Protein Secondary, Tertiary, Quaternary Structure (1A) - MCAT Biological and Biochemical Foundations of Living Systems

Hydrogen bonding between backbone C=O and Nb2H groups. These hydrogen bonds form between polar groups in the peptide backbone, providing stability to local folds without involving side chains.

Hydrophobic interactions at subunit interfaces. Hydrophobic interactions bury nonpolar surfaces at interfaces, providing energetic favorability similar to the hydrophobic effect in folding.

Homodimer. Homodimers consist of two identical monomers, often stabilizing through symmetric interfaces for functional cooperativity.

Reducing agents such as DTT or $b^2$-mercaptoethanol. These agents cleave disulfide bonds by reducing the sulfur-sulfur linkage, allowing separation of covalently linked chains or domains.

Secondary, tertiary, and quaternary structure (primary remains intact). Denaturation disrupts non-covalent interactions maintaining higher-order structures, but the covalent primary sequence remains unchanged.

Denaturation disrupts higher structure; hydrolysis cleaves peptide bonds. Denaturation unfolds the protein by breaking non-covalent interactions, while hydrolysis enzymatically breaks covalent peptide bonds into smaller fragments.

Two or more amphipathic $b^1$-helices wrapped around each other. Coiled-coils form stable bundles through hydrophobic interactions between helical surfaces, common in structural and regulatory proteins.

Recurring combination of secondary elements with a common 3D pattern. Motifs represent conserved spatial arrangements of secondary structures that recur across proteins, contributing to functional or structural roles.

Independently folding structural unit within a single polypeptide. Domains are modular units that can fold autonomously, often corresponding to specific functions within larger proteins.

Disulfide bond (covalent). As a covalent interaction, disulfide bonds provide greater strength compared to non-covalent forces like hydrogen bonds or hydrophobic interactions.

Lysine (or arginine) with aspartate (or glutamate). At pH around 7, lysine and arginine are positively charged, while aspartate and glutamate are negatively charged, enabling strong electrostatic attraction.

Electrostatic attraction between oppositely charged side chains. Salt bridges stabilize tertiary structure through ionic interactions that counterbalance charges and reduce electrostatic repulsion.

Nonpolar side chains cluster away from water, forming a hydrophobic core. This effect drives protein folding by minimizing the exposure of nonpolar residues to aqueous solvent, increasing entropy of water molecules.

Oxidizing environments such as the ER lumen (and extracellular space). Oxidizing conditions promote the formation of disulfide bonds by facilitating the oxidation of thiol groups in cysteine residues.

Covalent Sb2S bond between two cysteine side chains (cystine). Disulfide bonds covalently link distant parts of the protein, providing strong stabilization in oxidizing environments.

Glycine. Glycine's lack of a side chain allows excessive backbone flexibility, which destabilizes the rigid structure needed for an α-helix.

Proline. Proline's cyclic side chain restricts backbone flexibility, preventing the regular hydrogen bonding required for helical conformation.

$b^2$-turn (hairpin turn). β-turns provide a sharp 180-degree reversal in chain direction, facilitated by glycine and proline, linking adjacent antiparallel strands efficiently.

Strand Nb2b2C directions: same (parallel) versus opposite (antiparallel). The direction of polypeptide chains determines hydrogen bond geometry, with antiparallel sheets often more stable due to linear bonding patterns.

Multiple $b^2$-strands aligned with backbone hydrogen bonding between strands. β-sheets form extended, pleated structures where hydrogen bonds between adjacent strands create a rigid, planar arrangement.

Spatial arrangement of multiple polypeptide subunits in one complex. Quaternary structure involves the assembly of multiple polypeptide chains into a functional complex, often enhancing stability and function.

Overall $3$D fold of a single polypeptide chain. Tertiary structure encompasses the complete three-dimensional arrangement stabilized by interactions among side chains of a single chain.

C=O of residue $i$ bonds to Nb2H of residue $i+4$. This spacing allows for a tight coil with 3.6 residues per turn, optimizing hydrogen bonding and minimizing steric hindrance.

Backbone carbonyl oxygen and backbone amide hydrogen. In an α-helix, the hydrogen bond connects the carbonyl oxygen of one residue to the amide hydrogen four residues ahead, stabilizing the helical turn.