This tutorial will walk you through two different types of BD simulations on the Thrombin-Thrombomodulin system, which is an important component of the blood-clotting cascade. Thrombin consists of 295 amino acids, and thrombomodulin consists of 117 amino acids. (My thanks to Adam Van Wynsberghe for the necessary data on these two molecules.)
This tutorial assumes that you have Linux on your computer. If you have only Windows, a quick and easy way to get Ubuntu Linux onto your machine is Wubi (Windows Ubuntu Installer). Other necessary software is the Ocaml compiler, which is free and can be obtained from here . If you are using Ubuntu or a similar Linux based on Debian, you can just type
sudo apt-get install ocamlYou will also need APBS; this can also quickly be installed on Ubuntu by typing
sudo apt-get install apbsFinally, you can install BrownDye on your machine by going to the website , downloading the file
browndye.tar.gz
, moving the file to a directory of your choice,
and typing
tar xvfz browndye.tar.gz cd browndye make allAll of the executables are now in the directory
browndye/bin
; you need
to add this directory to your path. If you use a bash shell, this can
be done by typing
PATH=$PATH:fullpath/browndye/binwhere
fullpath
is the full path name of the directory containing the
browndye distribution.
If you now want to run the thrombin-thrombomodulin tutorial, go to
browndye/thrombin-example
.
The atomic coordinates for thrombin and thrombomodulin are in
files t.pqr
and m.pqr
. Because BrownDye works primarily with
XML files, you must convert this two files to an equivalent XML format:
pqr2xml < t.pqr > t-atoms.xml pqr2xml < m.pqr > m-atoms.xmlNext, you must generate the electrostatic grids, in dx format, using APBS:
apbs t.in apbs m.inwhere the input files
t.in
and m.in
are provided. The grids are
output to t-PE0.dx
and m-PE0.dx
.
Throughout this session, thrombin will
be denoted by the prefix "t" while thrombomodulin will be denoted
by prefix "m".
Be sure to take note of the Debye length
in the APBS output if you don't feel like calculating it by hand; it
will be needed later.
In addition to atomic coordinates and grids, the third key input
is the set of reaction criteria. This can be generated from the
two coordinate files t-atoms.pqrxml
and m-atoms.pqrxml
, and a file,
protein-protein-contacts.xml
, which describes which pairs of atom
types can define a contact
(It is actually stored as protein-protein-contacts.xml.bak
; you
need to copy from this file.)
The program make_rxn_pairs
takes these
three files and a search distance to generate a file of reaction
pairs. This assumes that the coordinates of the two molecules
are consistent with the bound state.
make_rxn_pairs -mol0 t-atoms.pqrxml -mol1 m-atoms.pqrxml -ctypes protein-protein-contacts.xml -dist 6.0 > t-m-pairs.xmlThe resulting file still is not suitable for input into the simulation programs, however. I've made the program general enough to have more than one reaction in a simulation, so one could envision having several such reaction pair files that would need to combined into a final reaction description file. For now, you can use the program
make_rxn_file
,
which generates an input file for the case of one reaction:
make_rxn_file -pairs t-m-pairs.xml -distance 5.5 -nneeded 3 > t-m-rxns.xmlThis generates a reaction description file which tells the simulation programs that if any 3 of the atom pairs approach within 5.5 Angstroms, a reaction occurs.
A note: if you don't feel like typing the above commands in, especially if you make changes and need to do it repeatedly, I have included a Makefile in the example directory. Typing
make allshould run the above commands.
The remaining pieces of information are contained in the file input.xml
.
(This must be copied from the file input.xml.bak
.)
It contains, among other things, information on the solvent,
information on each molecule, and parameters governing the simulation
itself.
For the sake of efficiency, the larger molecule
should be "molecule0" if there is a large size difference.
Also, each molecule is assigned a prefix as mentioned above; these
are used in naming the intermediate files that are generated in the
next step:
bd_top input.xmlThe bd_top program is written in Ocaml using a Unix "make"-like utility that I wrote to help orchestrate the creation of the files. Like "make", if an intermediate file is changed or replaced, running bd_top (which is analogous to a Makefile) will run only those commands necessary to re-generate files that depend on the updated file. Unlike "make", this utility, which I call the "Orchestrator", can also read information from xml files and have chains of dependent calculations (eventually I want to write a version in Python so it will look more familiar to most people). So, when the command is executed, the following files are generated for thrombin:
The following files are generated for both molecules:
bd_top
run again to update everything.
For example, right now I'm using a simple test-charge approximation by
default, but one could easily generate effective charges using another
program such as SDA, convert the output into the appropriate XML format,
replace t-charges.xml
and m-charges.xml
, and run bd_top
again.
Another useability note: if you want to clean things up, you
can delete all *.dx and *.xml files; the two xml files that you
need to get started are also available as input.xml.bak
and
protein-protein-contacts.xml.bak
.
At this point, you can choose to do a simulation of one trajectory at a time, or you can do a weighted-ensemble simulation. In general, the weighted-ensemble method is not as efficient at the single-trajectory method unless the probability of a reaction event is very low.
To perform the single-trajectory simulation, the following is executed:
nam_simulation t-m-simulation.xmlThe results end up in
results.xml
, as designated in input.xml
.
As the simulation proceeds, you can look at results.xml
to
see the simulation progress. (This is called nam_simulation
after
Northrup, Allison, and McCammon, who came up with the first algorithm
of this type. This code uses a fancy variation on the orginal
algorithm.).
At any point, you can use compute_rate_constant
to
analyze results.xml
and obtain an estimate of and 95% confidence bounds
on
the reaction rate constant in units of M/s. The file t-m-solvent.xml
must also be given to this program:
compute_rate_constant < results.xmlThis will put the rate constant results to standard output.
To perform the weighted-ensemble method, you must first generate the bins for the system copies:
build_bins t-m-simulation.xmlThe number of system copies used in the bin-building process are given in
input.xml
in the n-copies
tag.
As it runs, reaction coordinate numbers will go scrolling past; they should keep
getting smaller and eventually stop. If that does not happen, i.e.,
the numbers keep on going, you might need to increase the number of system
copies, or it might be that your reaction criterion is unattainable.
Assuming is converges, the bin information is place in t-m-bins.xml
.
The actual weighted-ensemble simulation is then run:
we_simulation t-m-simulation.xmlAs before, the results are output to
results.xml
. In each row of
output numbers, the right-most number is the flux of system copies that
escaped without reaction, while the other ones are reactive fluxes.
So, even for a rare reaction event, you should at least see small numbers for
the reactive fluxes after the system has reacted steady-state.
This can be visually examined at any point, and can also be analyzed as
above, but with a different program:
compute_rate_constant_we -sim results.xml -solvent t-m-solvent.xmlBecause the streams of numbers are autocorrelated, a more sophisticated approach for computing confidence intervals is used, and if there are not enough data points, the program
compute_rate_constant_we
will
simply refuse to provide an answer.
You can change the random number generator seed, under the seed
tag.
Good to do if you're bored but don't have the energy to do anything else.
One parameter to play with is the reaction criterion distance, which is the
-distance
input to the program make_rxn_file
. The number given in
the tutorial and the Makefile is 5.5, but you can change that by re-running
make_rxn_file
or by changing it in the Makefile.
You can also change the ionic strength in files t.in
and m.in
and
generate new APBS grids. Note: you must take note of the new Debye length
and put that value in the file input.xml
. So far, it is not possible to
automatically get the Debye length from the output DX file of APBS.
If your machine has several processors, you
can change the value under the tag n-threads
in file input.xml
and
see it run under several processors. So far, it runs only on shared-memory
machines using pthreads
.
Even on a single-processor
machine, you can still run several threads, but it does not make the
programs go any faster.
A final useability note: Most of the programs will output a description of themselves and their options if you type in
program -help
t.pqr
and t.in
found in the
input files,
generate the APBS grid t.dx
.
m.pqr
and m.in
found in the input files,
generate the APBS grid t.dx
.
Browndye: pqr2xml
, select the option Input URL
,
using another browser tab go to the input files, and copy and paste the URL of t.pqr
into the Input URL
box. Copy and pasting URL's can easily be done by moving the curser over the link, pressing the right mouse button and selecting Copy Link Location
.
Submit
button
t-atoms.pqrxml
in new browser tab, and do not delete the tab.
m.pqr
to generate the file m-atoms.pqrxml
in its own tab.
The following steps are used to generate the reaction pairs file:
Browndye: make_rxn_pairs
.
t-atoms.pqrxml
from Browndye: pqr2xml
service into the box for Molecule 0 Input URL
.
m-atoms.pqrxml
from Browndye: pqr2xml
service into the box for Molecule 1 Input URL
.
protein-protein-contacts.xml
in the input files into the
box for Contacts Type URL
.
Search Distance
box
t-m-rxn-pairs.xml
into the Output File
box.
t-m-rxn-pairs.xml
file.
Do not delete the tab.
The following steps are used to generate the reaction description file:
Browndye: make_rxn_file
.
t-m-rxn-pairs.xml
from the output of
make_rxn_pairs to the Pairs Input URL
box.
- Enter 5.5 into the
Reaction Distance
box.
- Enter 3 into the
Number of Required Contacts
box.
- Enter
t-m-rxns.xml
into the Output File
box.
- Submit the job, and inspect the results when finished.
The following steps are used to run bd_top
and nam_simulation
bd_top
.
t-atoms.pqrxml
and m-atoms.pqrxml
into
the boxes for Molecule 0 URL
and Molecule 1 URL
.
t-m-rxns.xml
into the Reaction File URL
box.
t.dx
and m.dx
into the APBS boxes
for Molecule 0 and Molecule 1.
input.xml
in the input files into the
Input File URL
box.
results.xml
finally appears in the Output Base URL
,
keep checking it if you want until 1000 trajectories have been run and
the job stops.
results.xml
into the input URL box
for the Browndye: compute_rate_constant
service, and check the file
stdout.txt
for the rate constant information.
Huber, GA and McCammon, JA. Browndye: A Software Package for Brownian Dynamics. Computer Physics Communications 181, 1896-1905 (2010)
Ermak, DL and McCammon, JA. Brownian Dynamics with Hydrodynamic Interactions, J. Chem. Phys. 69, 1352-1360 (1978)
Northrup SH, Allison SA and McCammon JA. Brownian Dynamics Simulation of Diffusion-Influenced Bimolecular Reactions J. Chem. Phys. 80, 1517-1526
Luty BA, McCammon JA and Zhou HX. Diffusive Reaction-Rates From Brownian Dynamics Simulations - Replacing the Outer Cutoff Surface by an Analytical Treatment, J. Chem. Phys. 97, 5682-5686 (1992)
Huber GA and Kim S. Weighted-ensemble Brownian dynamics simulations for protein association reactions, Biophys. J 70, 97-110 (1996)
Gabdoulline RR and Wade RC. Effective charges for macromolecules in solvent, J. Phys. Chem 100, 3868-3878 (1996)