Submit the file as a PDF file using the "submit" command. The
class locker is "bme206-kk.f06" and the assignment is "darling".
There are scanners in most of the ITS computer labs (not in Baskin
105, but next door in Baskin 109), if you want to convert something
hand-drawn into a digital image. See
http://ic.ucsc.edu/labs/hardware.shtml
for a list of which ITS labs have which equipment.
You can also you ChemDraw or similar software (see
http://ic.ucsc.edu/labs/software.shtml
for what software is in the instructional computing labs).
Note: we will only be accepting PDF files, *not* Microsoft word
files. Because this assignment requires some pictures, plain text
will not be adequate. Note: Acrobat is supposedly available in all
Mac and PC labs run by Instructional Computing, so PDF should be
fairly easy to produce. The Acrobat Distiller is also available on
School of Engineering SUN Sparc computers (as "distill"), but not on
the Linux machines or solopteron (run distill on "sundance",
"moondance", or "apache" from an SoE account).
- Build a protein backbone, at least 6 (and preferably 9) amino
acids long, following
the instructions I wrote.
Make sure you get the chirality right---this is determined by which
of the two available bond positions on C-alpha is used for the
hydrogen and which is used for the sidechain.
The CORN mnemonic (CO-R-N clockwise around the alpha carbon when
looking at the alpha hydrogen) helps get chirality right.
Fold the backbone into an alpha helix, remembering that there
should be a hydrogen bond between Oi and Ni+4.
Remember that the peptide plane with Oi has Ni+1.
Some things to check: do all the attachment points for side chains
point out of the helix?
Is the helix fairly rigid?
Have you got the twist going the right way (if you hold the helix
axis vertically, the backbone should be slanted like the middle
line of a Z)?
Measure the distances between C-alpha atoms separated by 1, 2, 3, ...
The scale of the Darling models is about 2 inches (or 5cm) to an
Angstrom.
(Turn in the table of distances in Angstroms between the centers of
C-alpha atoms.)
Add a C-beta carbon to each C-alpha, and measure the distances
for C-beta atoms separated by 1, 2, 3, ...
(Turn in the table of distances in Angstroms between the centers of
C-beta atoms.)
- Make a proline residue and add it to the C-terminal end of the
helix.
Note how the lack of a hydrogen on the backbone nitrogen prevents the
proline from participating in the hydrogen bonds that stabilize
the helix.
Try putting the proline at the N-terminal end of the helix.
Are there problems with the H-bond at this end?
Can the helix extend back before the proline?
How far back can it extend?
(That is can the proline be the first, second, third,
... position in an alpha helix?) I am not looking for a distance
here, but a count of the
number of residues.
That is, if proline is residue "i", can i-1 be part of the
helix, i-2, ... ?
Try converting the peptide of the proline from trans to cis
conformation.
There are two ways to do this: taking apart the peptide plane and
twisting the omega angle from 180o to 0o,
or changing which carbon you think of as Calpha and
which as Cdelta, changing which one the carbonyl
carbon bonds to.
The first method is probably closer to what happens with a
cis-trans-isomerase, but the second is a lot easier to do with
the models.
- Remove the proline, and add a serine side chain to the C-alpha
before the first residue of the helix (remember that "first"
means N-terminal). Make a hydrogen bond between the O of the
OH group and the HN of third residue of the helix.
This is a common N-cap motif for helices.
Try the same cap with a threonine instead of a serine (also
fairly common).
- Undo your helix H-bonds and split your protein backbone into two
chains. Align the chains as parallel beta strands, and arrange
the H bonds correctly. Which Hbonds are formed? Measure and
report the distance between the closest C-beta atoms from one
strand to the other. What is the distance from one C-beta to the
next one along the strand? To the C-beta that is two away on the strand?
- Now rearrange the strands to be anti-parallel. Again, which
H-bonds are formed, and what is the distance between the closest
C-beta atoms?
- Join the two anti-parallel strands with an aspartic acid (D) or
asparagine (N) followed by a glycine. If you want, you can
also make the two residues on either side be lysine (K), so
that you have the sequence KDGK or KNGK. This sequence makes
a tight bend about 41% of the time, XXK[DN]GXX forms a hairpin
about 36% of the time. (In past years, I suggested XPDG, which
produces a tight bend about 45% of the time, but XXXPDGXX only
forms a hairpin about 2% of the time.) Note that the K[DN]GK
turn is going to be a type I' turn (See http://www.cryst.bbk.ac.uk/PPS95/course/6_super_sec/super1.html
by j.cooper for an explanation of type I' and II' turns.
Come up with a pattern of hydrogen bonds that holds this
hairpin together. Hand in a sketch of the Hbonds.
Make sure that you take into consideration that the atoms are
actually space-filling—many of the conformations
obtainable with the models correspond to physically impossible
ones.
Also, make sure that the lysine side chains are both on the
same side of the hairpin.
- Build a pair of complementary DNA bases and find the Hbonds.
Note that the Hbonds may not come straight out from the double-bonded
oxygen (as in the proteins) but may be at closer to a 120 degree angle.
For the pair of bases you chose, make a sketch of the structure
showing the Hbonds, and indicate the angles for
carbon-donor-acceptor (CDA) and donor-acceptor-carbon (DAC).
Note: the nitrogen (or whatever the hydrogen is covalently bound
to) is the donor of the hydrogen for the
hydrogen bond, and the oxygen (or whatever electronegative atom
is not covalently bonded to the hydrogen) is the acceptor.
The CDA angle has its vertex at the donor atom, one ray to the
acceptor, and the other ray to the carbon that the donor is
covalently bonded to (there may be multiple such carbons, and so
multiple CDA angles).
Similarly the DAC angle has its vertex at the acceptor, and the
rays to the donor and to a carbon covalently bonded to the acceptor.
Note: no one outside the Karplus lab uses the CDA and DAC angles,
preferring to work with angles around the hydrogen atom.
Things learned to improve assignment next time
Still some confusion about the beta hairpin. Emphasize in class (or
somewhere) the need to have 2 H-bonds to stabilize a pair of beta
strands. Perhaps provide a pointer to a structure in PDB that has a
hairpin that they can copy the structure from???
Questions about page content should be directed to
Kevin Karplus
Biomolecular Engineering
University of California, Santa Cruz
Santa Cruz, CA 95064
USA
karplus@soe.ucsc.edu
1-831-459-4250
