Protein Structure Brianne Morgan, Adrienne Trotto, Alexis Angstadt
Secondary Structure 14.9 • A repetitive structure of the protein backbone. • The two most common secondary structures encountered in proteins are the α-helix and the β-pleated sheet. • The protein conformations that do not exhibit a repeated pattern are called random coils.
Helix • In the α-helix form, a single protein chain twists in such a manner that its shape resembles a right-handed coiled spring-that is, a helix. • The shape of the helix is maintained by numerous intramolecular hydrogen bonds that exist between the backbone – C=O and H-N- groups. • All the amino acid side chains point outward from the helix.
B-pleated sheet • In this case, the orderly alignment of protein chains is maintained by intermolecular or intramolecular hydrogen bonds. • The β-pleated sheet structure can occur between molecules when polypeptide chains run parallel (all N-terminal ends on one side) or antiparallel(neighboring N-terminal ends on opposite sides.)
Few proteins have predominately α-helix or β-sheet structures. • Most proteins, especially spherical ones, have only certain portions of their molecules in these conformations. The rest of the molecules consist of random coil.
Keratin is a fibrous protein of hair, fingernails, horns, and wool and it doesn’t have a predominately α-helix structure.
Extended Helix • Another repeating pattern classified as a secondary structure is the extended helix of collagen.
Tertiary Structure 14.10 • 3-D arrangement of every atom in the molecule • Includes interactions of side chains, not just the peptide backbone • Stabilized in 5 ways: Covalent Bonds, hydrogen bonding, salt bridges, hydrophobic interactions, metal ion coordination
Covalent Bonds • Disulfide bond is most often involved in the stabilization • When a cysteine residue is in 2 different chains, formation of a disulfide bond provides a covalent linkage that binds together the 2 chains • EX: structure of insulin
Hydrogen Bonding • Stabilized by hydrogen bonding between polar groups on side chains or between side chains and the peptide backbone
Salt Bridges • Also called electrostatic attractions • Occur between 2 amino acids with ionized side chains • Held together by simple ion-ion attraction
Hydrophobic Interactions • Result of polar groups turned outward toward the aqueous solvent and the nonpolar groups turned inward away from the water molecules • Weaker than hydrogen bonding or salt bridges • Acts over large surfaces
Metal Ion Coordination • 2 side chains can be linked with a metal ion • Human body requires certain trace minerals • Necessary components of proteins
Primary structure of a protein determines the secondary and tertiary structure • When particular R- groups are in proper position, all of the stabilization can form • The side chains allow some proteins to fold
Quaternary Structure of a Protein 14.11 • The highest level of protein organization • Applies to proteins with more than 1 polypeptide chain
Hemoglobin • Each chain surrounds an iron- containing heme unit • Proteins that contain non-amino portions are called conjugated proteins • The non-amino acid portion of a conjugated protein is called a prosthetic group • Early development stage of the fetus, hemoglobin contains 2 alpha and 2 gamma chains
Collagen • The triple helix units called tropocollagen constitute the soluble form of collagen • Structural protein of connective tissue • Provides strength and elasticity • Stabilized by hydrogen bonding between the backbones of the 3 chains • Most abundant protein in humans
Integral Membrane Proteins • Traverse partly • 1/3 of proteins
How are Proteins Denatured? 14.12 • Secondary and tertiary structures stabilize the native conformations of proteins • Physical and chemical agents destroy these structures and denature proteins • Protein functions depend on native conformation; when a protein is denatures, it can no longer carry out its function • Some is reversible, some chaperone molecules may reverse denaturation
