The big idea: The backbone of a chain has the same C=O and N-H groups again and again. Hydrogen bonds between them, at regular positions, coil the chain into helices or pleat it into sheets.
The points to remember
- Secondary structure is the coiling or pleating of parts of a chain.
- The backbone has a C=O and an N-H for every amino acid, at regular positions.
- Alpha helix: the chain coils, and each C=O bonds to the N-H four amino acids further along.
- Beta-pleated sheet: sections of chain lie side by side, with C=O on one bonding to N-H on the next.
- These are hydrogen bonds: each is weak, but many in regular positions make the structure stable.
Remember it as: Helix: coil, bonds up the coil. Sheet: pleats, bonds across.
Backbone, not R-groups: The hydrogen bonds of secondary structure are between C=O and N-H of the backbone. R-groups stick out and are not part of them; R-group bonds hold the tertiary structure (2.2.9).
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The big idea: Biologists draw proteins as ribbons: arrows for beta strands and coils for alpha helices, so you can see the secondary structure at a glance.
The points to remember
- In a ribbon diagram, a flat arrow is one beta strand; arrows side by side form a beta-pleated sheet.
- A coil or spiral is an alpha helix.
- Thin lines are loops that link helices and strands.
- Most proteins have both: helices and sheets, joined by loops, then folded further.
Real example: titin: Titin, the spring-like protein in muscle, is built from hundreds of repeating domains. Many of them are made almost entirely of beta-pleated sheets.
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How it is asked: A ribbon diagram of part of a protein, and a two-mark Describe on the structures its arrows or coils show.
A ribbon diagram of one domain of a muscle protein shows several flat arrows lying side by side. Describe the secondary structures represented by the arrows.
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And the same task for a coil:
Describe the secondary structure represented by the coils in a ribbon diagram of a protein.
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See the answer broken into clear points, with guidance on how each point earns marks.