Protein Folding and Levels of Structure (Chapter 3)

Levels of Protein Structure

• Primary structure

  • The order of amino acids determines everything that follows; starts at the N-terminus and ends at the C-terminus.
  • The primary sequence strongly influences the formation of secondary, tertiary, and quaternary structures, with particular emphasis on how the sequence sets up the potential interactions of side chains in later folding.
  • In class, the backbone is often represented by repeating NCC (N–C–C) units, where the central carbon bears the side chain (R group).

• Secondary structure

  • Definition: local folding patterns stabilized primarily by hydrogen bonds in the backbone.
  • Hydrogen bonds form between the carbonyl oxygen (C=O) and the amide hydrogen (N–H) of the backbone.
  • These interactions lead to:
  • Alpha helix
  • Beta pleated sheet
  • Key point: secondary structure involves backbone atoms, not the side chains (R groups).

Alpha helix

• Hydrogen bonding pattern: the carbonyl oxygen of residue i forms a hydrogen bond with the amide hydrogen of residue i+4.
  • This requires about 3–4 amino acids per turn; the helix can be visualized as coils where each turn spans roughly 4 residues.
• Visual intuition: the backbone participates in the helix through C=O … N–H interactions at i and i+4; side chains (R groups) extend outward and do not dictate the helical hydrogen-bonding pattern.
• Note on orientation: the helix orientation in diagrams may show the N-terminus on one side; hydrogen bonds consistently span four residues along the chain.

Beta pleated sheet

• Structure: hydrogen bonds form between strands that run in the same or opposite directions, creating a sheet-like arrangement.

• The strands can align in parallel or antiparallel configurations; hydrogen bonds form between backbone C=O and N–H across adjacent strands.

• Strands are held together by backbone hydrogen bonds and are generally in close proximity due to the folding pattern.

• In both alpha helices and beta sheets, hydrogen bonds are between backbone atoms (C=O and N–H) and are responsible for the local, rather than global, folding (local folding).

• Diagrammatic note: diagrams use lines or triangles to indicate where R groups project; triangles may indicate bonds pointing toward/away from the viewer, but this level of detail isn’t required for the course.

Transition to higher-order structure: tertiary structure

• Definition: tertiary structure is the global folding of a single polypeptide, i.e., how distant (in sequence) parts of the chain come together in 3D space.
• Important idea: interactions in tertiary structure often involve the side chains (R groups), which can be:
  • Hydrogen bonds (between polar side chains and/or backbone groups)
  • Ionic bonds (between positively and negatively charged side chains)
  • Hydrophobic interactions (nonpolar side chains clustering away from water)
  • Van der Waals interactions (non-specific, distance-dependent)
  • Covalent disulfide bonds (between sulfur atoms in cysteine residues)
• The backbone (secondary structure) is retained within the folded protein; tertiary structure describes how those elements and the side chains pack together in three dimensions.
• Examples of side-chain interactions:
  • Polar side chains can form hydrogen bonds with backbone carbonyls or with other side chains (e.g., a serine OH with a carbonyl group).
  • A hydrophobic core forms when nonpolar residues cluster away from water, stabilizing the folded state.
  • Ionic interactions can occur between acidic (negative) and basic (positive) side chains.
  • Disulfide bonds (S–S) covalently link cysteine residues and can stabilize the folded structure.
• A common motif: certain tertiary structures bring distant residues into proximity to create active sites, binding pockets, or structural frameworks.
• Overall significance: tertiary structure is the configuration that largely defines protein function, since it determines the geometry of active sites and interaction surfaces.

Quaternary structure

• Definition: quaternary structure arises when two or more polypeptide chains (subunits) assemble into a functional protein complex.
• Occurs only when multiple polypeptide units associate; the assembled complex can have new or augmented functionality.
• Examples:
  • Hemoglobin as a tetramer composed of four subunits (tetrameric protein):
  • 2 alpha and 2 beta subunits form a functional oxygen-transport protein.
  • Ribosomes contain multiple subunits that come together to perform their role.
• Note: quaternary structure depends on the same types of interactions as tertiary structure (and sometimes disulfide bonds or hydrophobic/hydrophilic packing between subunits).

Structure-function relationship in proteins

• Core idea: primary structure determines higher-order structures, which in turn determine function.
• If you change the amino acid sequence, you alter side chains and their potential interactions, which can disrupt folding and reduce or abolish function; occasionally, changes can improve function, but this is rarer.
• Examples illustrating structure-function links:
  • Collagen: long rod-like structure contributing to tissue strength.
  • Pore-forming proteins: shapes that create channels in membranes.
  • Hemoglobin (quaternary assembly) enabling cooperative oxygen binding.
  • Myosin/actin: motor proteins producing movement.
• Concept of spontaneous folding:
  • Most proteins fold spontaneously from unfolded states as they move from higher energy to lower energy states.
  • Denaturation increases energy; removal of denaturants or refolding can allow proteins to return to their native state if the primary sequence remains intact.
  • The native state is typically the lowest energy conformation, i.e., thermodynamically favorable.
• Primary structure and folding:
  • The information for correct folding is encoded in the amino acid sequence.
  • The primary structure largely dictates the ability to reach the native state through the network of side-chain interactions.
• Exceptions and helpers:
  • Some proteins require molecular chaperones (or chaperonins) to fold correctly; these proteins rely on other cellular factors to guide proper folding.
  • If a protein is denatured and its chaperone is missing, it may fail to refold properly.
• Conceptual takeaway: the organism’s proteome relies on the accurate folding of proteins to perform diverse cellular functions, from catalysis to structural support, signaling, transport, and defense.

Enzymes and proteins

• Definitions and relationships:
  • Enzymes are proteins that catalyze chemical reactions by stabilizing transition states and lowering activation energy.
  • All enzymes are proteins (thus, polypeptides), but not all proteins are enzymes; some have structural, signaling, or transport roles.
• Active site concept:
  • The active site is a region of the enzyme where substrate binding occurs and catalysis unfolds.
  • Specific amino acid residues in the active site interact with substrates to bind them and facilitate chemical transformation.
• Catalysis and structure:
  • The three-dimensional arrangement of active-site residues is critical for catalytic efficiency.
  • The folding state of the enzyme must position key residues correctly; changes in folding can disrupt catalysis.

Denaturation, refolding, and chaperones

• Denaturation:
  • Involves disruption of the protein’s native structure (secondary/tertiary/quaternary) while the primary sequence remains intact.
  • Conditions such as heat, pH changes, or chemical denaturants can unfold proteins.
  • Upon removal of the denaturing conditions, many proteins can refold to their native state, provided the primary structure remains:
  • Refolding demonstrates that the information required for folding is encoded in the sequence.
• Chaperones:
  • Molecular chaperones assist some proteins in achieving or maintaining their correct native fold.
  • They guide proper interactions and prevent misfolding during the folding process.
  • If a protein requires a chaperone and is unfolded without it, refolding to the native state may be impaired or incomplete.

Notes on learning goals and exam preparation (study guidance mentioned in the lecture)

• Learning goals (exam preparation):

  • Be able to describe the four levels of protein structure and their defining characteristics.
  • Be able to describe how protein folding relates to function.
  • Be able to draw and recognize an amino acid, and to describe/draw how amino acids polymerize to form polypeptides via peptide bonds.

• Practical study tips from the lecture:

  • Use the Moodle learning goals to guide studying: ensure you can perform tasks listed (e.g., draw amino acids, illustrate peptide bond formation).
  • Test yourself by drawing and labeling structures rather than just recognizing terms on slides.
  • If you’re anxious about exams, take a few minutes to breathe deeply and maintain composure before and during the exam.

• Quick pH note (lecture prompt about amino acids at pH 7):

  • At physiological pH (around 7), amino acids exist as zwitterions with
  • $ext{NH}_3^{+} ext{ (on the amino group) and } ext{COO}^{-} ext{ (on the carboxyl group).}$
  • In other words, the amino group is protonated and the carboxyl group is deprotonated at pH 7.

• Group activity and workflow (from the breakout):

  • Roles in groups: reporter (first alphabetically), note taker (second), reflector (third), facilitator (fourth).
  • Task: draw amino acids and predict peptide bond formation, including the condensation reaction where water is released.
  • A typical exercise: demonstrate formation of a dipeptide by linking the C terminus of one amino acid to the N terminus of the next via a peptide bond, releasing a molecule of water (H₂O).

• Additional clarifications discussed in class:

  • The difference between carbonyl and carboxyl groups: if a carbonyl group lacks hydroxyl (–OH) on the carbon, it is a carbonyl; if it bears an –OH, it is a carboxyl group (–COOH).
  • In many diagrams, R groups are shown as triangular extensions; the triangles indicate 3D orientation and do not require memorization for this course.
  • The canonical dipeptide formation involves a condensation reaction:
  • $ext{AA}<em>1- ext{COOH} + ext{NH}</em>2- ext{AA}<em>2 ightarrow ext{AA}</em>1- ext{CO}- ext{NH}- ext{AA}<em>2 + ext{H}</em>2 ext{O}.$

• Real-world relevance and perspective:

  • Protein misfolding or mutations can lead to dysfunctional proteins and diseases; proper folding is essential for cellular function and health.
  • The study of folding underpins understanding of metabolic pathways, enzyme kinetics, and physiology.

• Summary takeaways:

  • Four levels of protein structure: primary (amino acid sequence), secondary (local backbone interactions forming alpha helices and beta sheets), tertiary (global 3D folding involving side chains), and quaternary (assembly of multiple polypeptide subunits).
  • Protein folding is driven by a balance of interactions: hydrogen bonds, ionic bonds, hydrophobic effects, Van der Waals forces, and covalent disulfide bonds when present.
  • The correct folding determines function; the primary sequence encodes the information necessary for achieving the native structure.
  • Enzymes (proteins) accelerate reactions by placing substrates in active sites; chaperones assist folding for some proteins.

Quick reference formulas and key terms

• Peptide bond formation (condensation):
  $ext{AA}<em>1- ext{COOH} + ext{NH}</em>2- ext{AA}<em>2 ightarrow ext{AA}</em>1- ext{CO}- ext{NH}- ext{AA}<em>2 + ext{H}</em>2 ext{O}$

• Hydrogen bond in secondary structure (backbone):
  $ext{C=O}<em>{i} \cdots ext{N-H}</em>{i+4}$

• Alpha-helix hydrogen bond pattern: i to i+4

• Zwitterion at pH ~7:
  $ext{NH}_3^{+}, ext{COO}^{-}$

• Free energy relation for spontaneous folding:
  oxed{ riangle G_{ ext{fold}} < 0 }

• Primary structure influence on folding is strongest for tertiary structure due to side-chain interactions and available bonding opportunities.

• Functional contexts of protein structures:

  • Collagen: structural rod,
  • Hemoglobin: quaternary protein (tetramer) for oxygen transport,
  • Motor proteins (e.g., myosin) and cytoskeletal components (actin) for movement,
  • Signaling and transport proteins for cellular communication and material exchange.