Protein Folding, Stability, and Denaturation (1A) - Bio & Biochem

Identify the term for the set of all conformations a protein can sample during folding.

The conformational ensemble. Proteins fluctuate among various states, with the native form being the most populated under equilibrium conditions.
Which noncovalent interaction primarily stabilizes an $\alpha$ -helix and a $\beta$ -sheet backbone?

Backbone hydrogen bonding between $\text{C=O}$ and $\text{N-H}$ . These hydrogen bonds form regular patterns that stabilize the local folding in alpha-helices and beta-sheets.
Which bond type is directly broken by proteolysis but not by typical denaturation: peptide or hydrogen bond?

Peptide bonds are broken by proteolysis, not typical denaturation. Denaturation typically disrupts noncovalent interactions, whereas proteolysis enzymatically hydrolyzes covalent peptide bonds.
Which type of structure is typically preserved during denaturation: primary, secondary, tertiary, or quaternary?

Primary structure is typically preserved. Denaturation affects higher-order structures by breaking noncovalent bonds, but the amino acid sequence remains intact.
State the folding spontaneity criterion in terms of $\Delta G$ for $\text{U} \rightarrow \text{N}$ .

Folding is spontaneous when $\Delta G < 0$ . For the unfolded (U) to native (N) transition, spontaneity occurs when the free energy decreases, favoring the native state.
Which reagent denatures proteins by disrupting hydrophobic interactions and coating the polypeptide with negative charge?

SDS (sodium dodecyl sulfate). As an anionic detergent, it binds hydrophobically and imparts charge repulsion, unfolding proteins for electrophoresis.
Which level of protein structure describes the overall 3D fold of a single polypeptide chain?

Tertiary structure. It integrates secondary structures and side-chain interactions to form a compact, functional 3D arrangement.
What thermodynamic quantity determines whether protein folding is spontaneous at constant $T$ and $P$ ?

Gibbs free energy change, $\Delta G$ . At constant temperature and pressure, a negative ΔG indicates a spontaneous process, governing protein folding thermodynamics.
What is the equation relating $\Delta G$ , $\Delta H$ , $T$ , and $\Delta S$ for folding?

$\Delta G = \Delta H - T\Delta S$ . This equation captures the balance between enthalpic contributions and entropic effects scaled by temperature in protein folding.
What model describes folding as movement down a rugged energy landscape toward a minimum?

The folding funnel (energy landscape) model. It visualizes folding as a downhill process on a multidimensional surface, avoiding kinetic traps to reach the native minimum.
Which covalent bond can stabilize tertiary or quaternary structure by linking two cysteines?

A disulfide bond ( $\text{Cys-S-S-Cys}$ ). This covalent linkage provides structural rigidity by cross-linking distant parts of the protein chain or subunits.
What is the dominant driving force that stabilizes a protein core in aqueous solution?

The hydrophobic effect (burial of nonpolar side chains). Nonpolar residues cluster inside to avoid water, driven by entropy gain from releasing structured solvent molecules.
Which reagent reduces disulfide bonds to free thiols during protein denaturation?

β-mercaptoethanol (or DTT). These reducing agents cleave disulfide bridges, destabilizing structures reliant on cysteine cross-links during denaturation.
Identify the effect of increasing temperature on protein stability in terms of $\Delta S$ of unfolding.

Higher $T$ favors unfolding because $T\Delta S$ increases. Unfolding entropy becomes dominant at high temperatures, as the -TΔS term in ΔG shifts toward positive contributions.
Which change most directly increases electrostatic repulsion and can denature proteins: pH shift or adding salt?

A large pH shift (changes protonation and salt bridges). Extreme pH alters ionization of charged residues, disrupting ionic interactions and increasing repulsion between like charges.
Which reagent commonly denatures proteins by disrupting hydrogen bonds and the hydrophobic effect?

Urea (or guanidinium chloride). These chaotropes solvate nonpolar groups and weaken hydrogen bonds, promoting unfolding in biochemical experiments.
Which option best defines denaturation of a protein?

Loss of native structure and function without peptide bond hydrolysis. Denaturation disrupts noncovalent interactions, leading to unfolding while preserving the covalent peptide backbone.
Which cellular proteins assist folding by preventing aggregation without dictating final structure?

Molecular chaperones. They bind exposed hydrophobic regions on nascent or misfolded proteins, guiding proper folding and averting aggregates.
What is the term for a partially folded intermediate with native-like secondary structure but loose packing?

Molten globule. This state has compact secondary elements but lacks tight tertiary packing, serving as a folding intermediate.
Which option best defines the native state of a protein under physiological conditions?

The lowest Gibbs free energy conformation under those conditions. Under physiological conditions, proteins adopt the conformation that minimizes Gibbs free energy, ensuring stability and functionality.
Which level of protein structure refers to the association of multiple polypeptide subunits?

Quaternary structure. It involves noncovalent interactions between separate chains, enabling complex protein assemblies like hemoglobin.
Which amino acid is most likely to disrupt an $\alpha$ -helix due to conformational rigidity?

Proline. Its cyclic side chain restricts backbone flexibility, preventing the regular hydrogen bonding needed for helix formation.
What is the definition of $T_m$ (melting temperature) for a protein unfolding transition?

The temperature where $50\%$ is unfolded and $\Delta G = 0$ . At Tm, the folded and unfolded states are equally populated, with equilibrium constant K=1 and zero free energy change.
Which level of protein structure is defined by local backbone conformations like $\alpha$ -helices and $\beta$ -sheets?

Secondary structure. It arises from hydrogen bonding and torsional angles in the polypeptide backbone, independent of side-chain interactions.
Which option best describes why hydrophobic burial can increase solvent entropy during folding?

It releases ordered water molecules from nonpolar surfaces. Exposing nonpolar areas to water orders solvent molecules, so burial reduces this order and boosts entropy.

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