How to extract chains from a PDB file?

bash, bioinformatics, biopython, python

Solution

The following BioPython code should suit your needs well.

It uses `PDB.Select` to only select the desired chains (in your case, one chain) and `PDBIO()` to create a structure containing just the chain.

import os
from Bio import PDB


class ChainSplitter:
    def __init__(self, out_dir=None):
        """ Create parsing and writing objects, specify output directory. """
        self.parser = PDB.PDBParser()
        self.writer = PDB.PDBIO()
        if out_dir is None:
            out_dir = os.path.join(os.getcwd(), "chain_PDBs")
        self.out_dir = out_dir

    def make_pdb(self, pdb_path, chain_letters, overwrite=False, struct=None):
        """ Create a new PDB file containing only the specified chains.

        Returns the path to the created file.

        :param pdb_path: full path to the crystal structure
        :param chain_letters: iterable of chain characters (case insensitive)
        :param overwrite: write over the output file if it exists
        """
        chain_letters = [chain.upper() for chain in chain_letters]

        # Input/output files
        (pdb_dir, pdb_fn) = os.path.split(pdb_path)
        pdb_id = pdb_fn[3:7]
        out_name = "pdb%s_%s.ent" % (pdb_id, "".join(chain_letters))
        out_path = os.path.join(self.out_dir, out_name)
        print "OUT PATH:",out_path
        plural = "s" if (len(chain_letters) > 1) else ""  # for printing

        # Skip PDB generation if the file already exists
        if (not overwrite) and (os.path.isfile(out_path)):
            print("Chain%s %s of '%s' already extracted to '%s'." %
                    (plural, ", ".join(chain_letters), pdb_id, out_name))
            return out_path

        print("Extracting chain%s %s from %s..." % (plural,
                ", ".join(chain_letters), pdb_fn))

        # Get structure, write new file with only given chains
        if struct is None:
            struct = self.parser.get_structure(pdb_id, pdb_path)
        self.writer.set_structure(struct)
        self.writer.save(out_path, select=SelectChains(chain_letters))

        return out_path


class SelectChains(PDB.Select):
    """ Only accept the specified chains when saving. """
    def __init__(self, chain_letters):
        self.chain_letters = chain_letters

    def accept_chain(self, chain):
        return (chain.get_id() in self.chain_letters)


if __name__ == "__main__":
    """ Parses PDB id's desired chains, and creates new PDB structures. """
    import sys
    if not len(sys.argv) == 2:
        print "Usage: $ python %s 'pdb.txt'" % __file__
        sys.exit()

    pdb_textfn = sys.argv[1]

    pdbList = PDB.PDBList()
    splitter = ChainSplitter("/home/steve/chain_pdbs")  # Change me.

    with open(pdb_textfn) as pdb_textfile:
        for line in pdb_textfile:
            pdb_id = line[:4].lower()
            chain = line[4]
            pdb_fn = pdbList.retrieve_pdb_file(pdb_id)
            splitter.make_pdb(pdb_fn, chain)

One final note: don't write your own parser for PDB files. The format specification is ugly (really ugly), and the amount of faulty PDB files out there is staggering. Use a tool like BioPython that will handle parsing for you!

Furthermore, instead of using `wget`, you should use tools that interact with the PDB database for you. They take FTP connection limitations into account, the changing nature of the PDB database, and more. I should know - I updated `Bio.PDBList` to account for changes in the database. =)

Problem

I would like to extract chains from pdb files. I have a file named pdb.txt which contains pdb IDs as shown below. The first four characters represent PDB IDs and last character is the chain IDs. ``` 1B68A 1BZ4B 4FUTA ``` I would like to 1) read the file line by line 2) download the atomic coordinates of each chain from the corresponding PDB files. 3) save the output to a folder. I used the following script to extract chains. But this code prints only A chains from pdb files. ``` for i in 1B68 1BZ4 4FUT do wget -c "http://www.pdb.org/pdb/download/downloadFile.do?fileFormat=pdb&compression=NO&structureId="$i -O $i.pdb grep ATOM $i.pdb | grep 'A' > $i\_A.pdb done ```

Original source