Part 7: Protein Structure and Function
The Workhorses of the Cell
Proteins are the most versatile macromolecules in living systems. They catalyze reactions, provide structural support, transport molecules, transmit signals, and defend against pathogens. Their function is determined by their three-dimensional structure.
Levels of Protein Structure
Primary Structure
Linear sequence of amino acids linked by peptide bonds
Determined by gene sequence. Dictates all higher levels of structure.
Secondary Structure
Local folding stabilized by hydrogen bonds
Ī±-Helix
3.6 residues/turn, H-bonds iāi+4
Ī²-Sheet
Parallel or antiparallel strands
Tertiary Structure
Overall 3D shape of a single polypeptide
Stabilized by hydrophobic core, disulfide bonds, ionic interactions
Quaternary Structure
Association of multiple polypeptide chains
Example: Hemoglobin (Ī±āĪ²ā tetramer)
Protein Folding
Anfinsen's Dogma
The amino acid sequence contains all information needed for correct folding. Native structure is the thermodynamically most stable state.
Chaperones
Proteins that assist folding: Hsp70, Hsp60 (GroEL/GroES), Hsp90. Prevent aggregation, allow proper folding.
Levinthal's Paradox
Random sampling of all conformations would take longer than the age of the universe. Proteins fold in milliseconds via directed pathways.
Protein Functions
Enzymes
DNA polymerase
Structural
Collagen, keratin
Transport
Hemoglobin
Signaling
Insulin
Defense
Antibodies
Motor
Myosin, kinesin