Biochemistry Chapter 6: Proteins - Secondary, Tertiary, and Quaternary Structure

Vocabulary practice flashcards covering secondary, tertiary, and quaternary protein structure, folding thermodynamics, fibrous and globular motifs, structural techniques, and misfolding diseases based on Chapter 6 of Garrett and Grisham Biochemistry.

Proteus

A Greek sea god capable of assuming different forms, serving as a biological metaphor for how proteins act through changes in conformation.

Weak Noncovalent Forces in Protein Structure

The noncovalent interactions that form and stabilize secondary, tertiary, and quaternary protein structures: van der Waals interactions ($0.4\text{--}4\,\text{kJ/mol}$), hydrogen bonds ($12\text{--}30\,\text{kJ/mol}$), ionic bonds ($20\,\text{kJ/mol}$), and hydrophobic interactions ($<40\,\text{kJ/mol}$).

Amide Plane

The planar peptide backbone unit formed by the resonance stabilization energy of $88\,\text{kJ/mol}$ across the $\text{C-N}$ bond, restricting backbone rotation to the bonds connecting each $\alpha$-carbon.

Phi ($\phi$) Angle

The torsion angle of rotation about the bond connecting the $\alpha$-carbon and nitrogen atom ($\text{C}_\alpha\text{-N}$) in a polypeptide backbone.

Psi ($\psi$) Angle

The torsion angle of rotation about the bond connecting the $\alpha$-carbon and carbonyl carbon atom ($\text{C}_\alpha\text{-C}$) in a polypeptide backbone.

Ramachandran Diagram

A two-dimensional conformational map displaying sterically permissible combinations of backbone torsion angles $\phi$ and $\psi$ for amino acid residues in proteins.

$\alpha$-Helix

A prominent secondary structure motif stabilized by internal hydrogen bonds, characterized by $3.6$ residues per turn, a rise per residue of $1.5\,\text{Å}$ ($0.15\,\text{nm}$), a pitch of $5.4\,\text{Å}$, an H-bond-closed loop of $13$ atoms, and standard angles $\phi = -60^\circ$ and $\psi = -45^\circ$.

Helix Net Dipole Moment

A macroscopic dipole resulting from the parallel alignment of individual peptide bond dipole moments along the helix axis, giving the N-terminus a partial positive charge and the C-terminus a partial negative charge.

Helix Capping

The satisfaction of the four unpaired $\text{N-H}$ hydrogen-bond donors at the N-terminus and four unpaired $\text{C=O}$ acceptors at the C-terminus of an $\alpha$-helix via interactions with nearby donor/acceptor groups or nonpolar side-chain packaging.

$\beta$-Pleated Sheet

A secondary structure composed of parallel or antiparallel $\beta$-strands stabilized by inter-strand hydrogen bonds, featuring a rise per residue of $3.47\,\text{Å}$ for antiparallel strands and $3.25\,\text{Å}$ for parallel strands.

$\beta$-Turn (Tight Turn)

A four-residue secondary structure that directs a $180^\circ$ reversal in the polypeptide chain, stabilized by a hydrogen bond between the carbonyl carbon of residue $i$ and the amide nitrogen of residue $i+3$, frequently containing glycine and proline.

X-Ray Crystallography

A structural determination technique where protein crystals diffract an intense X-ray beam, generating a diffraction spot pattern that yields an electron density map to construct an atomic model.

Coiled Coil

A structural motif consisting of a bundle of $\alpha$-helices wound into a left-handed superhelix with $3.5$ residues per turn, mediated by a heptad repeat $(a\text{-}b\text{-}c\text{-}d\text{-}e\text{-}f\text{-}g)_n$ where positions $a$ and $d$ are nonpolar.

$\alpha$-Keratin

A mechanically strong, insoluble fibrous protein of hair, claws, fingernails, horns, and beaks, composed of central $311\text{--}314$ residue-long $\alpha$-helical rod segments with heptad repeats that associate into coiled coils, protofilaments, and filaments.

Fibroin

A fibrous $\beta$-sheet protein found in silk fibers with an alternating sequence of $\text{Gly-Ala/Ser-Gly-Ala/Ser}$, allowing glycines to pack against glycines and alanines/serines to pack against alanines/serines between adjacent sheets.

Tropocollagen

The basic structural unit of collagen connective tissue ($\text{MW} = 285{,}000$, length $300\,\text{nm}$, diameter $1.4\,\text{nm}$), comprising three intertwined left-handed helical chains assembled into a right-handed triple helix with $3.3$ residues per turn.

Prolyl Hydroxylase

An iron-dependent ($\text{Fe}^{2+}$) enzyme that hydroxylates proline residues into hydroxyproline in collagen using $\text{O}_2$, $\alpha$-ketoglutarate, and ascorbic acid (vitamin C), producing succinate and $\text{CO}_2$.

Packing Density of Globular Proteins

The ratio of the sum of the van der Waals volumes of a protein's atoms to its total occupied volume, typically ranging from $0.72$ to $0.77$, leaving approximately $25\%$ of volume as internal cavities that afford structural flexibility.

Helical Wheel Presentation

A two-dimensional axial projection of an $\alpha$-helix used to visualize the spatial arrangement of amino acid side chains and identify polar, nonpolar, or amphiphilic helical faces.

Protein Domain (Module)

A distinct, compact structural unit within a polypeptide (typically in proteins containing more than $250$ amino acids) capable of folding autonomously into a stable tertiary structure in aqueous solution.

Chimeric Antigen Receptor (CAR)

An engineered modular receptor expressed on patient T-cells featuring an antibody-derived extracellular target-binding domain specific for CD19, linked through a hinge to intracellular costimulatory and activating domains for cancer therapy.

Denaturation

The progressive loss of native secondary, tertiary, and quaternary protein structure and biological activity induced by external stresses such as heat or chemical denaturants.

Melting Temperature ($T_\text{m}$)

The characteristic transition temperature at which $50\%$ of a protein's native physiological structure is denatured (unfolded).

Anfinsen's Experiment

A classic demonstration that primary amino acid sequence dictates three-dimensional native structure, proven by denaturing ribonuclease A with urea and $\beta$-mercaptoethanol and observing spontaneous recovery of structure and enzymatic activity upon their removal.

Levinthal's Paradox

A principle establishing that a polypeptide cannot fold by randomly sampling every possible conformational state in reasonable biological time, proving that protein folding proceeds via directed pathways.

Hydrophobic Collapse

An early postulated event in globular protein folding during which nonpolar amino acid residues rapidly cluster and collapse together into the interior to exclude solvent water.

Folding Funnel

An energy landscape model proposed by Ken Dill wherein the wide rim represents numerous high-entropy unfolded conformations that progressively narrow into fewer, lower-energy states toward the native physiological structure.

Molten Globule

A compact, partially folded kinetic intermediate state during protein folding that features native-like secondary structural elements but lacks tightly packed tertiary side-chain interactions.

Proline Cis-Trans Isomerization

The interconversion between the cis and trans conformations of peptide bonds preceding proline residues, often acting as a slow, rate-limiting step in protein folding.

Metamorphic Proteins

Proteins that populate an ensemble of distinct native conformations of comparable stabilities in physiological equilibrium, where minor mutations can drastically reorganize the dominant fold.

Structural Classification of Proteins (SCOP2)

A database detailing structural and evolutionary relationships among known proteins, categorizing them into four major structural classes: all $\alpha$, all $\beta$, $\alpha/\beta$, and $\alpha+\beta$.

Molecular Chaperones

Proteins, such as heat-shock proteins Hsp60 and Hsp70, that assist nascent or denaturing polypeptides by preventing nonproductive aggregation and facilitating proper folding.

Intrinsically Unstructured Proteins (IUPs)

Proteins that lack a defined, stable three-dimensional fold under physiological conditions due to an abundance of polar residues and a lack of hydrophobic residues, often adopting defined structure upon binding partner targets.

p53 Protein

A tumor suppressor protein and transcription factor featuring an N-terminal disordered domain ($93$ residues), a central DNA-binding domain ($200$ residues), and a C-terminal disordered domain ($100$ residues) that interact with diverse partner proteins.

Prion

An infectious protein particle consisting of an altered, misfolded conformation ($\text{PrP}^\text{Sc}$) that templates conformational change in normal cellular prion molecules ($\text{PrP}^\text{C}$), causing transmissible neurodegenerative diseases.

Cryo-Electron Microscopy (Cryo-EM)

A structural technique wherein macromolecular complexes flash-frozen in vitreous ice are imaged with coherent electron beams to generate three-dimensional density maps and atomic models at $1.2\text{--}1.7\,\text{Å}$ resolution.

Quaternary Structure

The specific noncovalent spatial arrangement and interaction of multiple folded polypeptide subunits into an oligomeric protein complex, driven favorably by the entropy gained from burying nonpolar contact surfaces.

Immunoglobulin G (IgG)

A quaternary tetrameric glycoprotein consisting of two identical heavy chains and two identical light chains stabilized by inter- and intrachain disulfide bonds, featuring variable antigen-binding domains ($V_\text{L}, V_\text{H}$) and constant domains ($C_\text{L}, C_\text{H}1, C_\text{H}2, C_\text{H}3$).