The Structure and Function of Large Biological Molecules

The Structure and Function of Large Biological Molecules

Introduction

Large biological molecules, also known as macromolecules, are essential to the structure and function of all living organisms. The four major classes of macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Each class has unique properties and functions that are critical for life.

Macromolecules: Polymers Built from Monomers

Definition and Overview

• Macromolecules are large molecules composed of thousands of covalently connected atoms.

• Most macromolecules are polymers, long molecules consisting of many similar or identical building blocks called monomers.

• The four classes of life’s organic macromolecules are: carbohydrates, proteins, nucleic acids, and (with some exceptions) lipids.

Synthesis and Breakdown of Polymers

• Dehydration reaction: Monomers are connected by covalent bonds through the loss of a water molecule.

• Hydrolysis: Polymers are disassembled to monomers by the addition of water, a process catalyzed by enzymes.

• These reactions are fundamental to the metabolism of all living things.

Diversity of Polymers

• Each cell has thousands of different macromolecules.

• Polymers are constructed from only 40-50 common monomers, but their arrangement leads to great diversity.

Carbohydrates: Fuel and Building Material

Overview

• Carbohydrates include sugars and the polymers of sugars.

• Main functions: energy storage and structural support.

Monosaccharides

• Monosaccharides are the simplest carbohydrates (simple sugars), usually with a formula that is a multiple of CH2O.

• Glucose (C6H12O6) is the most common monosaccharide.

• Monosaccharides are classified by the location of the carbonyl group and the number of carbons in the carbon skeleton.

• They serve as major fuel for cells and as raw material for building molecules.

Disaccharides

• Disaccharides are formed when a dehydration reaction joins two monosaccharides.

• The covalent bond formed is called a glycosidic linkage.

Polysaccharides

• Polysaccharides are polymers of sugars, with storage and structural roles.

• The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages.

Storage Polysaccharides

• Starch: A storage polysaccharide of plants, consists entirely of glucose monomers.

• Glycogen: A storage polysaccharide in animals, mainly in liver and muscle cells.

Structural Polysaccharides

• Cellulose: A major component of the tough wall of plant cells, composed of β-glucose monomers.

• Chitin: Found in the exoskeleton of arthropods and the cell walls of fungi.

Lipids: A Diverse Group of Hydrophobic Molecules

Overview

• Lipids are the one class of large biological molecules that do not form true polymers.

• Lipids are hydrophobic because they consist mostly of hydrocarbons, which form nonpolar covalent bonds.

• Major types: fats, phospholipids, and steroids.

Fats

• 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.

• Fatty acid consists of a carboxyl group attached to a long carbon skeleton.

• Fats separate from water because water molecules hydrogen-bond to each other and exclude the fats.

• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol or triglyceride.

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds.

• Unsaturated fatty acids have one or more double bonds.

• Animal fats are usually saturated; plant fats and fish fats are usually unsaturated.

Functions of Fats

• Major function: energy storage.

• Adipose tissue also cushions vital organs and insulates the body.

Phospholipids

• Two fatty acids and a phosphate group are attached to glycerol.

• The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head.

• When added to water, they self-assemble into bilayers, with the hydrophobic tails pointing inward.

• Phospholipids are the major component of all cell membranes.

Steroids

• Steroids are lipids characterized by a carbon skeleton consisting of four fused rings.

• Cholesterol is an important steroid, a component in animal cell membranes.

• High levels of cholesterol may contribute to cardiovascular disease.

Proteins: Structure and Function

Overview

• Proteins account for more than 50% of the dry mass of most cells.

• Functions include structural support, storage, transport, cellular communication, movement, and defense against foreign substances.

Amino Acids and Polypeptides

• Amino acids are organic molecules with carboxyl and amino groups.

• Amino acids differ in their properties due to differing side chains, called R groups.

• Amino acids are linked by peptide bonds to form polypeptides.

• A polypeptide is a polymer of amino acids.

• A protein consists of one or more polypeptides folded into a unique shape.

Protein Structure

• The sequence of amino acids determines a protein’s three-dimensional structure.

• A protein’s structure determines its function.

Four Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coils and folds in the polypeptide chain, including α-helix and β-pleated sheet, stabilized by hydrogen bonds.

• Tertiary structure: Determined by interactions among various side chains (R groups), including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Quaternary structure: Results when a protein consists of multiple polypeptide chains.

Protein Folding and Denaturation

• Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel, a process called denaturation.

• A denatured protein is biologically inactive.

• Chaperonins are protein molecules that assist the proper folding of other proteins.

Example: Sickle-Cell Disease

• A slight change in primary structure can affect a protein’s structure and ability to function.

• In sickle-cell disease, a single amino acid substitution in hemoglobin causes the protein to aggregate into fibers, reducing its ability to carry oxygen.

Nucleic Acids: Store, Transmit, and Help Express Hereditary Information

Overview

• Nucleic acids are polymers called polynucleotides.

• Two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

• DNA provides directions for its own replication and directs synthesis of messenger RNA (mRNA), which controls protein synthesis.

Nucleotide Structure

• Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose.

• There are two families of nitrogenous bases: pyrimidines (cytosine, thymine, uracil) and purines (adenine, guanine).

Nucleotide Polymers

• Nucleotide polymers are linked together by phosphodiester bonds to build a polynucleotide.

• The sequence of bases along a DNA or mRNA polymer is unique for each gene.

DNA Structure

• A DNA molecule has two polynucleotides spiraling around an imaginary axis, forming a double helix.

• The two backbones run in opposite 5' → 3' directions from each other, an arrangement referred to as antiparallel.

• Only certain bases pair with each other: adenine (A) with thymine (T), and guanine (G) with cytosine (C).

DNA and Proteins as Measures of Evolution

• The linear sequences of nucleotides in DNA molecules are passed from parents to offspring.

• Two closely related species are more similar in DNA than are more distantly related species.

• Molecular biology can be used to assess evolutionary kinship.

Theme of Emergent Properties in the Chemistry of Life

• Higher levels of organization result in the emergence of new properties.

• Organization is key to the chemistry of life.

Summary Table: Major Classes of Biological Macromolecules

Class	Monomer	Polymer	Function	Example
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structural support	Starch, cellulose, glycogen
Lipids	Glycerol, fatty acids	Not true polymers	Energy storage, membranes, hormones	Fats, phospholipids, steroids
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, defense	Enzymes, hemoglobin
Nucleic Acids	Nucleotide	Polynucleotide	Genetic information storage and transfer	DNA, RNA