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

14.9- What is the secondary structure of a protein? Regular and repeating structural patterns 2 kinds of repeating patterns proposed by Linus Pauling and Robert Corey in 1940’s: 1. α-helix 2. 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 sheet Backbone of two protein chains is held together by hydrogen bonds

Secondary Proteins can be classified as: 1. Fibrous proteins 2. 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 shape Unlike secondary, it depends on interactions of amino acid side chains

Tertiary Structures are stabilized five ways: 1. Covalent Bonds 2. Hydrogen Bonding 3. Salt Bridges 4. Hydrophobic Interactions 5. 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. Hemoglobin 2. 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) Oxygen carrier in red blood cells

Collagen: Most abundant of all proteins in mammals Makes up 30% or more of the total Major constituent of skin, tendons, bones, blood vessels, and other connective tissues

How are proteins denatured? Denaturation- the loss of the secondary, tertiary, and quaternary structures of a protein by chemical or physical agent that leaves the primary structure intact Enzymes lose their catalytic activity and other proteins can’t carry out their biological functions when denatured

Denatured by: Heat Denaturing chemicals pH change Alcohol

Denaturation Does not affect primary structures Most denaturation is irreversible

Dietary Protein: Protein structure must be destroyed before it can provide the nutrition for the body Digestion involves denaturation and hydrolysis Stomach acids denature proteins Proteolytic enzymes in stomach and small intestine hydrolyze proteins to smaller fragments until free amino acids are formed and can be absorbed through intestinal membranes into the blood stream