Protein Structure and Function

Chapter 3: Protein Structure and Function

Overview of Proteins

  • Proteins are diverse macromolecules critical to biological functions.

  • A protein's conformation is its three-dimensional structure, essential for its function.

  • Correct folding is vital; function derives from structure.

Protein Composition

  • Monomer: Amino acids (e.g., H₂N-C-C-COOH)

  • Polymer: Peptides formed by peptide bonds.

  • Definitions:

    • Peptide: 2-30 amino acids.

    • Polypeptide: longer chains, can exceed 4000 residues.

    • Protein: a folded polypeptide with a specific conformation.

Levels of Protein Structure

  1. Primary Structure: Linear amino acid sequence; contains information for folding.

  2. Secondary Structure: Local folding into Alpha (α) helices and Beta (β) sheets.

    • Accounts for about 60% of a protein's 3D structure.

  3. Tertiary Structure: Overall folding into a specific 3D shape due to various interactions (hydrophobic, hydrogen, ionic bonds, etc.).

  4. Quaternary Structure: Multi-chain interactions (e.g., dimers, trimers).

Protein Motifs and Domains

  • Motifs: Regions with specific structures and functions, can be reused in different proteins.

  • Domains: Tertiary structure modules, often functional, with various classes based on size, function, and topology.

Protein Folding

  • Hydrophobic Effect: Main driving force for folding.

  • Self-assembly instructions are inherent in the amino acid sequence.

  • Chaperones: Assist in protein folding, preventing misfolding.

Regulating Protein Function

  • Proteins exhibit specificity (binding affinity) and can undergo conformational changes.

  • Enzymes: Increase reaction rates; catalyze biochemical reactions by stabilizing transition states.

  • Kinetics: Explored through Michaelis-Menten kinetics and affinity measurements.

  • Post-translational modifications (e.g., phosphorylation) regulate activity.

Methods for Protein Analysis and Purification

  • Techniques include centrifugation, electrophoresis, Western blotting, and mass spectrometry.

  • Proteomics involves identifying proteins in biological samples, examining structure and function.

Summary points:

  • The hierarchy of protein structure affects function at each level.

  • Correct folding is critical for the biological activity of proteins.

  • Misfolded proteins can be deleterious and are involved in diseases.

  • Techniques in protein study have diagnostic and therapeutic applications.