Study Notes: Protein Structure, Peptide Bonds, pKa, and Conformational Biology

Ionizable groups and pKa values in proteins

  • Table 3.1 (Typical pK_a values for ionizable groups in proteins)

    • Terminal α-carboxyl group (acid): pKa≈3.1pK_a \approx 3.1
    • Aspartate (Asp) side chain: pKa≈4.1pK_a \approx 4.1
    • Glutamate (Glu) side chain: pKa≈4.1pK_a \approx 4.1
    • Histidine (His) side chain: pKa≈6.0pK_a \approx 6.0
    • Terminal α-amino group (base): pKa≈8.0pK_a \approx 8.0
    • Cysteine (Cys) side chain: pKa≈8.3pK_a \approx 8.3
    • Tyrosine (Tyr) side chain: pKa≈10.9pK_a \approx 10.9
    • Lysine (Lys) side chain: pKa≈10.8pK_a \approx 10.8
    • Arginine (Arg) side chain: pKa≈12.5pK_a \approx 12.5

  • Important note on pK_a values

    • Values depend on temperature, ionic strength, and the microenvironment of the ionizable group.
    • These values determine whether a group is protonated or deprotonated at a given pH, influencing the net charge of the protein.

  • Three clusters of ionizable groups in proteins

    • Acidic cluster: terminal α-carboxyl group, Asp, Glu (tend to be deprotonated and negatively charged at physiological pH)
    • Neutral/near-neutral cluster around pK_a ~6–8 (e.g., His around 6.0, near physiological pH)
    • Basic cluster: terminal α-amino group, Lys, Arg, (sometimes His) with higher pK_a values, tend to be protonated and positively charged at physiological pH

  • Quick practice questions from the slides

    • True/False (Page 3):
    • “Acidic amino acids are negatively charged at pH 7?”
    • Answer: True
    • Electrostatic interaction (Page 4):
    • Question: Which amino acid would have an attractive electrostatic interaction with lysine’s R group (R group to R group)?
    • Options: Phenylalanine, Aspartate, Arginine, The R group of lysine is not capable of electrostatic interactions
    • Answer: Aspartate (Asp) would be attracted to the positively charged lysine; Arginine would repel; Phenylalanine is nonpolar; the R group of lysine can engage in electrostatics
    • Hydrogen bonding with tyrosine (Page 5):
    • Question: Which amino acid would form an H-bond with tyrosine (R group to R group)?
    • Options: Alanine, Glycine, Asparagine, The R group of tyrosine is not capable of forming H-bonds
    • Answer: Asparagine (Asn) can participate in H-bonds via its side-chain amide; Ala and Gly lack side-chain H-bonding capability; Tyr’s phenolic OH can also participate but the question targets the R-group to R-group interaction; Asn is the supported choice

Levels of protein structure

  • Primary structure
    • Amino acid sequence in a polypeptide
  • Secondary structure
    • Regular, specific 3D structures determined largely by backbone hydrogen bonding among carbonyl oxygens and amide N-H groups
  • Tertiary structure
    • Overall 3D structure of a single polypeptide chain; spatial arrangement of secondary structures and side chains
  • Quaternary structure
    • Assembly of multiple polypeptide chains into a functional protein complex

Peptide bonds and the peptide bond backbone

  • Peptide bonds
    • Covalent bonds between amino acids (amide bonds); link amino acids into a polypeptide
    • Residues: the term used for amino acids within a polypeptide
    • Formation: dehydration synthesis (removal of a water molecule during bond formation)
  • Bond planarity and geometry
    • The peptide bond is essentially planar: six atoms (Cα, C, O, N, H, Cα) lie in one plane
    • The peptide bond is uncharged and has partial double-bond character due to resonance; rotation about the bond is largely prohibited
    • Resonance gives the peptide bond a bond order between a single and a double bond (partial double-bond character), restricting rotation and stabilizing the planar arrangement
  • Cis/trans isomerism
    • Trans form is strongly favored for most peptide bonds due to reduced steric clashes between adjacent side chains; cis form introduces significant steric hindrance

The peptide backbone and directionality

  • Directionality
    • A polypeptide chain has a clear N-terminus (amino terminus) and a C-terminus (carboxyl terminus)
    • Primary structure is always written from the amino terminus to the carboxyl terminus (N to C)
  • Backbone vs side chains
    • The backbone/repeating part is called the main chain or backbone
    • The side chains (distinctive amino acid residues) branch off the backbone
  • Backbone hydrogen bonding potential
    • The backbone has hydrogen-bond donors (N-H) and acceptors (carbonyl O) capable of forming H-bonds that stabilize secondary structures
  • Important note about C-H bonds
    • There is no hydrogen bonding involving the H attached to the α-carbon (Cα–H); C–H bonds are relatively nonpolar and do not participate as H-bond donors in the backbone context

Peptide structure exercises (examples from the slides)

  • Page 11–12: Visual representations of peptide structures (backbone and side chains) used to practice recognizing residues (e.g., Phe, Tyr, Leu, Ser, Gly, Ala) and their placement along the backbone
  • Page 13: Exercise – amino acid sequence in single-letter code
    • Task: Determine the sequence for the given peptide structure and present it in single-letter code (example format: ABCD)
  • Page 14: Exercise – identifying the circled bond
    • Task: Identify which bond is circled in the peptide structure (e.g., a circled bond may be the peptide bond between amino acids 2 and 3, or another bond such as a C–N single bond outside the peptide linkage)
  • Page 15: Exercise – amino acid sequence in three-letter code
    • Task: Determine the sequence in three-letter code for the peptide (example format: Ala-Asn-Tyr-Thr)
  • Page 16: Clarification on bonds
    • Note: The circled item is not a peptide bond; it is the bond between the amide nitrogen and the α-carbon of residue 2 (this corresponds to the N–Cα bond used to define the φ (phi) torsion angle)
  • Page 17–18: Overall charge at pH 7 for shown peptides
    • Method: Count terminal groups and ionizable side chains; apply pK_a values to determine protonation states at pH 7; sum their contributions to obtain the net charge
    • Example outcomes from the slides
    • Page 17 example: Net charge at pH 7 = 0
    • Page 18 example: Net charge at pH 7 = −1
  • Practical takeaway: These exercises reinforce how pK_a values and sequence determine the net charge of a peptide/protein at a given pH

Phi (Φ) and Psi (Ψ) torsion angles; rotation around backbone bonds

  • Rotation around N–Cα bond (Φ or φ)
    • Defines the rotation of the peptide around the bond linking the amide nitrogen to the α-carbon
  • Rotation around Cα–C (carbonyl) bond (Ψ or ψ)
    • Defines the rotation of the peptide around the bond linking the α-carbon to the carbonyl carbon
  • Degrees of freedom
    • The rotations about Φ and Ψ provide the conformational flexibility needed for protein folding
    • Not all combinations of Φ and Ψ are physically permitted due to steric clashes and backbone constraints
  • Ramachandran considerations
    • The allowed regions in the Φ–Ψ space reflect favorable conformations for residues in proteins
    • Glycine and Proline have distinctive allowed regions due to their unique conformational preferences
  • Ramachandran resource references (for study)
    • Tutorials and plots available at Proteopedia and related resources (links provided in the slides)

Disulfide bonds

  • What they are
    • Covalent bonds between two cysteine residues via oxidation of their thiol groups: –S–S– (disulfide bond)
    • Occurs between cysteine R-groups, not within the peptide backbone
  • Formation and breaking
    • Formed by oxidation of two cysteines
    • Can be reduced to break the bond (e.g., in reductive environments or via reduction reagents)
  • Functional role and context
    • Disulfide bonds contribute to protein stability by covalently crosslinking parts of a chain or different chains
    • The chemical environment of the protein (subcellular compartment) influences disulfide formation
    • Not all cysteines participate in disulfide bonds; some remain in thiol form for catalytic or catalytic/structural roles
  • Example: Disulfide bonds in bovine insulin
    • Insulin contains disulfide bonds formed between cysteines that are close in three-dimensional space
    • The correct pairing often requires the cellular environment or helper proteins to ensure proper disulfide formation
    • Some cysteines do not participate in disulfide bonds in the final folded structure

Structures to know and memory aids (overview from the slide lyrics)

  • Hydrophobic side chains (common hydrophobic residues):
    • Glycine (Gly), Valine (Val), Leucine (Leu), Phenylalanine (Phe), Isoleucine (Ile), Alanine (Ala), Methionine (Met), Proline (Pro), Tryptophan (Trp)
  • Polar side chains capable of hydrogen bonding: Cysteine (Cys), Serine (Ser), Threonine (Thr), Histidine (His), Glutamine (Gln), Asparagine (Asn)
  • Charged residues
    • Negatively charged: Aspartate (Asp), Glutamate (Glu)
    • Positively charged: Lysine (Lys), Arginine (Arg) (and Histidine (His) can be partially positive depending on pH)
  • Backbone features
    • Amide N–φ (phi) to αC; ψ (psi) to carbonyl (C=O) – these describe the backbone dihedral angles that define the chain path
  • Charge calculation concept
    • The overall peptide charge at a given pH is the sum of charges from terminal groups and ionizable side chains, determined by their pK_a values and the pH of interest
  • Practical method summary
    • Identify all ionizable groups (N-terminus, C-terminus, side chains)
    • Determine expected charge state at the pH of interest using pK_a values
    • Sum contributions to obtain the net charge

Connections to broader concepts

  • Structure–function relationships
    • The net charge and the distribution of charged/polar residues influence protein stability, folding pathways, and interactions with other biomolecules
  • Real-world relevance
    • pH-dependent charge changes can affect enzyme activity, protein solubility, and protein–protein interactions in different cellular compartments
  • Ethical/philosophical/practical implications
    • Understanding protein structure informs drug design, enzyme engineering, and understanding disease-related misfolding; responsible application requires careful interpretation of experimental data and awareness of context (e.g., cellular environment, redox state)

Quick reference formulas and constants

  • Typical pK_a values (approximate):
    • pKaterminal  α−COOH≈3.1pK_a^{terminal\,\,\alpha-COOH} \approx 3.1
    • pK<em>aAsp≈4.1pK<em>a^{Asp} \approx 4.1, pK</em>aGlu≈4.1pK</em>a^{Glu} \approx 4.1
    • pKaHis≈6.0pK_a^{His} \approx 6.0
    • pK<em>aterminal  α−NH</em>3+≈8.0pK<em>a^{terminal\,\,\alpha-NH</em>3^+} \approx 8.0
    • pKaCys≈8.3pK_a^{Cys} \approx 8.3
    • pKaTyr≈10.9pK_a^{Tyr} \approx 10.9
    • pKaLys≈10.8pK_a^{Lys} \approx 10.8
    • pKaArg≈12.5pK_a^{Arg} \approx 12.5
  • Charge considerations (conceptual)
    • For acidic groups (pKa ≈ 3.1–4.1): charge tends to be 0 below pKa and −1 above pKa at pH values well above pKa
    • For basic groups (pKa ≈ 8.0–12.5): charge tends to be +1 below pKa and 0 above pK_a
    • Net peptide charge at pH p is the sum of the charges of all ionizable groups at that pH
  • Torsion angles definitions
    • \Phi(\phi) = \text{dihedral angle about the N-C_{\alpha}}
    • \Psi(\psi) = \text{dihedral angle about the C_{\alpha}-C'}

Note: The exact values and figures in the slide deck may vary slightly with context; use these as general guidelines and refer to your course materials for specifics.