Primary and Secondary Protein Structure
High-Yield Summary
- A peptide bond forms via a condensation (dehydration) reaction: one amino acid's carboxyl group + the next one's amino group → new C–N bond + released H₂O. Ribosome-catalyzed, requires energy.
- Hydrolysis is the exact reverse: adds water to break a peptide bond (acid/base conditions or proteases).
- Chains < ~50 amino acids = peptide; longer = protein. Always read/written N-terminus → C-terminus.
- 4 hierarchical levels: primary (sequence, peptide bonds) → secondary (local folding, backbone H-bonds) → tertiary (full 3D shape, R-group interactions) → quaternary (multi-chain assembly, same interactions as tertiary but between subunits).
- Secondary structure = alpha helix (right-handed coil, backbone C=O of residue n H-bonds to N–H of residue n+4, ~3.6 residues/turn) or beta-pleated sheet (parallel or antiparallel strands, H-bonds between strands).
- Proline's rigid ring disrupts both alpha helices and beta sheets — signals a bend/turn instead.
Key Terms
- Peptide bond
- Covalent bond between the carboxyl carbon of one amino acid and the amino nitrogen of the next, formed by condensation.
- N-terminus / C-terminus
- Free amino end / free carboxyl end of a peptide chain — the fixed reading direction.
- Primary structure
- The linear amino acid sequence, determined by DNA, held together by peptide bonds only.
- Secondary structure
- Local backbone folding (alpha helix, beta sheet) stabilized by backbone hydrogen bonds — not side chains.
Condensation vs. Hydrolysis
| Condensation | Hydrolysis |
|---|---|
| Forms a peptide bond | Breaks a peptide bond |
| Releases H₂O | Adds H₂O |
Alpha Helix vs. Beta-Pleated Sheet
| Alpha Helix | Beta-Pleated Sheet |
|---|---|
| Right-handed coil, ~3.6 residues/turn | Flat, extended strands |
| H-bonds within one strand: residue n to n+4 | H-bonds between adjacent strands |
| Single continuous spiral | Parallel or antiparallel arrangement |
Common MCAT Trap
- Secondary structure is stabilized by BACKBONE hydrogen bonds, not R-group (side-chain) interactions — that's tertiary structure. Easy to mix up.
- Proline is rarely found WITHIN an alpha helix or beta sheet (its ring rigidity breaks the pattern) but can appear at the START/cap of a helix.
- The analogy: primary = letters, secondary = words, tertiary = sentences, quaternary = paragraphs (multi-chain only).
Quick Recall
What reaction forms a peptide bond, and what is released?
What stabilizes secondary structure?
In an alpha helix, which residue's carbonyl bonds to which residue's amide hydrogen?
Why does proline disrupt alpha helices and beta sheets?