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Biochemistry Unit 3: Protein Structure and Folding
A protein's sequence folds into a precise 3D shape that determines its function. This unit explains the four levels of structure, the forces that stabilize them, how proteins fold and misfold, and how hemoglobin uses structure to deliver oxygen.
Covers: Levels of protein structure, α-helices, β-sheets and Ramachandran angles, Forces in tertiary structure, Quaternary structure, Protein folding, Anfinsen and chaperones, Denaturation and misfolding diseases, Myoglobin and hemoglobin oxygen binding
Study guide
Protein structure is described at four levels. Primary structure is the amino acid sequence. Secondary structure is local, repeating backbone patterns held by hydrogen bonds: the α-helix and the β-sheet. Tertiary structure is the complete 3D fold of one chain, and quaternary structure is the arrangement of several chains (subunits). Because the peptide bond is planar, the backbone can only rotate around the φ (N–Cα) and ψ (Cα–C) bonds. A Ramachandran plot shows which φ/ψ pairs avoid steric clashes; helices and sheets occupy the main allowed regions.
The α-helix is a right-handed coil with 3.6 residues per turn, a rise of 0.15 nm per residue and a pitch of 0.54 nm. Each backbone C=O hydrogen bonds to the N–H four residues ahead, and side chains point outward. Proline breaks helices because it lacks an N–H and its ring is rigid; glycine is too flexible. β-sheets are made of extended strands lying side by side, hydrogen bonded between strands, in parallel or antiparallel orientation. Turns and loops connect these elements, often using Gly and Pro.
Tertiary structure is stabilized mainly by the hydrophobic effect, which buries nonpolar side chains in the core, plus hydrogen bonds, salt bridges, van der Waals packing and, in many secreted proteins, disulfide bonds. Folding is a balance: the chain loses conformational entropy, but water gains entropy and many weak interactions form, giving a modest net ΔG of about −20 to −60 kJ/mol. Denaturants such as heat, urea, guanidinium, detergents and extreme pH disrupt these interactions. Anfinsen showed that denatured ribonuclease refolds on its own, proving that sequence encodes structure. Chaperones help by preventing aggregation, not by supplying information.
Misfolding causes disease. In amyloid diseases such as Alzheimer's and prion disorders, proteins adopt β-rich structures that stack into insoluble fibers. In sickle cell anemia, a single change, Glu6 to Val in the β chain, creates a hydrophobic patch so deoxyhemoglobin polymerizes. Levinthal's paradox notes that a random search of conformations would take far too long, so folding must follow an energy funnel in which local structure and hydrophobic collapse rapidly narrow the possibilities. Cells add quality control: chaperones such as Hsp70 and chaperonins such as GroEL give chains a protected chance to fold, and misfolded proteins are tagged with ubiquitin and degraded by the proteasome.
Myoglobin, a single chain with one heme, binds O₂ with a hyperbolic curve and high affinity (P50 ≈ 2.8 torr), suited for storage in muscle. Hemoglobin, a tetramer of two α and two β chains, binds O₂ cooperatively, giving a sigmoidal curve (Hill coefficient about 2.8, P50 ≈ 26 torr). O₂ binding shifts it from the low-affinity T state to the high-affinity R state. H⁺ and CO₂ (the Bohr effect) and 2,3-BPG stabilize the T state, shifting the curve right so more O₂ is unloaded in active tissue. Fetal hemoglobin binds 2,3-BPG weakly, so it has higher affinity.
Key ideas
- The amino acid sequence determines the native fold (Anfinsen).
- α-helices use i → i + 4 backbone hydrogen bonds; β-sheets use hydrogen bonds between strands.
- The hydrophobic effect is the main driving force for folding.
- Native proteins are only marginally stable, with ΔG of folding around −20 to −60 kJ/mol.
- Chaperones prevent aggregation but do not dictate the final structure.
- Misfolding and aggregation cause amyloid diseases and sickle cell anemia.
- Hemoglobin's cooperativity and allosteric regulation (H⁺, CO₂, 2,3-BPG) tune O₂ delivery.
Formulas and rules
Common mistakes
- Thinking backbone hydrogen bonds are the main driving force of folding; hydrophobic burial dominates.
- Confusing secondary hydrogen-bond patterns: helix within one segment (i → i + 4), sheet between strands.
- Saying chaperones carry the folding information instead of preventing aggregation.
- Shifting the O₂ curve the wrong way: H⁺, CO₂, 2,3-BPG and heat shift it right (lower affinity).
- Assuming urea or heat breaks peptide bonds rather than noncovalent interactions.
Before the test, make sure you can
- Describe the four levels of protein structure with examples.
- Compare α-helix and β-sheet hydrogen bonding and geometry.
- Explain why proline breaks helices and what Ramachandran plots show.
- Explain the forces that stabilize tertiary structure and how denaturants act.
- Calculate ΔG° of folding and fraction folded from K.
- Explain Anfinsen's experiment, chaperones and Levinthal's paradox.
- Explain cooperative O₂ binding and calculate O₂ delivery with the Hill equation.
- Predict effects of pH, CO₂, 2,3-BPG and CO on hemoglobin's O₂ curve.
Words to know
- Primary structure
- The amino acid sequence of a polypeptide.
- Secondary structure
- Local backbone conformations such as α-helices and β-sheets held by backbone hydrogen bonds.
- Tertiary structure
- The overall 3D fold of a single polypeptide chain.
- Quaternary structure
- The arrangement of multiple polypeptide subunits in a protein.
- α-helix
- A right-handed coil with 3.6 residues per turn and i → i + 4 hydrogen bonds.
- β-sheet
- Extended strands hydrogen bonded side by side, parallel or antiparallel.
- Ramachandran plot
- A plot of allowed backbone φ and ψ angles.
- Denaturation
- Loss of native structure without breaking peptide bonds.
- Anfinsen experiment
- Refolding of ribonuclease A showing sequence determines structure.
- Chaperone
- A protein that helps others fold by preventing aggregation, often using ATP.
- Levinthal's paradox
- A random search of all conformations would take too long, so folding must be guided.
- Amyloid
- Insoluble fibers of misfolded proteins with cross-β structure.
- Cooperativity
- Binding at one site changes affinity at other sites; gives a sigmoidal curve.
- T and R states
- Low-affinity (tense) and high-affinity (relaxed) forms of hemoglobin.
- Bohr effect
- Lower pH and higher CO₂ reduce hemoglobin's O₂ affinity.
- 2,3-BPG
- A red-cell metabolite that stabilizes T-state hemoglobin and raises P50.
- P50
- The O₂ pressure at which a protein is half saturated.
- Hill coefficient
- A measure of cooperativity; about 2.8 for hemoglobin and 1 for myoglobin.
Sample practice questions
30 more questions like these, from level 1 to 5, are in the practice set.
A protein's primary structure is:
- Its overall 3D fold
- Its pattern of α-helices and β-sheets
- The arrangement of its subunits
- Its sequence of amino acids
Show the answer and steps
Answer: D) Its sequence of amino acids
- Primary structure is the linear amino acid sequence, linked by peptide bonds.
- Secondary is local helices and sheets; tertiary is the whole chain's fold; quaternary is subunit arrangement.
- The sequence ultimately determines all higher levels.
In an α-helix, the backbone hydrogen bonds form between:
- The C=O of residue n and the N–H of residue n + 4
- Backbone groups on separate strands
- The C=O of residue n and the N–H of residue n + 1
- Side chains of adjacent residues
Show the answer and steps
Answer: A) The C=O of residue n and the N–H of residue n + 4
- An α-helix is held by backbone hydrogen bonds running parallel to the helix axis.
- Each C=O at residue n bonds the N–H four residues ahead (n + 4).
- Hydrogen bonds between separate strands describe β-sheets.
Hemoglobin's sigmoidal O₂-binding curve shows that:
- Each subunit binds O₂ independently
- Hemoglobin binds O₂ more tightly than myoglobin
- Binding one O₂ raises affinity at other sites
- Hemoglobin can bind only one O₂ molecule
Show the answer and steps
Answer: C) Binding one O₂ raises affinity at other sites
- A sigmoid curve signals positive cooperativity.
- O₂ binding shifts subunits from the T (tense) state toward the R (relaxed) state, raising affinity at the other sites.
- Myoglobin, with one heme and no cooperativity, gives a hyperbola and has higher affinity.