Macromolecules: Structure and Function

Macromolecules

• Living things consist of four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids
• Biological molecules are polymers made of repeating monomers

Intramolecular vs. Intermolecular Forces

• Macromolecules contain thousands of covalently bonded atoms
• Intramolecular forces within molecules are covalent bonds
• Intermolecular forces (IMFs) between molecules vary based on the molecule's polarity
• A molecule's structure and function are inseparable

Amino Acids

• Amino acids are the basic building blocks of proteins
• Amino acids contain two functional groups: an amine amino end (-NH2) and a carboxylic acid end (-COOH)
• These functional groups attach to a central alpha (α) carbon atom
• The side chain (R group) determines an amino acid's properties

Hydrophobic vs Hydrophilic

• Hydrophobic molecules do not have a charge
• Hydrophobic molecules do not dissolve in water
• Hydrophilic molecules are attracted to water and are water soluble

Dipeptides and Tripeptides

• The α-carboxyl group of one amino acid forms a peptide bond with the α-amino group of another by removing a water molecule
• The result is a dipeptide, linking two amino acids with one peptide bond
• A dipeptide can form a second peptide bond with a third amino acid resulting in a tripeptide

Polypeptides

• Polypeptides are unbranched polymers
• Polypeptides are built from a standardized set of 20 amino acids
• Polypeptide length ranges from a few to over a thousand monomers

Protein Structure - Primary

• Primary structure is the unique sequence of amino acids

Protein Structure - Secondary

• Secondary structure is where coils and folds within a polypeptide chain occur
• Secondary structures result from hydrogen bonds between repeating parts of the polypeptide backbone
• Alpha (α) helix is a coil
• Beta (β) pleated sheet is a folded structure
• Alpha helices form due to hydrogen bonds between the oxygen of a -CO group and the hydrogen of an -NH group four amino acids ahead
• Beta-pleated sheets are a looser shape versus an alpha helix
• Hydrogen bonds are fragile and break easily under high temperatures or changes in pH

Protein Structure - Tertiary

• Tertiary structure is determined by interactions between R groups rather than interactions between backbone constituents
• Hydrogen bonds, disulfide bonds, ionic bonds, and hydrophobic interactions help maintain protein tertiary structures
• Hydrogen bonds are formed between polar groups and disrupted by temperature or pH changes
• Disulfide bridges form between two cysteine molecules and can be broken by reducing agents
• Ionic bonds form between ionized amine and carboxylic acid groups and are broken by pH changes
• Hydrophobic interactions form between non-polar side groups

Protein Structure - Quaternary

• Quaternary structure results when two or more polypeptide chains form one macromolecule
• Chains are held together by hydrogen bonds, disulphide bonds, ionic bonds, and hydrophobic interactions
• Collagen is a fibrous protein of three polypeptides coiled like a rope (trimeric)
• Hemoglobin is a globular protein with four polypeptide chains (tetrameric)
• Insulin consists of two polypeptide chains (dimeric)

Hemoglobin

• Hemoglobin is a globular protein consisting of four polypeptides, with two alpha and two beta chains
• Hemoglobin carries oxygen and is found in red blood cells

Types of Proteins - Globular

• Globular proteins contain molecules that curl up into a ball shape
• The nonpolar (hydrophobic) groups are in the center
• The hydrophilic groups are on the outside
• Enzymes are an example

Types of Proteins - Fibrous

• Fibrous proteins do not curl into a ball shape
• Instead, they form long strands and are not soluble in water and mostly structural
• Collagen and keratin are examples

Protein Hierarchical Structure

• At first, there is a string of amino acids bound by peptide bonds
• As the string interacts in an aqueous environment, a functional protein forms of one or more polypeptides
• A protein's structure determines its function

Protein Functions

• Proteins are macromolecules important in nutrition
• Proteins include a diversity of structures, and functions
• Proteins account for >50% of cells' dry mass
• Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances

Protein Types - Enzymatic

• Enzymatic proteins selectively accelerate chemical reactions
• Digestive enzymes catalyze the hydrolysis of food molecule bonds
• Enzymes function best in a unique set of pH and temperature conditions because enzymes depend on the enzyme molecule's shape
• Changes in pH or temperature disrupt electrostatic forces responsible for an enzyme's specific shape
• Enzymes act as catalysts to speed up chemical reactions
• Enzymes perform their functions repeatedly

Protein types - Storage

• Storage proteins store amino acids
• Casein (milk protein) is the major amino acid source for baby mammals
• Plants have storage proteins in their seeds
• Ovalbumin (egg white protein) is an amino acid source for embryo development

Protein Types - Hormonal

• Hormonal proteins coordinate an organism's activities
• Insulin (a hormone from pancreas) causes tissues to take up glucose, regulating blood sugar concentration

Protein Types - Defensive

• Defensive proteins protect against disease
• Antibodies inactivate and destroy viruses and bacteria

Protein Types - Transport

• Transport proteins transport substances
• Hemoglobin transports oxygen
• Other proteins transport molecules across cell membranes

Protein Types - Receptor

• Receptor proteins respond to chemical stimuli
• Receptors in nerve cell membranes detect signaling molecules from other nerve cells

Protein Types - Structural

• Structural proteins provide support
• Keratin is a protein of hair, horns, feathers, and other skin appendages
• Collagen and elastin proteins provide animal connective tissues' framework

Sickle Cell Disease

• Sickle-cell disease is an inherited blood disorder from one amino acid substitution in the protein hemoglobin
• A slight change in the primary structure can affect a protein's structure and function

Protein Structure Deterrents

• Physical and chemical conditions can affect structure
• Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
• Loss of a protein's native structure is called denaturation
• A denatured protein is biologically inactive