Biological Polymers and Carbohydrate Chemistry: Monomers, Linkages, and Macromolecules

Condensation Reactions and Hydrolysis

• Biological polymers are constructed from individual monomer subunits through a chemical process known as a condensation reaction, which is also referred to as dehydration synthesis.
• Condensation reactions form chemical covalent bonds between individual monomers.
• During a condensation reaction, a hydrogen atom ($H$) is removed from one monomer and a hydroxyl group ($-OH$) is removed from an adjacent monomer, resulting in the formation of a water molecule ($H_2O$).
• The shortened reactive ends of the two monomers subsequently join together via a shared covalent bond.
• The term "condensation" fundamentally denotes the formation of a liquid, making it an appropriate name since water ($H_2O$) is produced during bond formation.
• Hydrolysis is the exact chemical reverse of a condensation reaction and functions to break the covalent bonds holding polymers together.
• The term "hydrolysis" literally translates to "water splitting."
• To cleave a polymer into individual monomer subunits during hydrolysis, a water molecule ($H_2O$) is added across the covalent bond, breaking the bond and restoring the hydrogen atom and hydroxyl group to their respective monomers.

General Composition, Structure, and Functions of Carbohydrates

• "Sugar" is a broad term applied to carbohydrates, though not all carbohydrate molecules exhibit a sweet taste.
• A microscopic image of a tissue cross-section is known as a micrograph.
• Structural distribution of stained carbohydrates in a plant root cross-section micrograph:
  • Cellulose stains green and forms the structural framework of plant cell walls.
  • Starch stains purple and is localized intracellularly within plant cells.
• Most carbohydrate molecules share a uniform chemical composition consisting of carbon ($C$), hydrogen ($H$), and oxygen ($O$) atoms.
• The stoichiometric ratio of atoms in most carbohydrates is two hydrogen atoms for every single carbon and oxygen atom, represented by the general chemical arrangement where subscript $n$ indicates the number of carbon atoms.
  • For example, setting $n = 5$ produces a pentose sugar molecule containing five carbon atoms, ten hydrogen atoms, and five oxygen atoms ($C_5H_{10}O_5$).
• As seen in most organic macromolecules, the structural backbone of carbohydrates is composed of a skeleton of carbon atoms.
• Attached to the carbon backbone are polar hydroxyl functional groups ($-OH$), which interact with water and allow most simple carbohydrates to readily dissolve in aqueous environments.
• Carbohydrates can exist as single isolated monomer units or as complex polymers composed of many monosaccharide units bound together.
• Primary biological functions of carbohydrates in living organisms:
  • Structural Support: Structural carbohydrates such as cellulose and chitin provide physical strength, rigidity, and stability to plant cell walls, fungal cell walls, and animal exoskeletons.
  • Cellular Identification: When carbohydrates are covalently bound to proteins, they form glycoproteins, which act as surface cellular identification markers for cell-cell recognition.
  • Energy Storage: Carbohydrate polymers like starch and individual monosaccharides like glucose store chemical energy that organisms can metabolize to power biological processes.

Functional Groups and Classification of Monosaccharides

• In addition to hydroxyl groups ($-OH$), carbohydrates contain carbonyl functional groups ($C=O$).
• Monosaccharides are classified into two specific structural categories based on the position of the carbonyl group along the carbon skeleton:
  • Aldose: A monosaccharide where the carbonyl oxygen atom is double-bonded to a carbon atom at the terminal end of the carbon backbone.
  • Ketose: A monosaccharide where the carbonyl oxygen atom is double-bonded to an internal carbon atom in the middle of the carbon backbone.
• Monosaccharides (the monomer building blocks of carbohydrates) typically contain three, five, or six carbon atoms:
  • Triose ($3\text{-carbon sugar}$):
  • Example: Glyceraldehyde ($C_3H_6O_3$), a three-carbon sugar produced during photosynthesis that serves as an intermediate precursor to synthesize more complex monosaccharides.
  • Pentose ($5\text{-carbon sugar}$):
  • Example: Deoxyribose ($C_5H_{10}O_4$), a five-carbon sugar that forms part of the nucleic acid backbone in DNA.
  • Hexose ($6\text{-carbon sugar}$):
  • Example: Glucose ($C_6H_{12}O_6$), the foundational hexose monosaccharide utilized by most living organisms for energy, alongside various other hexoses used for specialized biological purposes.

Isomerism, Energetics, and Structural Forms of Glucose

• Hexose sugars share the molecular formula containing 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms ($C_6H_{12}O_6$).
• Hexoses exhibit varied spatial arrangements of their atoms. Molecules that possess the exact same molecular formula but different structural configurations are defined as isomers.
• Structural comparison of glucose and galactose:
  • Both glucose and galactose possess the molecular formula $C_6H_{12}O_6$
  • Glucose has a hydroxyl group ($-OH$) attached to its fourth carbon ($C_4$) in the exact same spatial orientation as the hydroxyl group on its second carbon ($C_2$).
  • Galactose shares the formula $C_6H_{12}O_6$, but the hydroxyl group ($-OH$) attached to its fourth carbon ($C_4$) is oriented in a different spatial position.
  • This structural positional variation of the hydroxyl group provides glucose and galactose with distinct biological and physical properties.
• Chemical energetics of carbohydrate bonds:
  • Carbohydrates sustain life as the primary chemical products of photosynthesis, capturing solar energy and storing it in chemical bonds.
  • Carbohydrates are optimal energy-storage molecules because of their high proportion of shared electron pairs within nonpolar carbon-carbon ($C-C$) and carbon-hydrogen ($C-H$) bonds.
  • Carbon and hydrogen atoms possess similar electronegativities, meaning electrons are shared equally and are not held tightly by atomic nuclei.
  • When these chemical bonds are broken down enzymatically, a high amount of free energy is made available to power cellular endergonic reactions.
• Ring formation of glucose in aqueous solutions:
  • In biological aqueous solutions (such as cellular cytoplasm or extracellular fluid), glucose does not exist primarily in an open linear chain.
  • Carbons in the linear glucose molecule are numbered sequentially from the terminal carbon at one end ($C_1$) to the last carbon at the end of the backbone ($C_6$).
  • In aqueous solution, the hydroxyl group attached to the fifth carbon ($C_5$) interacts chemically with the carbonyl group at carbon 1 ($C_1$), causing the molecule to close into a ring structure.
  • The formation of this cyclic bond displaces the hydrogen atom from the $C_5$ hydroxyl group.
  • Concurrently, a hydrogen atom bonds with the carbonyl oxygen at $C_1$, converting that oxygen into a new hydroxyl group ($-OH$) attached to carbon 1.
• Structural isomers of glucose rings ($ ext{Alpha}$ vs. $ ext{Beta}$):
  • The spatial position of the newly formed hydroxyl group on carbon 1 ($C_1$) dictates whether the ring is alpha or beta glucose:
  • Alpha-Glucose ($ imes ext{-glucose}$ / $\alpha\text{-glucose}$): The hydroxyl group ($-OH$) attached to carbon 1 ($C_1$) points downwards relative to the plane of the sugar ring.
  • Beta-Glucose ($\beta\text{-glucose}$): The hydroxyl group ($-OH$) attached to carbon 1 ($C_1$) points upwards relative to the plane of the sugar ring.
  • Alpha-glucose and beta-glucose are structural isomers because they possess identical molecular formulas ($C_6H_{12}O_6$) but distinct three-dimensional structural arrangements.

Disaccharides and Glycosidic Linkages

• Monosaccharides join together through condensation reactions to build carbohydrate polymers.
• The specific covalent bond formed between two monosaccharides during a condensation reaction is called a glycosidic linkage.
• A disaccharide is a carbohydrate molecule composed of two monosaccharide units joined by a glycosidic linkage.
• Common disaccharides and their monomer compositions:
  • Sucrose: Composed of one glucose monosaccharide and one fructose monosaccharide.
  • Lactose: The main sugar found in milk, composed of one glucose monosaccharide and one galactose monosaccharide.
  • Maltose: A sugar derived from germinated barley used in fermentation to produce alcoholic beverages, composed of two glucose monosaccharide units.
• Synthesis and geometry of an $\alpha\text{-}1,4\text{-glycosidic linkage}$ in Maltose:
  • Formed from two $\alpha\text{-glucose}$ monomers, where hydroxyl groups ($-OH$) on carbon 1 ($C_1$) are in the down position and carbon 6 ($C_6$) groups are in the up position.
  • A condensation reaction extracts water ($H_2O$) from the hydroxyl group on carbon 1 ($C_1$) of the first glucose and the hydroxyl group on carbon 4 ($C_4$) of the second glucose.
  • The resulting covalent bond connects carbon 1 of the first monomer to carbon 4 of the second monomer, creating an $\alpha\text{-}1,4\text{-glycosidic linkage}$.
  • Following bond formation in maltose, the sixth carbons ($C_6$) of both glucose monomers remain oriented in the up position.
• Synthesis and geometry of a $\beta\text{-}1,4\text{-glycosidic linkage}$ in Lactose:
  • Formed by joining $\beta\text{-galactose}$ and $\beta\text{-glucose}$.
  • In $\beta\text{-glucose}$, the hydroxyl group ($-OH$) on carbon 1 ($C_1$) is oriented in the up position.
  • $\beta\text{-galactose}$ and $\beta\text{-glucose}$ are structural isomers with hydroxyl groups on carbon 4 ($C_4$) in differing spatial positions.
  • A condensation reaction removes water ($H_2O$) to yield a $\beta\text{-}1,4\text{-glycosidic linkage}$.
  • To accommodate the spatial requirements of the $\beta\text{-}1,4$ bond, the glucose molecule flips upside down relative to the galactose molecule.
  • As a consequence of this flip, the carbon 6 ($C_6$) groups of the two constituent monosaccharides end up in opposite spatial orientations (one pointing up, one pointing down) in the final lactose disaccharide.

Structural Characteristics and Roles of Polysaccharides

• Polysaccharides are long, macromolecular carbohydrate chains composed of hundreds to thousands of monosaccharides bound by glycosidic linkages.
• Major biological polysaccharides include starch, glycogen, cellulose, chitin, and peptidoglycan.

Starch

• Function: Storage polysaccharide synthesized by plants to store excess glucose for future metabolic energy needs.
• Cellular Storage: Stored inside specialized plant organelle structures called amyloplasts (e.g., within potato tuber tissue).
• Structural Composition: Consists of two molecular structural forms:
  • Amylose: An unbranched, helical polymer composed exclusively of glucose monomers joined by $\alpha\text{-}1,4\text{-glycosidic linkages}$.
  • Amylopectin: A branched polymer composed of glucose monomers joined by $\alpha\text{-}1,4\text{-glycosidic linkages}$ along main chains, with branch points formed by $\alpha\text{-}1,6\text{-glycosidic linkages}$.

Glycogen

• Function: Storage polysaccharide synthesized by animals to store glucose for future metabolic energy needs.
• Cellular Storage: Stored as insoluble granules within animal tissues, primarily in liver and skeletal muscle cells.
• Structural Composition: Composed of glucose monomers connected by $\alpha\text{-}1,4\text{-glycosidic linkages}$. It possesses a branched structure similar to amylopectin, but contains a significantly higher frequency of $\alpha\text{-}1,6\text{-glycosidic linkages}$, making glycogen more densely branched.

Cellulose

• Function: Major structural polysaccharide utilized by plants to construct rigid, tough cell walls surrounding plant cells.
• Structural Composition: Composed of glucose monomers connected exclusively by $\beta\text{-}1,4\text{-glycosidic linkages}$, forming long, straight, unbranched linear chains.
• Intermolecular Hydrogen Bonding: Multiple linear cellulose chains lie parallel to one another and form extensive cross-linking hydrogen bonds, providing exceptional mechanical strength, stability, and durability.
• Digestibility and Physiological Significance:
  • Most organisms cannot digest cellulose because they lack the specific enzymes required to hydrolyze $\beta\text{-}1,4\text{-glycosidic linkages}$, making the glucose energy locked inside cellulose unavailable to them.
  • Only specific bacterial species possess enzymes capable of breaking down $\beta\text{-}1,4\text{-glycosidic linkages}$.
  • Herbivorous organisms that consume large quantities of plant matter (such as ruminants and termites) maintain cellulose-digesting symbiotic bacteria within their digestive tracts.
  • In humans, indigestible plant cellulose acts as dietary fiber, which remains unabsorbed and aids proper digestive tract activity and passage.

Chitin

• Function: Structural polysaccharide used by fungi to construct cell walls and by arthropods (such as insects) to construct exoskeletons.
• Monomer Composition: Built from monomers of $N\text{-acetylglucosamine}$, a sugar monomer similar in structure to glucose that includes a nitrogen-containing functional group.
• Structural Composition: Monomers are connected by $\beta\text{-}1,4\text{-glycosidic linkages}$.
• Intermolecular Hydrogen Bonding: Extensive hydrogen bonds form between adjacent $N\text{-acetylglucosamine}$ chains, enhancing the overall physical toughness, structural stability, and durability of chitin.

Peptidoglycan

• Function: Structural polysaccharide utilized by bacteria to construct rigid cell walls.
• Monomer Composition: Composed of alternating units of $N\text{-acetylglucosamine}$ and a second specialized monosaccharide derivative.
• Structural Composition: Monosaccharide units within each chain are linked by $\beta\text{-}1,4\text{-glycosidic linkages}$.
• Cross-Linking Scaffold: Parallel peptidoglycan polysaccharide chains are covalently cross-linked to one another by short peptide strands composed of amino acids, forming a strong, continuous structural network surrounding the bacterial cell.