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  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  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?  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