Alpha-Helix: Right-Handed Staircase
The basic structure of the α-helix is like a
staircase with the polypeptide backbone winding in right-handed,
stair-step fashion around a central axis. The geometry of the α-helix is a little
easier to visualize if you build it yourself. In this exercise,
an eight-residue portion of the helix E segment of myoglobin
α-helix will be
assembled and disassembled in stepwise fashion.
In order to build this α-helical segment, one AA
residue at a time, click on the buttons below in sequence. In
this way you will move "up" the helix in the N-to-C direction sequentially joining the 8 adjacent AA residues in this
segment.
Start building the α-helix by pressing the first
residue button below and then add residues to the sequence. The position and orientation of the
α-helix is
set with the first AA button. Initially, the a-helix will be viewed
along its vertical axis from the side. However, you can build the α-helix
from any orientation by moving the image.
|
start Asp60 |
add Leu61 |
add Lys62 |
add Lys63 |
|
add His64 |
add Gly65 |
add Val66 |
add Thr67 |
Excercises:
- "Assemble" the a-helix a few more times after rotating it to new orientations in order to get a better overview of this structure from different orientations. Start with the Asp60 button above to set orientation, and and then rotate the structure before building the remainder of the α-helix.
- Now "disassemble" the
α-helix
by clicking the buttons above in the reverse order (bottom to top). Be sure to do this after changing orientation
several times.
Note how adjacent AA residues rise in parallel to the axis with a right-handed twist, regardless of whether the α-helix is "right-side up" or "upside down." The right-handed twist is inherently asymmetric; that is it is not super imposable on the mirror-image left-handed twist, just as your two hands are mirror images of each other but are not superimposable objects. - Turn the H bonds on and ofby using the Pop Up Menu.
- Go to "Style", "Hydrogen Bonds", "Calculate"
- Then "Style", "Hydrogen Bonds", "On"
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| © | Duane W. Sears Revised: August 19, 2012 |