protective functions and are essential for cell-to-cell recognition and adhesion, often acting as informational molecules on the cell surface.

Stereoisomerism in Monosaccharides

Monosaccharides exhibit enantiomerism, a form of stereoisomerism where molecules are non-superimposable mirror images of each other. The number of possible stereoisomers for a monosaccharide is calculated using the formula $2^n$, where $n$ represents the number of asymmetric carbon atoms. In Fischer projection formulas, horizontal bonds represent atoms pointing toward the viewer, while vertical bonds project away. The designation of a sugar as $D$ or $L$ is determined by the configuration of the asymmetric carbon atom furthest from the carbonyl group. In nature, $D$-forms are the most commonly occurring.

Epimerism is another specific type of isomerism where two monosaccharides differ in configuration around only one specific carbon atom. Examples include $D$-Glucose and $D$-Mannose, which are epimers at the $C-2$ position, and $D$-Glucose and $D$-Galactose, which are epimers at the $C-4$ position. In the ketohexose series, $D$-Ribulose and $D$-Xylulose are also considered epimers.

Specific Monosaccharide Examples

Trioses include glyceraldehyde ($C_3H_6O_3$, an aldotriose) and dihydroxyacetone (a ketotriose). Tetroses, such as erythrose and threose, contain two anomeric centers, leading to $2^2 = 4$ possible diastereomers. Pentoses are crucial for genetic material and metabolism, featuring molecules like $D$-Ribose, $D$-Ribulose, $D$-Xylulose, and $2\text{-deoxy-D-ribose}$. Hexoses are the most abundant monosaccharides, including glucose, mannose, galactose, and the ketohexose fructose. Heptoses, though less common, include compounds like $D$-Sedoheptulose.

Cyclization and Ring Formation

Monosaccharides with four or more carbon atoms (pentoses, hexoses, heptoses) typically exist in cyclic structures. This formation results from an intramolecular reaction between a hydroxyl group and the carbonyl (aldehyde or ketone) group, creating cyclic hemiacetals or hemiketals. These rings can be six-membered, known as pyranoses (named after pyran), or five-membered, known as furanoses (named after furan). For instance, ribose can form both $\beta\text{-D-ribopyranose}$ and $\beta\text{-D-ribofuranose}$.

The conversion from a Fischer projection to a Haworth projection involves specific spatial rules. To convert $D$-glucose to its cyclic \text{\alpha-pyranose} form, one must rotate the Fischer projection $90^{\circ}$ and fold it into a hexagon. For carbons $C-2$, $C-3$, and $C-4$, groups on the right side of the Fischer projection are placed down in the Haworth projection, while those on the left are placed up. For $D$-sugars, the $CH_2OH$ group at $C-5$ is always placed up. The anomeric carbon position determines the \text{\alpha} or \text{\beta} isomer: in $D$-sugars, the \text{\alpha}-anomer has the hydroxyl group pointing down, while the \text{\beta}-anomer has it pointing up.

Anomerism and Reducing Sugars

Anomers are isomers that differ only in their configuration at the hemiacetal or hemiketal carbon, which is called the anomeric carbon. This carbon is highly reactive and defines the chemical properties of the sugar. A reducing sugar is one that can reduce another compound while being oxidized itself; specifically, its carbonyl carbon is oxidized to a carboxyl group. This occurs only if the sugar possesses a free hemiacetal group or can exist in an open-chain form with an aldehyde group. All monosaccharides are reducing sugars, as are disaccharides of the maltose type (such as lactose and maltose). Non-reducing sugars, like sucrose, have their anomeric carbons locked in a glycosidic bond and cannot be oxidized by mild reagents like $Cu^{2+}$ ions.

Disaccharides and Glycosidic Bonds

Disaccharides are formed when two monosaccharides are linked by a glycosidic bond, a process involving condensation (the release of a water molecule). These bonds can be $O$-glycosidic (linked via oxygen) or $N$-glycosidic (linked via nitrogen). Disaccharides are classified into two types:

1. Maltose type: These possess a free glycosidic (anomeric) group and exhibit reducing properties. Examples include maltose, lactose (formed by \text{eta-D-galactopyranosyl-}(1\rightarrow 4)\text{-eta-D-glucopyranose}, or $Gal(\beta 1\rightarrow 4)Glc$), and cellobiose.
2. Trehalose type: These lack a free glycosidic group because the bond forms between the anomeric carbons of both sugars, resulting in no reducing properties. Examples include sucrose and trehalose. Sucrose is specifically described as \text{eta-D-fructofuranosyl \alpha-D-glucopyranoside}, symbolized as Fru(2eta \rightleftharpoons \text{\alpha 1})Glc or Glc(\text{\alpha 1} \rightleftharpoons 2eta)Fru.

Homopolysaccharides: Storage and Structure

Homopolysaccharides consist of a single type of monomeric unit and serve as either structural components or energy stores.

Cellulose is a linear, unbranched structural polysaccharide found in plants, consisting of $10,000$ to $15,000$ units of $\beta\text{-D-glucose}$ linked by $(\beta 1\rightarrow 4)$ $O$-glycosidic bonds. Cellobiose is its repeating disaccharide unit.

Starch is the primary energy reservoir in plants, composed of two forms: Amylose and Amylopectin. Amylose ($20\%$ of starch) consists of long, unbranched chains of \text{\alpha-D-glucose} units connected by (\text{\alpha 1} \rightarrow 4) bonds, ranging from $500$ to $20,000$ units. Amylopectin ($80\%$ of starch) is highly branched, with branch points occurring every $24$ to $30$ residues. The straight chains use (\text{\alpha 1} \rightarrow 4) bonds, while branches use (\text{\alpha 1} \rightarrow 6) bonds. Amylopectin can contain between $2,000$ and $2,000,000$ glucose residues.

Glycogen is the energy storage polysaccharide in animals, found mainly in liver and muscle cells. It is similar to amylopectin but more extensively branched (every $8$ to $12$ residues), which grants it compactness and a high number of non-reducing ends for rapid glucose mobilization. A single glycogen molecule can contain up to $120,000$ monomers.

Heteropolysaccharides and Glycoconjugates

Glycosaminoglycans (GAGs) are linear heteropolysaccharides made of repeating disaccharide units. Typically, one monomer is an $N$-acetylglucosamine or $N$-acetylgalactosamine, while the other is a uronic acid (such as $D$-glucuronic or $L$-iduronic acid). Many GAGs are sulfated, making them highly hydrophilic. Notable types include Hyaluronic acid, Chondroitin sulfate, Heparan sulfate, and Keratan sulfate.

Proteoglycans are macromolecules where one or more sulfated GAG chains are covalently bound to a core protein. They are major components of the extracellular matrix (ECM). In cartilage, proteoglycan aggregates form when proteoglycans bind to hyaluronic acid. These structures are crucial for mechanical support and hydration due to their carboxyl and sulfate groups. Heparan sulfate (HS) is found in all animal tissues and regulates processes like angiogenesis, blood clotting, and tumor metastasis. Recent studies have highlighted the role of HS as a co-receptor for viruses, including SARS-CoV-2, which interacts with the ACE2 receptor.

Glycoproteins and Glycosphingolipids are other forms of glycoconjugates. Glycoproteins feature oligosaccharides covalently linked to proteins and are found on the outer plasma membrane (as part of the glycocalyx), in the ECM, and in blood. Glycosphingolipids are membrane components where the hydrophilic head groups are oligosaccharides. These carbohydrate portions are highly informative, acting as specific recognition sites for proteins.