← LS 1a: Life Sciences

Unit 3: Protein Structure, Folding, and Function

How a chain of amino acids becomes a working machine, with hemoglobin as the worked example of structure driving function.

10 topics · 39 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

Primary Structure: The Sequence as Information

  1. How it worksHow does the order of amino acids strung along a chain get read out of a gene's sequence?
  2. Common mistakeWhy isn't a protein's primary structure just a random string of amino acids?
  3. Going furtherWhat happens to a protein's fate when one amino acid substitution lands in its active site instead of on its surface?
Your turn1 of 2
A student looks at a protein's amino acid sequence, hundreds of residues with no obvious repeating pattern, and concludes the order must be essentially random, the way letters would be if typed by chance. Why is this wrong?

Secondary Structure: Alpha Helices and Beta Sheets

  1. How it worksHow do hydrogen bonds between backbone atoms spaced four residues apart twist a chain into an alpha helix?
  2. How it worksWalk me through how backbone hydrogen bonds between two separate chain segments zip them together into a beta sheet.
  3. Common mistakeWhy isn't secondary structure held together by the amino acid side chains?
  4. Going furtherWhat happens to a stretch of secondary structure when a proline shows up in the middle of it?
Your turn1 of 2
A student assumes that an alpha helix must be held in its coiled shape by attractions between the amino acid side chains sticking off the backbone, the same way tertiary structure is held together. What actually holds a helix or sheet in shape?

Tertiary Structure and the Forces That Fold a Protein

  1. How it worksHow do hydrophobic side chains burying themselves in a protein's core pull the whole chain into a compact tertiary shape?
  2. How it worksWalk me through how hydrogen bonds, ionic bridges, and disulfide bonds lock a folded protein's tertiary structure in place.
  3. Going furtherWhat happens to a protein's tertiary structure when a disulfide bond linking two cysteines gets chemically reduced?
  4. Ties togetherHow does the same hydrophobic-core logic that folds a small protein also drive a lipid membrane to self-assemble?
Your turn1 of 2
A newly synthesized protein chain contains helices and sheets, but a student assumes these local structures alone must already be the finished, functional shape, since they are already neatly folded. What is missing?

Hydrophobic Collapse and the Folding Funnel

  1. How it worksHow does burying hydrophobic residues away from water drive a protein chain to collapse toward its folded shape?
  2. How it worksWalk me through a protein's folding funnel, from a high-energy unfolded chain down to its low-energy native state.
  3. Worked exampleFind how many conformations a 100-residue chain could sample at 3 conformations per residue, and how long a random search at 1013 tries per second would take.
  4. Common mistakeWhy isn't a protein finding its folded shape by randomly trying every possible conformation until one fits?
  5. Going furtherWhat happens to a protein's folding funnel when a mutation fills its hydrophobic core with a charged residue instead?
Your turn1 of 2
A chain of 75 residues could sample about 3 conformations per residue. Searching randomly at 10 to the 13th tries per second, find how many years an exhaustive search of every conformation would take.

Quaternary Structure: Multi-Subunit Proteins

  1. How it worksHow do hydrophobic patches on a protein's surface let separate subunits recognize and bind each other?
  2. Common mistakeWhy isn't a protein's quaternary structure just its subunits sitting loosely next to each other?
  3. Going furtherWhat happens to a multi-subunit protein's function when one subunit's binding interface gets mutated?
Your turn1 of 2
A student sees a diagram of hemoglobin's four separately folded subunits packed together and assumes they must simply be sitting loosely side by side, like four marbles touching in a box. Why is this description wrong?

Protein Denaturation and Renaturation

  1. How it worksHow does heat or a shift in pH break the weak forces holding a protein's tertiary structure without touching its peptide bonds?
  2. Going furtherWhat happens when a denatured protein is given the chance to refold slowly under gentle conditions?
  3. Common mistakeWhy isn't denaturing a protein the same thing as hydrolyzing it?
  4. Ties togetherHow does a chaperone protein use the same hydrophobic-shielding trick as folding itself to rescue a protein that's folding wrong?
Your turn1 of 2
A student watches an egg white turn from clear and runny to white and solid when heated, and describes this as the proteins being broken down into their individual amino acids by the heat. Why is this description of denaturation wrong?

Myoglobin and Oxygen Binding

  1. How it worksHow does myoglobin's iron-containing heme group grab and release a single oxygen molecule?
  2. Worked exampleFind the fractional saturation of myoglobin at an oxygen pressure of 2 torr, given a P50 of 1 torr and a hyperbolic binding curve.
  3. Going furtherWhat happens when myoglobin's hyperbolic oxygen-binding curve is compared directly against hemoglobin's?
Your turn1 of 2
Myoglobin has a P50 of 2.24 torr for oxygen and binds oxygen with a simple hyperbolic curve. Find its fractional saturation at an oxygen pressure of 7.89 torr.

Hemoglobin Cooperativity and the Sigmoid Binding Curve

  1. How it worksHow does oxygen binding to one hemoglobin subunit make the next subunit grab oxygen more easily?
  2. How it worksWalk me through how hemoglobin's four subunits shift together from a tense low-affinity shape to a relaxed high-affinity shape.
  3. Worked exampleFind hemoglobin's oxygen saturation at a partial pressure of 26 torr, given a P50 near 26 torr.
  4. Common mistakeWhy isn't hemoglobin's cooperative binding the same as its four subunits just independently grabbing oxygen?
  5. Going furtherWhat happens to the binding curve's shape if you could isolate just one of hemoglobin's four subunits alone?
Your turn1 of 2
Hemoglobin has a P50 near 28 torr and a Hill coefficient of 2.8. Find its oxygen saturation at a partial pressure of 49 torr.

Allostery and the T-State-to-R-State Transition

  1. How it worksHow does hemoglobin's T state hold its subunits in a low-affinity conformation until the first oxygen binds?
  2. How it worksWalk me through how the T-to-R transition ripples through hemoglobin's subunit interfaces once oxygen binds.
  3. Common mistakeWhy isn't allosteric regulation just a second active site competing with the first one?
  4. Ties togetherHow does hemoglobin's T-state-to-R-state switch use the same allosteric logic as an enzyme regulated by a molecule far from its active site?
Your turn1 of 2
A drug binds hemoglobin at a site far from any of its four oxygen-binding hemes, yet it still changes how readily those hemes bind oxygen. A student assumes this must mean the drug is actually competing directly with oxygen at each heme, just approaching from a different angle. Why is this assumption wrong?

The Bohr Effect and Oxygen Delivery

  1. How it worksHow does a drop in pH inside active tissue shift hemoglobin toward its low-affinity T state?
  2. Going furtherWhat happens to hemoglobin's oxygen affinity when red blood cells raise their 2,3-BPG levels at high altitude?
  3. Common mistakeWhy isn't the Bohr effect just carbon dioxide directly shoving oxygen off of hemoglobin?
  4. Ties togetherHow does the Bohr effect's pH sensitivity connect back to the same protonation chemistry that sets an amino acid side chain's charge?
Your turn1 of 2
In an exercising muscle, carbon dioxide and acid build up, and hemoglobin releases more oxygen there than it does in the lungs. A student explains this by saying carbon dioxide molecules must be physically bumping oxygen molecules off of hemoglobin's hemes. What is the actual mechanism?