Chapter 3: Amino Acids, Peptides, and Proteins

Amino Acids: Fundamental Building Blocks of Proteins

• Proteins are linear heteropolymers composed of $\alpha$-amino acids linked together in a specific sequence.

• Amino acids possess chemical properties that make them uniquely suited to fulfill diverse biological functions:

  • Capacity to polymerize into long polypeptide chains.

  • Useful acid-base properties across physiological and extreme $pH$ ranges.

  • Varied physical properties, including solubility, polarity, and steric volume.

  • Diverse chemical functionality provided by distinct side chains ($R$ groups).

• Definition of an Amino Acid Residue:

  • According to IUPAC definition, when two or more amino acids combine to form a peptide, the elements of water ($H_2O$) are removed.

  • What remains of each amino acid unit is termed an amino-acid residue.

  • Residues in a peptide chain lack a hydrogen atom of the amino group ($-NH-CHR-COOH$), the hydroxyl moiety of the carboxyl group ($NH_2-CHR-CO-$), or both ($-NH-CHR-CO-$).

• Condensation vs. Hydrolysis Reactions:

  • Formation of peptide bonds occurs via a condensation reaction where water is eliminated.

  • Cleavage of peptide bonds occurs via a hydrolysis reaction, which represents the exact reverse of condensation.

• General Structure of $\alpha$-Amino Acids:

  • Every amino acid contains a central carbon known as the $\alpha$-carbon ($C_a$).

  • Attached covalently to the $\alpha$-carbon are four groups: a primary amino group (or secondary in proline), a carboxylic acid group, a hydrogen atom, and a distinctive side chain ($R$ group).

  • There are $20$ standard amino acids commonly found in all ribosomal proteins, each defined by its unique $R$ group.

Nomenclature, Carbon Numbering, and Stereochemistry of Amino Acids

• Carbon Nomenclature Systems:

  • Standard Numerical System: Carbon atoms are numbered starting from the carboxylic acid carbon as carbon-1 ($C-1$), the $\alpha$-carbon as $C-2$, followed sequentially along the side chain as $C-3$, $C-4$, $C-5$, $C-6$, etc.

  • Greek Letter Designation System: Carbon atoms are designated starting from the carbon adjacent to the carboxyl group, designated as $\alpha$ ($C_a$), followed down the side chain by $\beta$, $\gamma$, $\delta$, $\epsilon$, etc.

  • Example (Lysine): Carbon-1 is the carboxyl carbon ($-COO^-$), Carbon-2 ($C_a$) carries the primary $\alpha$-amino group ($-NH_3^+$), Carbon-3 ($\beta$) is a $-CH_2-$ group, Carbon-4 ($\gamma$) is a $-CH_2-$ group, Carbon-5 ($\delta$) is a $-CH_2-$ group, and Carbon-6 ($\epsilon$) carries the side-chain amino group ($-NH_3^+$).

• The $D,L$ System of Stereochemistry:

  • The $D,L$ stereochemical nomenclature system relates the absolute configuration of a chiral molecule to the reference molecule glyceraldehyde.

  • $L$-glyceraldehyde rotates plane-polarized light in the levorotatory (left) direction.

  • $D$-glyceraldehyde rotates plane-polarized light in the dextrorotatory (right) direction.

  • The $D$ and $L$ designations specify absolute spatial configuration and do NOT consistently indicate the physical direction of optical rotation of plane-polarized light; actual optical rotation must be measured experimentally.

  • All $\alpha$-amino acids found naturally in proteins possess the $L$ stereochemistry.

• The $R/S$ Cahn-Ingold-Prelog System:

  • When applying the absolute $R/S$ configuration rules to the chiral standard amino acids, $18$ of the $19$ chiral amino acids have the $(S)$ configuration.

  • Glycine is achiral because its $\alpha$-carbon is bound to two identical hydrogen atoms.

  • Cysteine is the sole exception among chiral amino acids; $L$-cysteine has the $(R)$ configuration because the sulfur-containing side chain ($-CH_2SH$) takes priority over the carboxyl group ($-COOH$) under Cahn-Ingold-Prelog priority rules.

• Amino Acids with Multiple Chiral Centers:

  • Two standard amino acids possess a second chiral center in their side chains: Isoleucine and Threonine.

Classification and Chemical Properties of Standard Amino Acid Side Chains

• Nonpolar, Aliphatic Amino Acids:

  • Characteristics: Lack polar functional groups; drive protein stability and folding through hydrophobic interactions; predominantly buried within the interior of folded globular proteins away from water.

  • Glycine ($Gly$, $G$): The only achiral amino acid; possesses the smallest side chain (a single hydrogen atom, $-H$); does not contribute hydrophobic driving force; provides high conformational flexibility to polypeptide backbones; derived from Greek glykos ("sweet") due to its taste; commonly utilized as a buffer component.

  • Alanine ($Ala$, $A$): Possesses a simple methyl side chain ($-CH_3$).

  • Valine ($Val$, $V$): Possesses a branched aliphatic side chain ($-CH(CH_3)_2$).

  • Leucine ($Leu$, $L$): Possesses a branched aliphatic side chain ($-CH_2CH(CH_3)_2$).

  • Isoleucine ($Ile$, $I$): Possesses a branched aliphatic side chain ($-CH(CH_3)CH_2CH_3$) containing a second chiral center.

  • Methionine ($Met$, $M$): Contains a nonpolar thioether side chain ($-CH_2-CH_2-S-CH_3$); hydrophobic in nature; unlike a thiol group, a thioether cannot undergo oxidation to form covalent disulfide bonds.

  • Proline ($Pro$, $P$): Features a distinctive cyclic structure where the aliphatic side chain forms a covalent ring with the backbone $\alpha$-amino group, forming a secondary amino (imino) group; holds polypeptide backbones in a rigid conformation that restricts structural flexibility; lacks a hydrogen atom on the backbone nitrogen atom when incorporated into peptide bonds, limiting its hydrogen-bonding capacity.

• Aromatic R Groups:

  • Characteristics: Relatively nonpolar; participate in hydrophobic interactions; engage in $\pi-\pi$ stacking interactions with other aromatic rings.

  • Phenylalanine ($Phe$, $F$): Nonpolar aromatic side chain containing a phenyl ring.

  • Tyrosine ($Tyr$, $Y$): Contains a phenolic hydroxyl group ($-OH$); more polar than phenylalanine; can form hydrogen bonds; features an ionizable side chain with $pK_a = 10.5$; named from Greek tyros ("cheese") after first being isolated from cheese.

  • Tryptophan ($Trp$, $W$): Bulky, nonpolar indole-like side chain; slightly more polar than phenylalanine.

  • Ultraviolet Light Absorption:

  • Aromatic side chains, predominantly Tryptophan and Tyrosine (with minor contribution from Phenylalanine), absorb UV light in the wavelength range of $270\,nm$ to $280\,nm$.

  • Absorption spectra plots display Absorbance ($A$) on the y-axis versus Wavelength (in $nm$) on the x-axis.

  • Protein concentration ($c$) in solution is determined via UV-visible spectrophotometry using Beer's Law:

$A = \varepsilon \cdot c \cdot l$

where $A$ is the measured absorbance, $\varepsilon$ is the molar extinction coefficient, $c$ is molar concentration, and $l$ is the optical path length.

• Polar Uncharged R Groups:

  • Characteristics: Soluble in water due to functional groups that readily form hydrogen bonds with aqueous solvent.

  • Serine ($Ser$, $S$): Contains a primary hydroxyl side chain ($-CH_2OH$).

  • Threonine ($Thr$, $T$): Contains a secondary hydroxyl side chain ($-CH(OH)CH_3$) with a second chiral center.

  • Cysteine ($Cys$, $C$): Contains a sulfhydryl (thiol) group ($-CH_2SH$).

  • The sulfhydryl group is less polar than a hydroxyl group; acts as a weak acid ($pK_a = 8.1$); can form dipole-dipole interactions with oxygen or nitrogen.

  • Oxidation of two cysteine sulfhydryl groups yields a covalent disulfide bond ($-S-S-$), forming the dimeric amino acid cystine.

  • Disulfide cross-links stabilize tertiary and quaternary protein structures across different chains or within a single chain.

  • Asparagine ($Asn$, $N$) and Glutamine ($Gln$, $Q$): Amide derivatives of aspartate and glutamate.

  • Glutamine possesses one additional methylene group ($-CH_2-$) compared to asparagine.

  • Single-letter codes follow alphabetical order by side-chain length: $N$ for Asn, $Q$ for Gln.

• Positively Charged (Basic) R Groups:

  • Characteristics: Possess side chains with net positive charges at physiological $pH\approx 7.0$.

  • Lysine ($Lys$, $K$): Features a primary amino group at the $\epsilon$-position ($pK_a \approx 10.5$).

  • Arginine ($Arg$, $R$): Contains a positively charged guanidino group ($pK_a \approx 12.5$); one-letter abbreviation is $R$.

  • Histidine ($His$, $H$): Contains an ionizable imidazole ring with $pK_a \approx 6.0$.

  • Histidine is the only standard amino acid with a side chain $pK_a$ near physiological $pH$, allowing it to shift between protonated (charged) and unprotonated (neutral) states to catalyze enzymatic acid-base reactions.

• Negatively Charged (Acidic) R Groups:

  • Characteristics: Possess side chains with net negative carboxylic acid charges at physiological $pH\approx 7.0$.

  • Aspartic Acid / Aspartate ($Asp$, $D$): Carboxyl side chain ($-CH_2COO^-$).

  • Glutamic Acid / Glutamate ($Glu$, $E$): Carboxyl side chain containing one additional methylene group ($-CH_2CH_2COO^-$).

  • Single-letter codes follow alphabetical order by side-chain length: $D$ for Asp, $E$ for Glu.

Non-Standard, Modified, and Biological Amino Acids

• Nutritional Classification and Auxotrophy:

  • Auxotrophy: The inability of an organism to synthesize a specific organic compound necessary for its growth and development.

  • Humans are auxotrophic for essential amino acids and must acquire them through diet.

  • Prototrophy: The ability of organisms (e.g., plants) to synthesize all required organic compounds from inorganic precursors.

  • Complete (Whole) Proteins: Animal protein sources (milk, eggs, dairy) supply all essential amino acids in proportions matching human nutritional needs.

  • Complementary Plant Proteins: Most plant items lack adequate proportions of specific essential amino acids (e.g., Rice is deficient in Lysine; Beans are deficient in Methionine; ingested together, they form a complete protein profile).

• Cotranslationally Incorporated Non-Standard Amino Acids:

  • Selenocysteine ($Sec$, $U$):

  • Considered the "21st amino acid"; cotranslationally incorporated during protein synthesis across all three domains of life; present in at least $25$ human proteins.

  • Contains selenium ($-SeH$ group) instead of sulfur.

  • Side chain $pK_a = 5.2$, making it $99\%$ deprotonated at neutral physiological $pH$

  • Incorporation mechanism in prokaryotes: Context-dependent suppression of the opal ($UGA$) stop codon.

• Pyrrolysine ($Pyl$, $O$):

  • First identified in methanogenic archaea; encoded by the amber ($UAG$) stop codon.

• Post-Translationally Modified Amino Acids:

  • Formed via enzymatic modification after the polypeptide chain has been synthesized on the ribosome:

  • $4$-hydroxyproline: Found in plant cell walls and collagen structural fibers.

  • $5$-hydroxylysine: Found in collagen structural fibers.

  • $6-N$-methyllysine: Found in muscle contractile protein myosin.

  • $\gamma$-carboxyglutamate: Found in blood-clotting protein prothrombin and other calcium-binding ($Ca^{2+}$) proteins.

  • Desmosine: A complex derivative found in the fibrous protein elastin.

  • Reversible / Transient Modifications for Protein Regulation:

  • Phosphorylation: Most common regulatory modification; targets hydroxyl groups of Serine, Threonine, and Tyrosine.

  • Acetylation: $6-N$-acetyllysine found in histone proteins; regulates gene expression and chromatin structure at an epigenetic level.

  • Other transient groups: Methyl, adenylyl, and ADP-ribosyl groups.

• Non-Protein Amino Acid Intermediates:

  • Ornithine and Citrulline: Metabolic intermediates in the biosynthesis of arginine and the urea cycle; not incorporated into proteins during translation.

Acid-Base Chemistry and Ionization Behavior of Amino Acids

• Polyprotic Acid Behavior:

  • Amino acids act as weak polyprotic acids containing at least two ionizable protons, each with a distinct $pK_a$ value.

  • $\alpha$-Carboxylic Acid Dissociation:

$COOH \rightleftharpoons COO^- + H^+$

• $\alpha$-Amino Group Dissociation:

$NH_3^+ \rightleftharpoons NH_2 + H^+$

• At strongly acidic $pH$: Amino acids exist in a fully protonated cationic form ($net\ charge = +1$ or greater).

• At strongly basic $pH$: Amino acids exist in a fully deprotonated anionic form ($net\ charge = -1$ or lower).

• At intermediate $pH$: Amino acids exist as dipolar zwitterions carrying both a positive and a negative charge ($net\ charge = 0$).

• Amphoteric Nature:

  • Zwitterions can donate a proton (acting as an acid) or accept a proton (acting as a base); compounds with this dual capability are termed amphoteric or amphiolytes.

• Chemical Environment Effects on $pK_a$:

  • The $pK_a$ of an $\alpha$-carboxyl group is $\approx 2.3$, significantly lower than carboxylic acids like acetic acid ($pK_a = 4.8$).

  • Explanation: The positively charged $\alpha$-amino group ($-NH_3^+$) exerts an electron-withdrawing inductive effect on the carboxyl group, stabilizing the negatively charged carboxylate form and lowering its $pK_a$

  • The $pK_a$ of an $\alpha$-amino group is $\approx 9.6$, slightly lower than simple primary amines like methylamine ($pK_a = 10.6$).

  • Explanation: Electronegative oxygen atoms in the carboxyl group withdraw electron density from the amino group, facilitating proton dissociation and lowering its $pK_a$

  • Ionization State Thresholds:

  • Above $pH = 3.5$, $\alpha$-carboxyl groups are entirely in their carboxylate ($-COO^-$) form.

  • Below $pH = 8.0$, $\alpha$-amino groups are entirely in their protonated ammonium ($-NH_3^+$) form.

Titration Curves and Isoelectric Point Determination

• Henderson-Hasselbalch Equation:

  • Quantitative relationship between $pH$, $pK_a$, and conjugate base/acid ratio:

$pH = pK_a + \log\left(\frac{[A^-]}{[AH]}\right)$

• Isoelectric Point ($pI$):

  • The characteristic $pH$ at which the net electric charge of an amino acid or peptide is exactly zero.

  • Calculation for Diprotic Amino Acids (e.g., Glycine):

$pI = \frac{1}{2}(pK_1 + pK_2)$

• For Glycine:

$pK_1 (\alpha\text{-COOH}) = 2.34$

$pK_2 (\alpha\text{-NH}_3^+) = 9.60$

$pI = \frac{1}{2}(2.34 + 9.60) = 5.97$

• Physical Properties at the Isoelectric Point ($pH = pI$):

  • Net electric charge equals zero.

  • Amino acid exhibits minimum solubility in aqueous solvent.

  • Molecule experiences zero net directional migration within an applied electric field.

• Net Charge Dependencies:

  • When $pH > pI$, the molecule carries a net negative charge.

  • When $pH < pI$, the molecule carries a net positive charge.

Formation, Structure, and Nomenclature of Peptides

• Peptide Bond Formation:

  • Formed via a condensation reaction (nucleophilic acyl substitution).

  • The unprotonated $\alpha$-amino group of one amino acid acts as a nucleophile, attacking the $\alpha$-carboxyl carbon of another amino acid to displace a hydroxyl group, releasing $H_2O$

  • While amino groups are effective nucleophiles, hydroxyl groups are poor leaving groups; thus, at physiological $pH$, this condensation reaction does not proceed spontaneously to a significant extent without energy input and enzymatic catalysis.

• Size Classifications:

  • Dipeptide: Contains $2$ amino acid residues.

  • Tripeptide: Contains $3$ amino acid residues.

  • Oligopeptide: General term for short peptide chains containing $4$ to $10$ amino acid residues.

  • Polypeptide: Chain containing greater than $10$ amino acid residues.

  • Protein: Polypeptide chain containing hundreds or thousands of amino acid residues with molecular mass ($M_w$) exceeding $10\,kDa$

• Naming and Conventions for Peptides:

  • Sequence Directionality: Written and read from the N-terminus to the C-terminus (left to right).

  • N-Terminus (Amino Terminus): Residue with the free $\alpha$-amino group ($-NH_3^+$).

  • C-Terminus (Carboxyl Terminus): Residue with the free $\alpha$-carboxylate group ($-COO^-$).

  • Full Systematic Nomenclature: Systematic names link constituent amino acids sequentially from N-terminus to C-terminus, changing suffixes -ine or -ate to -yl, leaving the C-terminal residue name unchanged.

  • Three-Letter Notation: Linked with dashes (e.g., $Gly-Ala-Leu$).

  • One-Letter Notation: Linked continuously without dashes (e.g., $GAL$).

Biologically Active Peptides and Protein Complexity

• Peptide Ionization Behavior:

  • Peptides carry only one free ionizable $\alpha$-amino group at the N-terminus and one free ionizable $\alpha$-carboxyl group at the C-terminus.

  • Total charge is determined by terminal groups and the number of ionizable $R$ groups.

  • Side chain $pK_a$ values buried within folded protein environments differ from values of free amino acids.

• Examples of Biologically Active Peptides:

  • Commercial Sweetener: Aspartame ($L\text{-Aspartyl-}L\text{-phenylalanine methyl ester}$).

• Vertebrate Hormones & Pheromones: Insulin, Oxytocin.

• Neuropeptides: Substance P (mediates pain perception signal transmission).

• Antimicrobial Peptides:

  • Polymyxin B: Active against Gram-negative bacteria (produced by Bacillus polymyxa).

  • Bacitracin: Active against Gram-positive bacteria (produced by Bacillus subtilis).

  • Dermcidin-1L: Human antimicrobial defense peptide (PDB ID: 2KSG).

• Potent Toxins: Amanitin (death cap mushrooms), Conotoxin (cone snails), Chlorotoxin (scorpions).

  • Protein Composition and Structural Components:

• Polypeptides consist of covalently linked $\alpha$-amino acid residues, sometimes bound to non-amino acid components:

  • Cofactors: Functional non-amino acid components, such as metal ions ($Fe^{2+}$, $Mg^{2+}$, $Zn^{2+}$) or small organic molecules.

  • Coenzymes: Complex organic cofactors (e.g., $NAD^+$ in lactate dehydrogenase).

  • Prosthetic Groups: Covalently or tightly bound cofactors essential for activity (e.g., heme group in myoglobin).

• Multisubunit Proteins: Contain two or more polypeptide chains associated noncovalently.

• Oligomeric Proteins: Multisubunit proteins in which at least two polypeptide chains are identical.

• Protomers: Identical repeating structural units consisting of one or more polypeptide chains in an oligomeric protein.

  • Example: Hemoglobin contains two $\alpha$ chains and two $\beta$ chains ($\alpha_2\beta_2$); can be defined as a tetramer of four subunits or a dimer of $\alpha\beta$ protomers.

• Conjugated Proteins: Proteins containing permanently associated non-amino acid chemical groups.

  • Estimation of Amino Acid Residue Count:

• The average molecular weight ($M_w$ or $M_r$) of an amino acid residue in a protein is estimated at $110\,Da$ ($110\,g/mol$).

  • Derived from average amino acid mass ($128\,Da$) weighted for abundance of smaller amino acids, minus $18\,Da$ for water lost during condensation.

$\text{Estimated Number of Residues} = \frac{\text{Protein } M_w \text{ (in Da)}}{110\,Da/\text{residue}}$

Methods for Protein Purification and Separation

• Physicochemical Properties Exploited in Purification:

  • Size, charge, binding specificity, solubility, primary sequence, molecular shape, hydrophobicity, thermal stability.

• Initial Fractionation and Centrifugation:

  1. Cell Lysis: Mechanically or chemically break open cells to produce a crude lysate extract.

  2. Differential Centrifugation: Spin extract at increasing gravitational force to pellet structural rubbish or isolate subcellular organelles.

• Fractional Precipitation ("Salting Out"):

  • Protein solubility varies with $pH$, temperature, and ionic strength.

  • Addition of high salt concentration (typically ammonium sulfate, $(NH_4)_2SO_4$) reduces water available to hydrate protein surfaces, causing proteins to selectively precipitate out of solution.

  • Dialysis: Semipermeable membrane tubing allows small salt ions to diffuse out while retaining large protein macromolecules.

• Column Chromatography Techniques:

  • Stationary Phase: Solid matrix material packed into a cylindrical column.

  • Mobile Phase: Buffered liquid phase carrying protein sample through matrix.

  • Fractions are collected continuously, and protein concentration is monitored by UV absorbance at $280\,nm$ ($A_{280}$).

  • Ion-Exchange Chromatography:

  • Separates proteins based on net surface charge sign and magnitude at a given $pH$

  • Anion Exchangers: Positively charged resin matrix that binds negatively charged proteins.

  • Cation Exchangers: Negatively charged resin matrix that binds positively charged proteins.

  • Elution: Performed by applying a salt gradient ($NaCl$) or $pH$ shift in mobile phase.

  • Gel Filtration / Size-Exclusion Chromatography:

  • Separates proteins according to size and hydrodynamic radius.

  • Column packed with engineered porous beads.

  • Small proteins enter porous interior, delaying their transit.

  • Large proteins are excluded from pores and travel rapidly through interstitial spaces, eluting first.

  • Affinity Chromatography:

  • Separates proteins based on highly specific binding interactions with ligands immobilized on resin matrix beads.

  • Bound proteins are eluted using high concentrations of free ligand or altered ionic strength.

  • High-Performance Liquid Chromatography (HPLC):

  • High-pressure mechanical pumps drive mobile phase through fine particle matrix columns.

  • Reduces transit time and diffusional broadening, resulting in high chromatographic resolution.

• Enzyme Activity vs. Specific Activity:

  • Activity: Total units of enzyme activity present in a solution.

  • Specific Activity: Number of enzyme units per milligram of total protein ($\text{units}/\text{mg}$).

  • Specific Activity measures sample purity; it increases progressively as unwanted proteins are removed during purification.

Analytical Techniques: Electrophoresis and Isoelectric Focusing

• Polyacrylamide Gel Electrophoresis (PAGE):

  • Analytical separation based on the migration of charged protein molecules through a polyacrylamide gel matrix within an applied electric field.

• Native PAGE:

  • Performed under nondenaturing conditions preserving native folded states.

  • Noncovalent interactions, tertiary structure, and disulfide bonds remain intact.

  • Migration velocity depends complexly on net charge, physical size, and overall molecular shape.

  • Used to check protein sample purity and analyze intact protein-protein complexes.

• SDS-PAGE (Sodium Dodecyl Sulfate PAGE):

  • Analytical denaturing method used to estimate molecular weight ($M_w$) and purity.

  • Detergent Chemical Structure: Sodium dodecyl sulfate ($SDS$).

• Action of SDS: Binds proteins at a uniform ratio of approximately one $SDS$ molecule per two amino acid residues.

• Charge Masking and Unfolding: The high negative charge of $SDS$ overwhelms intrinsic protein charges, conferring a uniform mass-to-charge ratio and rod-like unfolded conformation to all proteins.

• Separation Mechanism: Proteins separate almost purely by molecular mass, with smaller proteins moving faster through polyacrylamide gel pores.

• Plotting $\log(M_r)$ versus relative migration distance ($R_f$) yields a linear plot for determining unknown protein molecular weights.

  • Isoelectric Focusing (IEF):

• Separates proteins according to their individual isoelectric points ($pI$).

• Protein mixture is loaded onto a gel strip containing a stable immobilized $pH$ gradient.

• Under an applied electric field, proteins migrate until reaching the exact $pH$ matching their $pI$

• At $pH = pI$, net charge equals zero ($net\ charge = 0$), halting further migration.

  • Two-Dimensional (2D) Electrophoresis:

• Sequentially combines Isoelectric Focusing (First Dimension, separation by $pI$) and SDS-PAGE (Second Dimension, separation by $M_w$ perpendicular to the first strip).

• Resolves complex protein mixtures containing thousands of distinct protein species.

Protein Structure Hierarchy and Chemical Cleavage / Modification

• Hierarchical Levels of Protein Structure:

  • Primary Structure: Linear amino acid sequence joined covalently by peptide bonds.

  • Secondary Structure: Regular local spatial arrangements of backbone atoms (e.g., $\alpha$-helices, $\beta$-sheets), defined by specific backbone dihedral angles ($\phi$ and $\psi$) and repeating hydrogen bonds between main-chain amide groups.

  • Tertiary Structure: Complete three-dimensional spatial folding of a polypeptide chain, driven by hydrophobic core burial and stabilized by noncovalent interactions (salt bridges, hydrogen bonds) and covalent disulfide bonds.

  • Quaternary Structure: Three-dimensional arrangement of multiple polypeptide chains (subunits) in a multisubunit protein complex.

• Cleavage and Modification of Disulfide Bonds:

  • Oxidation with Performic Acid: Irreversibly cleaves disulfide bonds ($-S-S-$), converting cystine into two cysteic acid residues ($-SO_3^-$).

  • Reduction with Thiol Reagents: Reversibly reduces disulfide bonds back to free sulfhydryl ($-SH$) groups using dithiothreitol (DTT) or $\beta$-mercaptoethanol ($\beta\text{-ME}$).

  • Alkylation with Iodoacetate: Follows reduction to covalently modify reactive $-SH$ groups via carboxymethylation, preventing re-oxidation and re-formation of disulfide bonds.

• Identification of N-Terminal Amino Residues:

  • Reagents:     a) $1$-fluoro-$2,4$-dinitrobenzene (FDNB, Sanger's reagent).     b) Dansyl chloride.     c) Dabsyl chloride.

  • Mechanism: Reagents react specifically with primary unprotonated amines to covalently label the $\alpha$-amino group at the N-terminus. Peptide hydrolytic cleavage releases the labeled N-terminal derivative, which is identified chromatographically.

• Enzymatic Cleavage: Proteases catalyze hydrolytic cleavage of peptide bonds at specific amino acid sequence sites.

Protein Sequencing Methods: Edman Degradation and Mass Spectrometry

• Edman Degradation (Classical Chemical Method):

  • Performs cyclic rounds of N-terminal amino acid modification, selective cleavage, and chromatographic identification.

  • Sequences short peptide chains step-by-step from the N-terminus.

• Mass Spectrometry (Modern High-Precision Method):

  • Measures molecular mass of intact proteins and peptide fragments with high accuracy.

  • Sequences short peptides ($20$ to $30$ residues), identifies post-translational modifications, and analyzes complete cellular proteomes.

  • Four General Steps in Mass Spectrometry:

  1. Ionization: Analytes are converted into gas-phase ions in a high vacuum.

  2. Acceleration: Charged ions are introduced into electric and/or magnetic fields.

  3. Field Separation: Charged ions drift or orbit through fields as a function of mass-to-charge ratio ($m/z$).

  4. Detection & Calculation: Ion trajectories translate into precise mass ($m$) deduction.

  • Ionization Techniques:

  • Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI MS): Proteins embedded in a light-absorbing organic matrix are ionized and desorbed into the gas phase by laser pulses.

  • Electrospray Ionization Mass Spectrometry (ESI MS): Macromolecules are forced directly from liquid solution into gas-phase ions through a charged capillary tip.

  • Mass-to-Charge ($m/z$) Analyzers:

  • Time of Flight (TOF): Ion acceleration velocity through an electric drift tube depends directly on $m/z$

  • Orbitrap: Traps electrostatic ions in orbital motion around a central spindle; ion frequencies are converted to $m/z$ via Fourier transform.

  • Tandem Mass Spectrometry (MS/MS):

  • Uses two mass filters arranged in series.

  • First Mass Filter: Sorts and selects a specific target peptide ion produced by initial cleavage.

  • Collision Cell: Target peptide is fragmented by collisions with inert gas.

  • Second Mass Filter: Measures $m/z$ ratios of generated charged peptide fragments to reconstruct the sequence.

  • Liquid Chromatography-Tandem MS (LC-MS/MS):

  • Couples liquid chromatography separation directly to tandem mass spectrometry.

  • Resolves complex peptide mixtures continuously, enabling comprehensive proteome quantification.

Solid-Phase Peptide Synthesis and Bioinformatics

• Solid-Phase Peptide Synthesis (SPPS):

  • Chemical synthesis technique developed by Bruce Merrifield.

  • Polypeptide chains are assembled while anchored covalently to an insoluble solid resin support.

  • Direction of Synthesis: Chemical synthesis proceeds stepwise from the C-terminus to the N-terminus direction (exact opposite of biological ribosomal synthesis).

• Bioinformatics and Evolutionary Sequence Analysis:

  • Amino acid sequences provide insights into:

  • Three-dimensional tertiary structure prediction.

  • Biological function and active-site mechanisms.

  • Cellular localization.

  • Evolutionary history and phylogenetic relationships.

  • Conserved vs. Variable Residues:

  • Essential amino acid residues critical for catalytic activity or folding are conserved across evolution.

  • Non-essential structural residues vary across divergent species over time.

  • Consensus Sequences: Representative sequences displaying the most common amino acid residue at each position among aligned homologous sequences (visualized via Sequence Logos).

  • Lateral (Horizontal) Gene Transfer: Process in which an organism incorporates genetic material from another organism without being its offspring.

  • Homologous Proteins:

  • Homologs: Proteins sharing detectable sequence similarity.

  • Paralogs: Homologous proteins occurring within the same species (e.g., human $\alpha$-globin and $\beta$-globin).

  • Orthologs: Homologous proteins occurring in different species performing equivalent functions (e.g., human hemoglobin and bovine hemoglobin).

  • Signature Sequences: Contiguous amino acid motifs ($10$ to $50$ residues long) associated with specific structural domains or biological functions across protein families.