Calculate volume of the binding site and molecules

Written by Trent Balius, Dec. 2016.

The script to calculate volume is available here: volume_cal_sph.py

syntax: python volume_cal_sph.py input.sph spacing output_prefix

example: python volume_cal_sph.py binding_site.sph 0.5 binding_site 

how the volume calculation works.

• First, Lay a grid over the spheres.
• Count the number or points contained in the spheres (Ns).
• Count the number of points in the grid box (Ng).
• Calculate the volume of the grid box (Vb).

 Vs = Ns/Ng * Vb

• This method also produces dx files, so you can visualize (using chimera or VMD)the volume as grids.

Calculating the volume of a binding site.

You can run blastermaster.py which is distributed with DOCK3.7 and then use the lowdielectric.sph to define the pocket.

Alternatively, you can do the following:

• run dms (or you can also generated the molecular surface with Chimera) to generate a molecular surface.

• Use the sphgen program(distributed with all versions of DOCK) to flood the surface of the protein with spheres, which are then cluster by distance.
• Select the cluster that defines the binding site of interest by visualization in Chimera.
• Copy the sphere file. Using a text editor (vim) remove all clusters except the one of interest.

• Calculate the volume using the following script: volume_cal_sph.py

Calculating the volume of a small molecule.

Convert ligands to spheres using the following script: mol2toSPH_radius.py

Say, you would like to calculate how much two docking poses overlap in volume:

Convert the mol2 files to sph files:

python ~/zzz.scripts/mol2toSPH_radius.py molone.mol2 molone.sph
python ~/zzz.scripts/mol2toSPH_radius.py moltwo.mol2 moltwo.sph

Create a combined sphere file:

cat molone.sph > bothmol.sph
sed -e 's/cluster     1 /cluster     2 /g' moltwo.sph | grep -v DOCK >> bothmol.sph

>> python ~/zzz.scripts/volume_cal_sph.py molone.sph 0.5 molone
input file =  molone.sph
scale = 0.5
outputprefix = molone
max corner =  51.964 41.34 37.041
min corner =  39.322 31.367 28.854
0.5 26 20 17 0.5 0.5 0.5 [39.321999999999996, 31.366999999999997, 28.853999999999999]
molN= 1502   boxN= 8840   boxV= 1105.0
molV= 187.75

>> python ~/zzz.scripts/volume_cal_sph.py moltwo.sph 0.5 moltwo
input file =  moltwo.sph
scale = 0.5
outputprefix = moltwo
max corner =  49.176 38.767 36.529
min corner =  39.059 31.2 27.753
0.5 21 16 18 0.5 0.5 0.5 [39.058999999999997, 31.199999999999996, 27.753]
molN= 1257   boxN= 6048   boxV= 756.0
molV= 157.125

>> python ~/zzz.scripts/volume_cal_sph.py bothmol.sph 0.5 bothmol
input file =  bothmol.sph
scale = 0.5
outputprefix = bothmol
max corner =  51.964 41.34 37.041
min corner =  39.059 31.2 27.753
0.5 26 21 19 0.5 0.5 0.5 [39.058999999999997, 31.199999999999996, 27.753]
molN= 1868   boxN= 10374   boxV= 1296.75
molV= 233.5

157.125 + 187.75 - 233.5 = 111.375