Protein Structure and Folding

Secondary Structure

Proteins adopt specific local structures known as secondary structures, stabilized primarily by hydrogen bonds in the backbone. The most common types are the α-helix and β-sheet.

• α-Helix: A right-handed coil where each backbone N-H group forms a hydrogen bond with the C=O group four residues earlier. Side chains project outward from the helix.

• β-Sheet: Composed of β-strands connected laterally by at least two or three backbone hydrogen bonds, forming a sheet-like structure. Can be parallel or antiparallel.

• Ramachandran Plot: A graphical representation of the allowed regions of backbone dihedral angles (φ and ψ) in a polypeptide. Common secondary structures occupy specific regions on this plot.

• MALEK: Single-letter codes for amino acids often found in α-helices (Methionine, Alanine, Leucine, Glutamate, Lysine).

• Helical Dipole Moment: The net dipole created by the alignment of individual peptide bond dipoles along the helix axis.

• Wenxiang Diagram: A helical wheel diagram depicting the spatial arrangement of amino acids in an α-helix, useful for visualizing amphipathic helices.

• β-Turn: A tight turn in the polypeptide chain, often connecting strands of antiparallel β-sheets. Glycine and proline are commonly found in turns.

Supersecondary Structure and Motifs

Supersecondary structures are combinations of secondary structure elements that form recognizable patterns, such as β-α-β motifs and helix-turn-helix motifs. These motifs contribute to the overall folding and function of proteins.

• Coiled-coil: Two or more α-helices wound around each other, stabilized by hydrophobic interactions.

• β-barrel: A large β-sheet that twists and coils to form a closed structure in which the first strand is hydrogen-bonded to the last.

Fibrous Proteins

Fibrous proteins, such as keratin and collagen, have elongated structures and serve structural roles in cells and tissues.

• Keratin: Composed of α-helices coiled together, stabilized by disulfide bonds and hydrophobic interactions.

• Collagen: Consists of three left-handed helices wound into a right-handed triple helix. Contains a repeating Gly-X-Y sequence (X and Y are often proline and hydroxyproline).

Protein Folding and Stability

Protein folding is the process by which a polypeptide chain attains its functional three-dimensional structure. The folding pathway is determined by the amino acid sequence and is influenced by various intramolecular and environmental factors.

• Folding Funnel: The energy landscape of protein folding is often depicted as a funnel, with many possible conformations at the top and the native state at the bottom.

• Levinthal's Paradox: The observation that proteins fold much faster than would be expected if they sampled all possible conformations randomly. This suggests the presence of folding pathways and intermediates.

• Molten Globule: A partially folded intermediate with significant secondary structure but lacking the tight packing of the native state.

• Hydrophobic Effect: The tendency of nonpolar side chains to avoid water, driving the folding of proteins into compact, globular shapes.

• Disulfide Bonds: Covalent bonds formed between the sulfur atoms of cysteine residues, stabilizing the tertiary structure.

• Denaturation: The loss of native structure and function due to external stress (e.g., heat, chemicals, pH changes).

• Renaturation: The process by which some proteins can refold into their native structure after denaturation.

• Chaperones: Proteins that assist in the folding of other proteins, especially under stress conditions (e.g., heat shock proteins).

Protein Misfolding and Aggregation

Proteins may fail to fold properly due to mutations, environmental stress, or cellular crowding, leading to aggregation and loss of function. Misfolded proteins can cause diseases such as Alzheimer's and Parkinson's.

• Protein Aggregation: The clumping of misfolded proteins, often leading to cellular toxicity.

• Heat Shock Proteins (Hsp): Molecular chaperones that help prevent aggregation by assisting in proper folding.

Protein Purification Techniques

Size Exclusion Chromatography (SEC)

Separates proteins based on size by passing them through a column packed with porous beads. Larger proteins elute first because they are excluded from the pores, while smaller proteins enter the pores and elute later.

Ion Exchange Chromatography

Separates proteins based on charge by passing them through a column containing charged beads. Proteins with opposite charge to the beads are retained and later eluted by increasing salt concentration.

• Anion Exchange: Beads are positively charged; bind negatively charged proteins (e.g., DEAE columns).

• Cation Exchange: Beads are negatively charged; bind positively charged proteins (e.g., CM columns).

SDS-Polyacrylamide Gel Electrophoresis (SDS-PAGE)

An analytical technique used to separate proteins by size. Proteins are denatured and coated with SDS, giving them a uniform negative charge. They are then separated by electrophoresis through a polyacrylamide gel matrix.

Affinity Chromatography

Separates proteins based on specific binding interactions with a ligand attached to the column matrix. The protein of interest binds to the ligand, while other proteins are washed away.

Key Terms and Definitions

• Native State: The functional, folded conformation of a protein with the lowest free energy.

• Denatured State: The unfolded, inactive form of a protein.

• Disulfide Bonds (Bridges): Covalent bonds between cysteine residues that stabilize protein structure.

• β-Mercaptoethanol: A reducing agent used to break disulfide bonds in proteins.

• SDS: An anionic detergent that denatures proteins by disrupting hydrophobic interactions.

• Urea: A chaotropic agent that denatures proteins by disrupting hydrogen bonds.

• Subunit: A single polypeptide chain within a multi-chain protein complex (quaternary structure).

• Irregularly Structured Regions: Protein regions lacking regular secondary structure, often involved in flexibility or function.

Summary Table: Protein Purification Methods

Additional Info

• Protein Domains: Independently folded regions of a protein, often associated with specific functions (e.g., DNA binding domain).

• Conformational Entropy: The entropy associated with the number of possible conformations a polypeptide can adopt; folding reduces entropy but increases stability via enthalpic interactions.

• Symmetry in Quaternary Structure: Many multi-subunit proteins exhibit symmetry (e.g., cyclic, dihedral, helical).