b -Week 9 - L4 - Protein Conformational Flexibility, Dynamics, and Energy Landscapes

Main Chain and Side Chain Conformational Flexibility

• Protein Domain Interactions and Mobility:

  • Protein domains do not remain static; they interact and move with respect to one another within a single protein molecule.

  • Large-scale conformational changes occur frequently in structural elements, such as loops opening and closing.

  • Conformational flexibility and internal structural movements are essential for protein function.

• Rotational Freedom in the Main Chain:

  • Protein flexibility stems from possible rotations around single bonds within the chemical structure.

  • For every standard amino acid residue in the polypeptide main chain, there are exactly $2$ single bonds around which rotation is allowable to alter protein conformation.

  • Peptide bonds in the main chain do not permit rotation due to their partial double-bond character, maintaining local rigidity between single-bond rotatable positions.

• The Proline Exception:

  • Proline is a unique structural exception among amino acid residues.

  • Due to its cyclic side chain structure bonded back to the main chain amine, proline possesses only $1$ single bond in the main chain capable of rotation.

  • Proline residues introduce localized rigidity into the polypeptide chain.

  • Mutating a residue to proline frequently alters the overall conformation of the main chain.

• Side Chain Flexibility:

  • In addition to main chain flexibility, amino acid side chains exhibit extreme flexibility due to single-bond rotations.

  • Glutamic Acid (Glutamate) Example:

  • Glutamic acid contains $3$ single bonds within its side chain.

  • Combined with the $2$ rotatable single bonds in the main chain, a single glutamic acid residue possesses a total of $5$ single bonds around which rotation can theoretically occur.

Secondary Structure Rigidity vs. Loop Flexibility

• Structural Stabilization and Rigidity:

  • Despite possessing numerous rotational degrees of freedom, proteins remain stable and maintain defined three-dimensional shapes because non-flexible structural elements constrain movement.

  • Secondary structure elements are tightly compacted and densely packed with close atomic contact.

  • Multiple hydrogen bonds stabilize secondary structures, and steric clashes prohibit rotation around main chain angles.

  • $\alpha$-helices: Behave functionally as rigid rods.

  • $\beta$-sheets: Behave functionally as rigid plates.

• Flexible Structural Elements:

  • Flexible loops connecting secondary structure elements retain high rotational mobility.

  • Unstructured connecting segments of the polypeptide chain that link distinct domains serve as flexible linkers.

Functional Importance of Conformational Changes and Energy Coupling

• Functional Roles of Flexible Loops:

  • Protruding, flexible loop regions frequently form critical parts of binding sites for target molecules.

  • Receptor Signaling Example:

  • A receptor detects a specific small molecule.

  • Binding of the small molecule causes a flexible loop to close over the ligand-binding pocket.

  • Loop closure induces a conformational change throughout the molecule, signaling the presence of the bound ligand to the interior of the cell.

• Cystic Fibrosis Transmembrane Conductance Regulator (CFTR):

  • CFTR exhibits two distinct conformational states that have both been captured via protein crystallization.

  • One crystal structure represents the open conformation of the chloride channel, whereas the other represents the closed conformation.

  • The conformational transition between these domain states controls the channel's chloride conductivity.

  • Movement between these distinct states is mediated directly by the binding of magnesium ($\text{Mg}^{2+}$) and adenosine triphosphate ($\text{ATP}$).

• Thermodynamic Driving Forces for Conformational Transitions:

  • Large-scale conformational transitions can present high energy barriers.

  • High energy transitions require external energy inputs, such as:

  • The energy released from $\text{ATP}$ hydrolysis.

  • The binding energy released upon the exergonic association of a ligand with its receptor.

Energy Landscapes and the Free Energy Funnel Model

• Thermal Energy and Conformational Equilibrium:

  • For many proteins, the energy difference between distinct structural conformations is small.

  • Proteins in solution naturally and rapidly fluctuate between a ensemble of slightly different conformations.

  • These fluctuations do not alter the overall three-dimensional fold or secondary structure elements; minor twists around rotatable single bonds generate slightly different molecular shapes.

  • Conformational interconversion requires no external chemical energy input; it is driven entirely by intrinsic thermal energy possessed by all molecules in solution (including solvent water molecules).

  • Thermal energy is sufficient to overcome the low energy barriers separating similar free energy states.

• Revision of the Idealized Folding Funnel:

  • Idealized Model: Posits that as a protein leaves the ribosome and folds through intermediate states, it progresses monotonically down a free energy funnel to a single global minimum corresponding to a single native conformation.

  • Realistic Model: Protein folding converges to a set of multiple native sub-conformations sharing very similar free energy levels.

  • The bottom of the free energy funnel contains multiple sub-minima (subminimums), each representing a native state variant.

  • Open and closed conformations of a protein reside adjacent to each other at comparable free energy levels within these sub-minima.

  • Transitions between adjacent native sub-minima occur readily by utilizing ambient thermal energy or binding energy released during ligand association.