Proteins and Protein Structure

Proteins

• Composed of carbon, hydrogen, oxygen, nitrogen, and small amounts of other elements, notably sulfur
• Amino acids: the building blocks of proteins
    * 20 different amino acids
    * Common structure with variable sidechain that determines structure and function

Secondary Structure

• Chemical and physical interactions cause protein folding
• α helices and β pleated sheets
    * Key determinants of a protein’s characteristics
    * The protein chains are held together by intermolecular hydrogen bonding. 
    * In a β sheet, strands of protein lie adjacent to one another, interacting laterally via 
    * H bonds between backbone carbonyl oxygen and amino H atoms. 
    * The strands may be parallel (N-termini of both strands at the same end) or antiparallel.
• “Random coiled regions” 
    * Not α helix or β pleated sheet
• Shape is specific and important to function

Tertiary Structure

• Folding gives protein a complex 3D shape
• This is the final level of structure for a single polypeptide chain

Quaternary Structure

• Made up of two or more polypeptides
    * Individual polypeptide chains are protein subunits
    * Protein can be formed from several copies of the same polypeptide
    * Or may be multimeric – composed from different polypeptides

Protein Folding In The Cell

• It is hard to predict a protein’s structure from its primary sequence
• Most proteins probably go through several states on their way to a stable structure
• Chaperones: protein molecules that assist the proper folding of other proteins

Chaperone Functions

• Folding and refolding
• Preservation of the unfolded state
• Facilitation to transport
• Block to aggregation
• Disaggregation
• Degradation
• Assembly/disassembly of oligomeric complexes
• Facilitation of signaling processes