Chapter 14.9-14.12Proteins: Secondary, Tertiary, Quaternary, and Denaturation

14.9- What is the secondary structure of a protein?Regular and repeating structural

patterns2 kinds of repeating patterns proposed

by Linus Pauling and Robert Corey in 1940’s:1. α-helix2. β-pleated sheet

Hydrogen bonds hold polypeptide chain in place

Hydrogen bond connects carbonyl oxygen with amide hydrogen atom of another(–C=O---H-N-)

α-helix

α-helix

single protein chain coiled in a spiral with a right-handed twist

held together by hydrogen bonds parallel to the axis of the coil

β-pleated sheet

β-pleated sheetBackbone of two protein chains is

held together by hydrogen bonds

Secondary Proteins can be classified as:1. Fibrous proteins2. Globular proteins

Fibrous proteins:Tough, insoluble proteins in which

chains form long fibers or sheets◦Wool, hair, and fingernails made of

α-keratins(fibrous protein)◦α-keratins are composed of α-helixes◦Natural silk and spider webs are

made of fibroin, proteins mainly composed of β-pleated sheets

Globular proteins:Water-proteins whose chains are

folded into compact, globelike shapes

Presence of hydrophilic side chains on outer surfaces account for water solubility –allowing them to travel through blood and other body fluids to sites where activity is needed

14.10- What is the tertiary structure of a protein?Three-dimensional shapeUnlike secondary, it depends on

interactions of amino acid side chains

Tertiary Structures are stabilized five ways:1. Covalent Bonds2. Hydrogen Bonding3. Salt Bridges4. Hydrophobic Interactions5. Metal Ion Coordination

14.11 What is the Quaternary Structure of a protein?The way in which 2 or more

protein chains form a single three-dimensional unit

2 important quaternary proteins:1. Hemoglobin2. Collagen

Hemoglobin:Composed of 4 polypeptide

chains 2α chains (141 amino acids) and

2 β chains(146 amino acids)Held together by interaction

hydrophobic groups and heme groups (iron in center of heterocyclic ring)

Conjugated proteins- contain non-amino acid portions

Oxygen carrier in red blood cells

Collagen:Most abundant of all proteins in

mammalsMakes up 30% or more of the

totalMajor constituent of skin,

tendons, bones, blood vessels, and other connective tissues

14.12 How are proteins denatured?Denaturation:

◦Loss of secondary, tertiary, or quarternary protein structure by a chemical or physical agent

◦Leaves peptide bonds and primary structure intact

Agents that cause denaturation:HeatpH changeInorganic saltsOrganic compounds

Most denaturation is:Irreversible, hard boiled eggs

don’t soften much when their temperature is lowered