Converting segments of large .cif files to smaller .pdb files - casting

Question

I'm trying to carve out some binding sites with ligands from cif-files of ribosome crystal structures, and have encountered an annoying problem involving a type error.
TypeError: %c requires int or char
Using the code below,
from Bio.PDB import *
from Bio import PDB
class save_res(Select):
def accept_residue(self, residue):
if residue in keep_res_list:
print(residue)
return 1
else:
return 0
keep_res_list = []
parser = MMCIFParser()
structure = parser.get_structure("1vvj.cif", "./1vvj.cif")
structure = structure[0]
atom_list = Selection.unfold_entities(structure, "A") # A for atoms
ns = NeighborSearch(atom_list)
for residue in structure.get_residues():
if residue.get_resname() == "PAR":
for atom in residue:
center = atom.get_coord()
neighbors = ns.search(center, 5.0)
neighbor_residue_list = Selection.unfold_entities(neighbors, "R")
for res in neighbor_residue_list:
if res not in keep_res_list:
keep_res_list.append(res)
io = PDBIO()
io.set_structure(structure)
io.save("1vvj_bs.pdb", save_res())
gives me the error:
File "/scratch/software/anaconda3/envs/my-devel-3.6/lib/python3.6/site-packages/Bio/PDB/PDBIO.py", line 112, in _get_atom_line
return _ATOM_FORMAT_STRING % args
TypeError: %c requires int or char
This code works well when changing the pdb-id to 1fyb, which also has the same ligand id.
I'm thinking the problem stems from the vast amounts of chains and their IDs in the original file. Am I completely wrong in this assumption or does anyone know how to fix this? I've been trying to find a way to rename the chain IDs, but haven't found a viable method to do this.
Your help is appreciated.

Answer 1

The chain name format in _ATOM_FORMAT_STRING is %c, while in this case you have chain named QA.
Chain names in PDB files were traditionally single characters.
But there are only so many letters and digits. For ribosome it's necessary to use longer names. The pdb format has space for a second letter -- empty column on the left from the 1-character chain name. Many programs support it, but not all, and this is not part of the official specification.
So you can either use PDB files with 2-character chains (if the rest of your workflow supports it) or rename chains in the output (your output is only a tiny part of the original structure).
Here is how to do it in gemmi:
import gemmi
structure = gemmi.read_structure('1vvj.cif')
model = structure[0]
ns = gemmi.NeighborSearch(model, structure.cell, 5.0).populate()
for chain in model:
for residue in chain:
if residue.name == 'PAR':
for atom in residue:
for nb in ns.find_neighbors(atom):
nb.to_cra(model).residue.flag = 'y'
sel = gemmi.Selection().set_residue_flags('y')
new_structure = sel.copy_structure_selection(structure)
#new_structure.remove_empty_chains()
#new_structure.shorten_chain_names()
new_structure.write_minimal_pdb('1vvj-par.pdb')
The two commented out lines are renaming the chains.
One difference comparing with your code is that the NeighborSearch in gemmi is symmetry-aware. It finds also nearby atoms from symmetry mates. In BioPython you search only in asymmetric unit (asu).
Both are different than the biological assembly --
PDB-101 covers it nicely.
If you'd like to search in asu only -- replace structure.cell with gemmi.UnitCell() above, i.e. don't pass the unit cell information.
(You can ask such questions on bioinformatics.SE -- it should get answer sooner there).