Chapter Five: The Structure and Function of Macromolecules
Polymer Principles
Most Macromolecules are Polymers, such as Carbohydrates, Lipids, Proteins, and Nucleic Acids. These are long molecules of similar building blocks connected by covalent bonds. The building blocks are known as Monomers, which are small molecules. The connection is achieved through condensation (dehydration) reactions, where water is lost. On the other hand, hydrolysis reverses dehydration, breaking polymers into monomers.
An Immense Variety of Polymers Can Be Built from a Small Set of Monomers, as 40 to 50 common monomers create diverse polymers. The linear sequence variations result in unique macromolecules.
Carbohydrates: Fuel and Building Material
Sugars, the smallest carbohydrates, serve as fuel and carbon sources. Monosaccharides such as glucose have CH20 molecular formulas. They can be classified as aldoses or ketoses based on the carbonyl group location and by size as trioses, pentoses, or hexoses. In aqueous solutions, monosaccharides form rings. Disaccharides such as maltose and sucrose are formed by glycosidic linkage, while polysaccharides like starch, glycogen, and cellulose serve as storage and structure.
Lipids: Diverse Hydrophobic Molecules
Fats store large amounts of energy and are formed by glycerol and fatty acids, forming triacylglycerol. They can be saturated fats (animal fats) or unsaturated fats (oils). Their hydrophobic nature is due to nonpolar carbon-hydrogen bonds. Phospholipids in cell membranes self-assemble into micelles, and steroids such as cholesterol have a four-fused-ring structure.
Four Levels of Protein Structure
Proteins have multiple structures and functions. A polypeptide is a polymer of amino acids connected in a specific sequence, and peptide bonds are formed by dehydration between adjacent amino acids. The secondary structure consists of coiled or folded segments due to hydrogen bonds, with a helix and pleated sheet being common structures. The tertiary structure has irregular contortions due to side chain interactions like hydrophobic interactions, van der Waals forces, disulfide bridges, and ionic bonds. Finally, the quaternary structure involves the arrangement of multiple polypeptide chains into fibrous (e.g., collagen) or globular (e.g., hemoglobin) proteins.
What Determines Protein Conformation?
Protein conformation is determined by the pH, salt concentration, and temperature, and any structural disruption leading to inactivity is known as denaturation. Proper protein folding is assisted by chaperonins.
Nucleic Acids: Informational Polymers
Nucleic acids, such as DNA and RNA, store and transmit hereditary information. A nucleic acid strand is a polymer of nucleotides, which consist of a nitrogenous base, pentose, and phosphate. DNA and RNA differ in their pentose composition with deoxyribose in DNA and ribose in RNA. Replication of the DNA double helix is the basis for inheritance, as well as the reflective relationship between DNA, RNA, and proteins.
