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Structure of Proteins:
Primary and Secondary Structure
Biochem 4511
Figures: Essentials of Biochemistry, 3rd Ed
...
, Moran et al
...
, Nelson & Cox
Fundamentals of Biochemistry 2nd Ed
...
6 residues per turn

•

Pitch (distance helix rises per turn)
of 5
...
8 Å

•

ϕ = -57°, ψ = -47°

Right-Handed α-Helix

•
•
•

Side chains point “out” and “back” compared to turn of helix
Very few steric clashes
Maximized hydrogen bonding of backbone amides

Amino Acids: Helical propensity by frequency

Helical Wheel Views and Heptad Repeats

•

Ways to plot the view of an α-helix down the
helical axis

Drawing a Helical Wheel
•

Draw a helical wheel for the peptide segment THIRSTY

Helical Wheel Views and Heptad Repeats

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Helix: Salt bridge stabilization

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Electrostatic interaction across a turn of the helix

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Side chains a and d (i and i + 3) or a and e (i and i
+ 4) are close in space

Super-secondary Structure: Coiled-Coil

• Coiled-Coil helices are quite common
• Occur when an amphipathic helix is primarily hydrophilic, but has
hydrophobic residues in the i and i + 3 positions which allow two helices
to interact with each other
• Can be parrallel or antiparrallel
• Can be homo- or hetero-dimers

Supersecondary Structure: Coiled-Coil
Myosin

Tropomyosin
Single R133W mutation
in human tropomyosin
leads to
muscle weakness and
distal limb deformities

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