Lecture 4: Macromolecules

Lecture 4: Macromolecules

• Macromolecules are large molecules composed of covalently connected atoms, numbering in the thousands or more
• Each cell contains many thousands of different macromolecules
• Macromolecule variation exists among cells within an organism, and even more between species
• All living organisms are composed of four classes of macromolecules: carbohydrates, proteins, nucleic acids, and lipids
• Which of the four macromolecules is different from the others?

Polymers Built from Monomers

• A polymer is a large molecule consisting of many similar building blocks, called monomers
• When part of a polymer, the subunits are often referred to differently (e.g., amino acids in polypeptides)
• A wide variety of polymers can be built from a small set of monomers
• A dehydration reaction occurs when two monomers bond together via the loss of a water molecule
• Polymers break down into monomers through hydrolysis, a reaction that is the opposite of dehydration 

Carbohydrates

• Carbohydrates serve as fuel and building materials
• Carbohydrates include simple sugars and polymers of sugars
• Monosaccharides are the simplest carbohydrates (single sugars)
• Two linked sugar molecules are called disaccharides
• Short chains of sugars are called oligosaccharides
• The most complex carbohydrate macromolecules are polysaccharides (polymers composed of many sugar building blocks)

Monosaccharides

• Monosaccharides have molecular formulas that are usually multiples of CH₂O
• Monosaccharides are classified by the location of the carbonyl group (aldose or ketose) and the number of carbons in the carbon skeleton
• Monosaccharides serve as fuel for cells and raw material 

Carbohydrates: Monosaccharides

• Monosaccharides exist as linear skeletons, but rings dominate in aqueous solutions
• Spontaneous isomerizations occur, and equilibrium favors the ring structure

Carbohydrates: Disaccharides

• A disaccharide is formed when a dehydration reaction joins two monosaccharides
• This covalent bond is called a glycosidic linkage

Carbohydrates: Oligosaccharides

• Oligosaccharides are short polymers of sugars with diverse roles (building blocks for more complex carbohydrates and signaling molecules, involved in cell-cell recognition)

Carbohydrates: Polysaccharides

• Polysaccharides, long polymers of sugars, have storage and structural roles
• The structure and function of a polysaccharide depend on its sugar monomers and the positions of glycosidic linkages.
• Starch: a storage polysaccharide in plants (composed of glucose monomers linked in a way called 1-4 linkage)
• Cellulose: a structural polysaccharide in plants (composed of glucose monomers; different linkage arrangement from starch, leading to a different structure and function)
• Chitin is a structural polysaccharide in arthropods and fungi (different from cellulose)

Carbohydrates: Storage Polysaccharides

• Starch, a storage polysaccharide of plants, consists entirely of glucose monomers
• Plants store surplus starch as granules within chloroplasts and other plastids.
• The simplest form of starch is amylose, whereas a branched form of plant starch is amylopectin
• Glycogen is the storage polysaccharide in animals
• Humans store glycogen mainly in liver and muscle cells

Carbohydrates: Structural Polysaccharides

• Cellulose is a major component of plant cell walls
• Cellulose differs from starch in the glycosidic linkages; this difference impacts the structure and function of the polymer.
• Cellulose has straight chains, whereas starch is helical; cellulose’s hydrogen bonds allow parallel cellulose molecules to group into microfibrils, which provide strength
• Chitin is a structural polysaccharide in the exoskeletons of arthropods and in the cell walls of many fungi.

Proteins

• Proteins include a wide variety of structures, resulting in diverse functions.
• Proteins account for more than 50% of the dry mass of most cells

Proteins: Huge Diversity of Structures and Functions

• Enzymatic proteins: Function in selective acceleration of chemical reactions
• Defensive proteins: Function in protection against disease
• Storage proteins: Function in storage of amino acids
• Transport proteins: Function in transport of substances
• Hormonal proteins: Function in coordination of an organism's activities
• Receptor proteins: Function in response of cell to chemical stimuli
• Contractile and motor proteins: Function in movement
• Structural proteins: Function in support

Proteins: Amino Acid Monomers

• Proteins are polymers of amino acids, linked by peptide bonds
• Polypeptides range in length from a few to thousands of amino acids
• Each polypeptide has a unique linear sequence of amino acids, with an amino end and a carboxyl end

Proteins: Structure and Function

• A protein's unique three-dimensional shape determines its function
• A protein's structure can be affected by environmental changes (pH, salt concentration, temperature, etc.)

Proteins: Proper Folding

• Physical and chemical conditions affecting proteins include pH, salt concentration, temperature, and the presence of detergents
• Loss of a protein's native structure is called denaturation, and an unfolded protein is biologically inactive

Nucleic Acids

• Nucleic acids store, transmit, and help express hereditary information
• Genes consist of DNA and a nucleic acid made of monomers called nucleotides
• Two types exist: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA)

Nucleic Acids: Nucleotides

• Nucleic acids are polymers of monomers called nucleotides
• A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphates
• Polynucleotides have sugar-phosphate backbones linked by covalent bonds
• Adjacent nucleotides are joined by covalent bonds

Nucleic Acids: Base Pairing

• The nitrogenous bases in DNA pair up (adenine with thymine, and guanine with cytosine), forming hydrogen bonds
• Complementary base pairing also occurs between two RNA molecules

Nucleic Acids: Structures of DNA and RNA

• DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helix.
• In the DNA double helix, the two backbones run in opposite 5' → 3' directions
• RNA molecules usually exist as single polynucleotide chains, and these chains can fold back on themselves, which stabilizes their three-dimensional structures

Lipids

• Lipids are a diverse group of hydrophobic molecules that do not form polymers
• The unifying feature of lipids is the little or no affinity for water
• The most biologically important lipids are fats, phospholipids, and steroids 

Lipids: Fats

• Fats are constructed from two types of smaller molecules (glycerol and fatty acids)
• Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon.
• A fatty acid consists of a carboxyl group attached to a long carbon skeleton
• Fats separate from water due to hydrophobic nature 
• Fats are typically solid at room temperature when made from saturated fatty acids
• Fats are typically liquid at room temperature when made from unsaturated fatty acids. 

Lipids: Phospholipids

• In a phospholipid, two fatty acids and a phosphate group are attached to glycerol.
• The phosphate group can be further modified
• The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head

Lipids: Steroids

• Steroids are lipids characterized by a carbon skeleton consisting of four fused rings
• Cholesterol is an essential steroid component in animal cell  membranes
• High cholesterol levels may contribute to cardiovascular disease