AP BiologyUnit 1: Chemistry of Life1.7
AP Biology · Unit 1 · Topic 1.7

Proteins

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson4 min read

Denatured means unfolded, not cut. The sequence survives; the shape and the job don't.

Every protein starts as a chain of amino acids, and their order decides how it folds. The fold does the job: unfold it with heat or chemicals and the job stops, though the chain stays in one piece.

Key terms

  • R group — the side chain that makes each of the 20 amino acids different. It can be nonpolar, polar, or charged.
  • Peptide bond — the covalent bond joining one amino acid's carboxyl group to the next one's amino group. Water is released.
  • Primary structure — the order of amino acids, set by the gene. It carries the information for every higher level.
  • Disulfide bridge — a covalent link between the R groups of two cysteines. It staples parts of the fold together.
  • Denaturation — loss of shape from heat, extreme pH, or chemicals. Peptide bonds and sequence stay intact.

How it works

  1. Peptide bonds link amino acids into a chain with an amino end and a carboxyl end. R groups stick out from this N–C–C backbone.
  2. Secondary structure: hydrogen bonds between backbone atoms coil the chain into α-helices or fold it into β-pleated sheets. R groups are not part of these bonds.
  3. Tertiary structure: R groups interact. Nonpolar ones cluster inside, away from water; polar and charged ones mostly face out. Hydrogen bonds, ionic bonds, and disulfide bridges hold the fold.
  4. Quaternary structure: two or more folded chains join, held by the same kinds of R-group interactions. Hemoglobin has four chains; many proteins are a single chain.
  5. Shape sets function: an enzyme's active site is built by the fold. Lose the fold and the site is gone.
  6. Remove the stress and some proteins refold on their own, since the sequence still holds the instructions. Others misfold or clump together and stay inactive.

Why it matters

Gene → sequence → fold → function runs through the whole course. Enzymes stop working at high temperature or extreme pH because they denature. One swapped amino acid can change how a protein folds or what it sticks to. In sickle cell hemoglobin, a charged surface R group becomes nonpolar. The sticky patch makes the molecules clump into fibers when oxygen is low.

Exam traps

TrapActually
Heat breaks peptide bonds and scrambles the sequence.Heat breaks only weak interactions. Peptide bonds and amino acid order survive; the fold is what's lost.
The cell or the disulfide bridges set the shape.The sequence holds the information for the fold. Bridges and normal conditions help the chain reach and keep it.
A peptide bond joins R groups, like a disulfide bridge does.A peptide bond joins carboxyl to amino group along the backbone. A disulfide bridge cross-links two cysteine R groups.
Polar and charged R groups are the same thing.Polar R groups have partial charges and hydrogen-bond. Charged ones carry a full charge and form ionic bonds. Nonpolar ones do neither.
Worked example

An enzyme's core is packed with nonpolar R groups, and its surface is covered with polar and charged ones. A researcher tests three single substitutions. Change 1 swaps a positive R group on the surface for a different positive one. Change 2 swaps a large nonpolar R group in the core for a smaller nonpolar one. Change 3 swaps a nonpolar R group in the core for a negatively charged one. Which change is most likely to destroy the enzyme's activity?

  1. Ask what each original R group was doing. Nonpolar groups in the core hold the fold by clustering away from water. Charged groups on the surface interact with water.
  2. Changes 1 and 2 keep the same chemistry in the same place: charge for charge on the surface, nonpolar for nonpolar in the core.
  3. Change 3 buries a full negative charge among nonpolar R groups, with no water to interact with. The core can't pack as before, so the fold and the active site are likely to be disrupted.

Change 3, the charged R group placed in the nonpolar core.

Folding follows R-group chemistry, so the most damaging swap puts the wrong chemistry in the wrong place. The near-miss is Change 2: a different-size R group looks drastic, but the core stays nonpolar, so the fold is far more likely to hold.

Now try it on real exam-style questions

Ten questions on Proteins, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 1.7 Proteins

Two cysteine R groups in a folded protein form a disulfide bridge. What type of bond is this?

  1. A A hydrogen bond
  2. B An ionic bond
  3. C A peptide bond
  4. D A covalent bond
Show the answer

D — The sulfur atoms of two cysteines share electrons in a covalent S-S bond, the strongest of the interactions that stabilize tertiary structure. It links two R groups rather than two backbone groups, so it is not a peptide bond.

Question 2 · 1.7 Proteins

A polypeptide chain has directionality. What are its two ends called?

  1. A The 5' end and the 3' end
  2. B The amino end and the R-group end
  3. C The amino end and the carboxyl end
  4. D The polar end and the nonpolar end
Show the answer

C — One end of a polypeptide has a free amino group (the N-terminus) and the other a free carboxyl group (the C-terminus), because each peptide bond uses the carboxyl group of one amino acid and the amino group of the next. R groups project from every amino acid along the chain, so there is no R-group end.

Question 3 · 1.7 Proteins

Which element is present in every amino acid but absent from glucose?

  1. A Sulfur
  2. B Nitrogen
  3. C Phosphorus
  4. D Iron
Show the answer

B — The amino group of every amino acid contains nitrogen, while glucose contains only carbon, hydrogen, and oxygen. Sulfur is tempting, but it appears only in a few R groups, such as those of cysteine and methionine.