LECT 9-11: Biochemistry: Protein Purification, Peptide Therapeutics, and Secondary Structure

Comprehensive practice flashcards covering protein purification methods, chromatography, electrophoresis, insulin discovery, peptide drug engineering, thermodynamics of folding, and secondary structural features.

Crude Cellular Extract

A solution prepared by lysing tissue or microbial cells, which releases all soluble intracellular proteins as the initial step in protein purification.

Fractionation

The second general step in protein purification in which a crude extract is separated into fractions based on physical properties such as molecular size or electrical charge.

Salting In

The phenomenon observed at low ionic strength where an increase in the concentration of dissolved salt ions increases protein solubility by weakening interactions between individual protein molecules.

Salting Out

The precipitation of proteins at high ionic strength caused by added salt ions outcompeting protein surfaces for water molecules, leading to protein aggregation and insolubility.

Ammonium Sulfate ($(\text{NH}_4)_2\text{SO}_4$)

A highly soluble salt commonly used in protein salting out precipitation procedures because it effectively precipitates proteins without harming or denaturing them.

Isoelectric Point ($\text{pI}$)

The specific $\text{pH}$ at which a protein carries a net electric charge of zero ($\text{pH} = \text{pI}$), representing the condition at which the protein is least soluble.

Dialysis

A purification procedure utilizing a semipermeable membrane bag to separate large macromolecules from small solute ions and exchange buffer components.

Column Chromatography

A fractionation method wherein a buffered mobile phase containing a protein mixture migrates through a stationary phase porous solid matrix packed within a column.

Stationary Phase

The solid, porous matrix packed into a chromatography column that retards the migration of specific proteins based on charge, size, or binding affinity.

Mobile Phase

The buffered solution that flows through the stationary phase of a chromatography column, carrying the protein sample with it.

Ion-Exchange Chromatography

A chromatographic method that separates proteins based on the sign and magnitude of their net electric charge at a designated buffer $\text{pH}$.

Cation Exchanger

An ion-exchange resin containing negatively charged functional groups (such as carboxymethyl) that binds positively charged proteins while allowing negatively charged proteins to elute faster.

Anion Exchanger

An ion-exchange resin functionalized with positively charged groups (such as diethylaminoethyl) that binds negatively charged proteins.

Carboxymethyl (CM)

The most frequently used functional group in cation-exchange chromatography matrices, possessing the structure $\text{Matrix-CH}_2\text{-COO}^-$.

Diethylaminoethyl (DEAE)

The most frequently used functional group in anion-exchange chromatography matrices, possessing the structure $\text{Matrix-CH}_2\text{-CH}_2\text{-NH(CH}_2\text{CH}_3)_2^+$.

Size-Exclusion Chromatography (Gel Filtration)

A chromatographic technique that separates proteins according to size and shape, where large molecules are excluded from resin pores and elute first, while small molecules enter pores and elute last.

Affinity Chromatography

A highly specific chromatography technique based on reversible noncovalent binding between a target protein and a complementary ligand covalently immobilized on a solid resin matrix.

Imidazole

A compound structurally analogous to the histidine side chain, used at increasing concentrations to elute polyhistidine-tagged proteins from nickel affinity columns by competitive displacement.

$\text{His}_6$-Tag

A genetically engineered sequence of six consecutive histidine residues appended to a recombinant protein to allow selective purification on immobilized divalent metal columns.

Immobilized Metal Affinity Chromatography (Ni-NTA / Ni-TED)

An affinity capture system where immobilized nickel ions ($\text{Ni}^{2+}$) coordinate with the imidazole rings of an engineered polyhistidine tag on a target protein.

Specific Activity

The ratio of total enzyme activity units to the total protein mass in milligrams ($\text{units/mg}$), which increases as a protein sample becomes progressively purer.

Purification Factor

The quantitative parameter calculated as the ratio of the final specific activity of a purified protein sample to the initial specific activity of the starting crude extract.

Percent Yield

The percentage of total enzyme activity recovered after a purification procedure, defined as $(\text{final activity} / \text{starting activity}) \times 100\%$.

Polyacrylamide Gel Electrophoresis (PAGE)

An analytical method that separates macromolecules within a cross-linked polyacrylamide gel matrix under the influence of an electric field based on charge-to-mass ratio and shape.

Sodium Dodecyl Sulfate (SDS)

An anionic detergent with the formula $\text{Na}^+ \, ^-\text{O-S}(=\text{O})_2\text{-O-(CH}_2)_{11}\text{CH}_3$ that denatures proteins and confers a uniform negative charge proportional to polypeptide length.

SDS-PAGE

A denaturing electrophoretic technique in which proteins treated with SDS migrate through a polyacrylamide gel strictly according to their molecular weights ($\text{M}_\text{r}$).

Electrophoretic Mobility ($\mu$)

The physical rate of migration of an ion in an electric field, given by $\mu = \frac{V}{E} = \frac{Z}{f}$, where $V$ is velocity, $E$ is potential, $Z$ is net charge, and $f$ is frictional coefficient.

Isoelectric Focusing (IEF)

An electrophoretic technique that separates proteins along a stable, immobilized $\text{pH}$ gradient until each protein reaches the specific position where its net charge equals zero ($\text{pH} = \text{pI}$).

Two-Dimensional (2D) Gel Electrophoresis

A high-resolution separation method combining isoelectric focusing along a $\text{pH}$ gradient in the first dimension with perpendicular SDS-PAGE separation by molecular weight in the second dimension.

Coomassie Blue

A dye that binds stoichiometrically to proteins in a polyacrylamide gel matrix, allowing visualization of migrated protein bands.

Average Molecular Weight of an Amino Acid

The mean molecular mass of a standard free amino acid, taken as approximately $128\,\text{Da}$.

Average Molecular Weight of an Amino Acid Residue

The estimated contribution of a single amino acid residue within a polypeptide chain ($\approx 110\,\text{Da}$), calculated by subtracting the mass of water ($18\,\text{Da}$) from the free amino acid average ($128\,\text{Da}$).

Protease

An enzyme that catalyzes the hydrolytic cleavage of peptide bonds linking amino acid residues in a polypeptide chain.

Trypsin

A pancreatic endopeptidase that hydrolyzes peptide bonds specifically on the carboxyl-terminal side of lysine ($\text{Lys}$) and arginine ($\text{Arg}$) residues.

Chymotrypsin

A pancreatic endopeptidase that specifically cleaves peptide bonds on the carboxyl-terminal side of aromatic residues: phenylalanine ($\text{Phe}$), tryptophan ($\text{Trp}$), and tyrosine ($\text{Tyr}$).

Staphylococcus aureus V8 Protease

A bacterial endoprotease that specifically cleaves peptide bonds on the carboxyl-terminal side of aspartate ($\text{Asp}$) and glutamate ($\text{Glu}$) residues.

Asp-N-Protease

A bacterial protease from Pseudomonas fragi that cleaves peptide bonds specifically on the amino-terminal ($\text{N}$-terminal) side of aspartate ($\text{Asp}$) and glutamate ($\text{Glu}$) residues.

Pepsin

A porcine stomach protease that hydrolyzes peptide bonds on the amino-terminal ($\text{N}$-terminal) side of leucine ($\text{Leu}$), phenylalanine ($\text{Phe}$), tryptophan ($\text{Trp}$), and tyrosine ($\text{Tyr}$).

Endoproteinase Lys C

A bacterial protease from Lysobacter enzymogenes that cleaves peptide bonds strictly on the carboxyl-terminal side of lysine ($\text{Lys}$) residues.

Cyanogen Bromide ($\text{CNBr}$)

A chemical reagent that cleaves polypeptide chains specifically on the carboxyl-terminal side of methionine ($\text{Met}$) residues.

Polypeptide Fragmenting Specificity (Table 3-6)

The catalog of reagent cleavage points showing target residues and cleavage orientations ($\text{C}$ vs. $\text{N}$ terminus) used to digest polypeptide chains for sequence analysis.

280 nm UV Spectrophotometry

The analytical detection method for measuring protein concentration based on light absorption by conjugated double bonds in aromatic amino acid side chains.

Tryptophan ($\text{Trp}$)

The aromatic amino acid with an indole side chain that exhibits the highest molar absorption of ultraviolet light in the $270\text{--}280\,\text{nm}$ range.

Conservative Substitution

An amino acid replacement in homologous proteins that maintains similar chemical properties, size, and charge, such as replacing serine ($\text{Ser}$) with threonine ($\text{Thr}$).

Micelle

A thermodynamically stable spherical aggregate formed by amphiphilic molecules in water, with polar groups oriented outward and nonpolar hydrocarbon chains sequestered internally.

2-Mercaptoethanol ($\beta\text{-ME}$)

A reducing agent commonly used to reduce and cleave covalent disulfide bonds between cross-linked polypeptide chains.

Urushiol

The allergenic alkyl catechol component of poison ivy ($\text{p}K_\text{a} = 8$), most effectively removed by washing with basic baking soda ($\text{NaHCO}_3$) to convert it to an ionized, water-soluble form.

Buffering Region

The relatively flat zone of a titration curve spanning $\text{pH} = \text{p}K_\text{a} \pm 1$, over which a weak acid-conjugate base pair effectively resists changes in $\text{pH}$.

PIPES Buffer

A biological buffer with a dissociation constant of $K_\text{a} = 1.74 \times 10^{-7}$ ($\text{p}K_\text{a} \approx 6.76$), making it optimal for maintaining enzymatic activity at $\text{pH } 6.8$.

Frederick Banting

The Canadian surgeon who conceived the idea to isolate internal pancreatic secretions and led the research effort that successfully discovered insulin.

Charles Best

The medical student who assisted Dr. Frederick Banting with experimental surgeries and glucose testing during the initial discovery of insulin at the University of Toronto in 1921.

John Macleod

The physiology professor at the University of Toronto who provided laboratory space, experimental animals, and resources to Banting and Best.

James Collip

A visiting biochemist from the University of Alberta who developed the critical fractional precipitation protocol to purify crude pancreatic extracts into clinical-grade insulin.

Frederick Banting and Charles Best

The co-discoverers of insulin photographed together around 1924 following their historic breakthrough at the University of Toronto.

GLP-1(7-37)

An endogenous incretin peptide hormone that stimulates glucose-dependent insulin secretion, having a very short in vivo elimination half-life of $t_{1/2} \approx 2\,\text{minutes}$.

Dipeptidyl Peptidase-4 (DPP-4)

The circulating protease that rapidly inactivates native $\text{GLP-1}$ by cleaving the $N$-terminal dipeptide after alanine at position 8 ($\text{Ala}^8$).

Liraglutide

A once-daily acylated $\text{GLP-1}$ receptor agonist featuring a fatty acid modification on lysine, resulting in an extended elimination half-life of $t_{1/2} = 13\,\text{hours}$.

Semaglutide

A long-acting $\text{GLP-1}$ agonist engineered with $\alpha$-aminoisobutyric acid ($\text{Aib}$) at position 8 and a $\text{C-18}$ fatty acid chain at $\text{Lys}^{26}$, resulting in a half-life of $t_{1/2} = 1\,\text{week}$.

$\alpha$-Aminoisobutyric Acid (Aib)

A non-proteinogenic amino acid incorporated at position 8 of semaglutide to sterically prevent cleavage and inactivation by $\text{DPP-4}$.

$\text{C-18}$ Fatty Diacid Modification

The lipophilic side chain attached via an $\text{L-}\gamma\text{-Glu}$ spacer to lysine 26 in semaglutide that mediates reversible binding to albumin, preventing rapid renal clearance.

Pancreatic Islets (Islets of Langerhans)

Endocrine clusters within the pancreas containing specialized alpha and beta cells that synthesize and release regulatory metabolic hormones directly into the bloodstream.

Beta Cells

Endocrine cells located in the pancreatic islets responsible for synthesizing and secreting insulin in response to elevated blood glucose levels.

Alpha Cells

Endocrine cells in the pancreatic islets that synthesize and secrete glucagon to raise circulating glucose levels.

Acinar Cells

Exocrine cells of the pancreas arranged in clusters that produce and secrete digestive enzymes into the pancreatic duct system feeding into the duodenum.

Protein Conformation

The spatial arrangement of atoms in a protein that can be altered without breaking covalent bonds, primarily mediated by rotations about single bonds.

Native State

The biologically active, fully folded three-dimensional structure of a protein that occupies the thermodynamic state of lowest Gibbs free energy ($G$).

Conformational Entropy

The high thermodynamic entropy associated with the multitude of random coil conformations in an unfolded polypeptide chain, which opposes folding into a defined structure.

Solvation Layer

A structured, ice-like cage of water molecules surrounding exposed nonpolar solute groups; its collapse upon protein folding releases ordered water and drives folding via an entropy increase.

Hydrophobic Effect (Protein Folding)

The predominant noncovalent thermodynamic driving force that stabilizes globular protein folds by sequestering nonpolar amino acid side chains within an internal core away from water.

Peptide Bond Resonance

The electronic delocalization between the carbonyl oxygen and the amide nitrogen that gives the peptide bond approximately $40\%$ double-bond character.

Peptide Electric Dipole

The small electrostatic dipole moment generated across a peptide bond due to the partial negative charge on the carbonyl oxygen and the partial positive charge on the amide nitrogen.

Planar Peptide Group

The rigid structural unit comprising six atoms ($\text{C}_\alpha\text{--C}(=\text{O})\text{--N}(\text{--H})\text{--C}_\alpha$) constrained to lie within a single plane because of restricted rotation around the peptide bond.

Dihedral Angle (Torsion Angle)

The rotation angle between two intersecting planes defined by four sequentially bonded backbone atoms ($\text{A--B--C--D}$) in a polypeptide chain.

Phi Angle ($\phi$)

The backbone dihedral angle describing rotation around the single covalent bond between the alpha carbon and the amide nitrogen ($\text{C}_\alpha\text{--N}$).

Psi Angle ($\psi$)

The backbone dihedral angle describing rotation around the single covalent bond between the alpha carbon and the carbonyl carbon ($\text{C}_\alpha\text{--C}$).

Fully Extended Conformation

The idealized staggered polypeptide backbone conformation where both dihedral angles $\phi$ and $\psi$ are defined as $180^\circ$ (or $-180^\circ$).

Steric Interference

The van der Waals overlap between backbone atoms or side chain atoms that restricts rotational freedom and forbids many combinations of $\phi$ and $\psi$, including $\phi = 0^\circ, \psi = 0^\circ$.

Ramachandran Plot

A two-dimensional graphical diagram plotting $\psi$ on the y-axis against $\phi$ on the x-axis to display sterically allowed backbone dihedral angle conformations.

Idealized Dihedral Angles (Table 4-1)

The defined theoretical pairs of $\phi$ and $\psi$ values characteristic of standard regular secondary structures including helices, sheets, and turns.

Secondary Structure

The local three-dimensional spatial arrangement of the main-chain atoms in a defined segment of a polypeptide chain, independent of side chain conformations.

Regular Secondary Structure

A secondary structure segment in which the dihedral angles $\phi$ and $\psi$ remain identical or repetitive throughout the entire sequence.

Alpha Helix ($\alpha$ Helix)

A ubiquitous rod-like regular secondary structure characterized by a tightly coiled backbone around an imaginary central axis with outward-projecting amino acid side chains.

Helical Pitch

The linear axial distance traveled along an $\alpha$ helix per complete $360^\circ$ turn, equal to exactly $5.4\,\text{Å}$ ($0.54\,\text{nm}$).

Helical Residues per Turn ($N$)

The precise number of amino acid residues needed to complete one full turn of an $\alpha$ helix, defined as $3.6$ residues.

Rise per Residue

The linear advance along the longitudinal axis of an $\alpha$ helix contributed by each residue, calculated as $(5.4\,\text{Å}) / (3.6) = 1.5\,\text{Å}$ per residue.

Intrahelical Hydrogen Bond

The primary stabilizing interaction of an $\alpha$ helix formed between the carbonyl oxygen of residue $n$ and the electronegative amide nitrogen proton of residue $n+4$.

Right-Handed $\alpha$ Helix

The dominant helical conformation observed in native proteins, characterized by idealized dihedral angles of $\phi = -57^\circ$ and $\psi = -47^\circ$.

Left-Handed $\alpha$ Helix

An extended helical conformation with dihedral angles near $\phi = +60^\circ$ and $\psi = +45^\circ$ that is theoretically less stable due to steric strain and rarely found in natural proteins.

Proline Kink

A structural destabilization in an $\alpha$ helix induced by proline because its rigid pyrrolidine ring restricts $\text{N--C}_\alpha$ rotation and lacks a backbone amide proton for hydrogen bonding.

Helix Breakers

Amino acids that occur infrequently within $\alpha$ helices; specifically, proline due to its rigid ring structure and glycine due to its excessive conformational flexibility.

Beta Conformation ($\beta$ Conformation)

An extended, pleated secondary structure where the polypeptide backbone follows a zigzag path with side chains alternating above and below the sheet plane.

Beta Strand ($\beta$ Strand)

A single stretch of amino acids arranged in an extended zigzag conformation that associates laterally with adjacent strands to form a $\beta$ sheet.

Beta Sheet ($\beta$ Sheet)

A secondary structure formed by interstrand hydrogen bonds linking the backbone carbonyl and amide groups of multiple adjacent $\beta$ strands.

Antiparallel $\beta$ Sheet

A $\beta$ sheet in which adjacent strands run in opposite $N$-to-$C$ directions ($\phi = -139^\circ, \psi = +135^\circ$), yielding linear, collinear hydrogen bonds and a repeat period of $7\,\text{Å}$.

Parallel $\beta$ Sheet

A $\beta$ sheet in which adjacent strands run in the same $N$-to-$C$ direction ($\phi = -119^\circ, \psi = +113^\circ$), resulting in bent, distorted hydrogen bonds and a repeat period of $6.5\,\text{Å}$.

$\beta$ Sheet Strand Connections

The loop connections linking adjacent strands in a pleated sheet, which can be direct and short in antiparallel sheets or longer crossover loops in parallel sheets.

Beta Turn ($\beta$ Turn)

A four-residue structural motif executing a tight $180^\circ$ reversal in polypeptide direction that connects the ends of two adjacent strands in an antiparallel $\beta$ sheet.

Type I $\beta$ Turn

A $\beta$ turn with dihedral angles of $\phi = -60^\circ, \psi = -30^\circ$ at position $i+1$ that frequently features a proline residue at position 2 ($i+1$).

Type II $\beta$ Turn

A $\beta$ turn characterized by dihedral angles of $\phi = +80^\circ, \psi = 0^\circ$ at position $i+2$ that almost invariably contains a glycine residue at position 3 ($i+2$).

Collagen Triple Helix

A structural motif composed of three intertwined left-handed helical chains with characteristic backbone dihedral angles of $\phi = -51^\circ$ and $\psi = +153^\circ$.