MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1a Protein Folding Denaturation

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This deck focuses on 1a Protein Folding Denaturation, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Flashcard 1: Identify the term for the set of all conformations a protein can sample during folding.

Answer: The conformational ensemble. Proteins fluctuate among various states, with the native form being the most populated under equilibrium conditions.

Flashcard 2: Which noncovalent interaction primarily stabilizes an $\alpha$-helix and a $\beta$-sheet backbone?

Answer: 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.

Flashcard 3: Which bond type is directly broken by proteolysis but not by typical denaturation: peptide or hydrogen bond?

Answer: Peptide bonds are broken by proteolysis, not typical denaturation. Denaturation typically disrupts noncovalent interactions, whereas proteolysis enzymatically hydrolyzes covalent peptide bonds.

Flashcard 4: Which type of structure is typically preserved during denaturation: primary, secondary, tertiary, or quaternary?

Answer: Primary structure is typically preserved. Denaturation affects higher-order structures by breaking noncovalent bonds, but the amino acid sequence remains intact.

Flashcard 5: State the folding spontaneity criterion in terms of $\Delta G$ for $\text{U} \rightarrow \text{N}$.

Answer: 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.

Flashcard 6: Which reagent denatures proteins by disrupting hydrophobic interactions and coating the polypeptide with negative charge?

Answer: SDS (sodium dodecyl sulfate). As an anionic detergent, it binds hydrophobically and imparts charge repulsion, unfolding proteins for electrophoresis.

Flashcard 7: Which level of protein structure describes the overall 3D fold of a single polypeptide chain?

Answer: Tertiary structure. It integrates secondary structures and side-chain interactions to form a compact, functional 3D arrangement.

Flashcard 8: What thermodynamic quantity determines whether protein folding is spontaneous at constant $T$ and $P$?

Answer: Gibbs free energy change, $\Delta G$. At constant temperature and pressure, a negative ΔG indicates a spontaneous process, governing protein folding thermodynamics.

Flashcard 9: What is the equation relating $\Delta G$, $\Delta H$, $T$, and $\Delta S$ for folding?

Answer: $\Delta G = \Delta H - T\Delta S$. This equation captures the balance between enthalpic contributions and entropic effects scaled by temperature in protein folding.

Flashcard 10: What model describes folding as movement down a rugged energy landscape toward a minimum?

Answer: 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.

Flashcard 11: Which covalent bond can stabilize tertiary or quaternary structure by linking two cysteines?

Answer: 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.

Flashcard 12: What is the dominant driving force that stabilizes a protein core in aqueous solution?

Answer: The hydrophobic effect (burial of nonpolar side chains). Nonpolar residues cluster inside to avoid water, driven by entropy gain from releasing structured solvent molecules.

Flashcard 13: Which reagent reduces disulfide bonds to free thiols during protein denaturation?

Answer: β-mercaptoethanol (or DTT). These reducing agents cleave disulfide bridges, destabilizing structures reliant on cysteine cross-links during denaturation.

Flashcard 14: Identify the effect of increasing temperature on protein stability in terms of $\Delta S$ of unfolding.

Answer: 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.

Flashcard 15: Which change most directly increases electrostatic repulsion and can denature proteins: pH shift or adding salt?

Answer: 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.

Flashcard 16: Which reagent commonly denatures proteins by disrupting hydrogen bonds and the hydrophobic effect?

Answer: Urea (or guanidinium chloride). These chaotropes solvate nonpolar groups and weaken hydrogen bonds, promoting unfolding in biochemical experiments.

Flashcard 17: Which option best defines denaturation of a protein?

Answer: Loss of native structure and function without peptide bond hydrolysis. Denaturation disrupts noncovalent interactions, leading to unfolding while preserving the covalent peptide backbone.

Flashcard 18: Which cellular proteins assist folding by preventing aggregation without dictating final structure?

Answer: Molecular chaperones. They bind exposed hydrophobic regions on nascent or misfolded proteins, guiding proper folding and averting aggregates.

Flashcard 19: What is the term for a partially folded intermediate with native-like secondary structure but loose packing?

Answer: Molten globule. This state has compact secondary elements but lacks tight tertiary packing, serving as a folding intermediate.

Flashcard 20: Which option best defines the native state of a protein under physiological conditions?

Answer: 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.

Flashcard 21: Which level of protein structure refers to the association of multiple polypeptide subunits?

Answer: Quaternary structure. It involves noncovalent interactions between separate chains, enabling complex protein assemblies like hemoglobin.

Flashcard 22: Which amino acid is most likely to disrupt an $\alpha$-helix due to conformational rigidity?

Answer: Proline. Its cyclic side chain restricts backbone flexibility, preventing the regular hydrogen bonding needed for helix formation.

Flashcard 23: What is the definition of $T_m$ (melting temperature) for a protein unfolding transition?

Answer: 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.

Flashcard 24: Which level of protein structure is defined by local backbone conformations like $\alpha$-helices and $\beta$-sheets?

Answer: Secondary structure. It arises from hydrogen bonding and torsional angles in the polypeptide backbone, independent of side-chain interactions.

Flashcard 25: Which option best describes why hydrophobic burial can increase solvent entropy during folding?

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