Version Compatibility

Reference examples tested with: BioPython 1.83+

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Structure Navigation

"Access residues and atoms in a PDB structure" → Navigate the Structure-Model-Chain-Residue-Atom hierarchy to iterate over components, extract sequences, and access atomic coordinates.

• Python: structure[0]['A'][100]['CA'].get_vector() for direct access

Navigate the Structure-Model-Chain-Residue-Atom (SMCRA) hierarchy to access and iterate over structure components.

Required Imports

python
from Bio.PDB import PDBParser, PPBuilder, Selection
from Bio.Data.PDBData import protein_letters_3to1

SMCRA Hierarchy

Structure
    |
    +-- Model (0, 1, ...)      # NMR ensembles, crystal asymmetric unit
          |
          +-- Chain (A, B, ...) # Polypeptide chains, ligands
                |
                +-- Residue     # Amino acids, nucleotides, hetero groups
                      |
                      +-- Atom  # Individual atoms

Accessing Hierarchy Levels

python
from Bio.PDB import PDBParser

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

# Access by index/ID
model = structure[0]          # First model
chain = model['A']            # Chain A
residue = chain[100]          # Residue 100 (simple numbering)
residue = chain[(' ', 100, ' ')]  # Full residue ID (hetfield, resseq, icode)
atom = residue['CA']          # C-alpha atom

Iterating Over Structure

python
# Iterate all levels
for model in structure:
    for chain in model:
        for residue in chain:
            for atom in residue:
                print(f'{chain.id}:{residue.id[1]}:{atom.name}')

# Shortcut iterators (all levels below current)
for chain in structure.get_chains():
    print(f'Chain: {chain.id}')

for residue in structure.get_residues():
    print(f'Residue: {residue.resname}')

for atom in structure.get_atoms():
    print(f'Atom: {atom.name} at {atom.coord}')

Residue Identification

python
# Residue ID is a tuple: (hetfield, resseq, icode)
for residue in chain:
    hetfield, resseq, icode = residue.id
    print(f'Residue {resseq}{icode}: {residue.resname}')

# hetfield values:
#   ' '  - standard amino acid
#   'W'  - water
#   'H_xxx' - hetero residue (ligand, modified residue)

# Filter standard residues only
standard_residues = [r for r in chain if r.id[0] == ' ']

# Filter water
waters = [r for r in chain if r.id[0] == 'W']

# Filter hetero atoms (ligands)
hetero = [r for r in chain if r.id[0].startswith('H_')]

Atom Properties

python
for atom in residue:
    print(f'Name: {atom.name}')
    print(f'Element: {atom.element}')
    print(f'Coordinates: {atom.coord}')
    print(f'B-factor: {atom.bfactor}')
    print(f'Occupancy: {atom.occupancy}')
    print(f'Full ID: {atom.full_id}')
    print(f'Serial number: {atom.serial_number}')

Getting Full Identifiers

python
# Full hierarchical ID from any entity
atom = structure[0]['A'][100]['CA']
print(atom.get_full_id())
# ('protein', 0, 'A', (' ', 100, ' '), ('CA', ' '))

# Components: (structure_id, model_id, chain_id, residue_id, atom_id)

Checking for Children

python
# Check if entity has child
if chain.has_id(100):
    residue = chain[100]

# Check if residue has atom
if residue.has_id('CA'):
    ca = residue['CA']

# Get list of all children
chains = structure[0].get_list()
residues = chain.get_list()
atoms = residue.get_list()

Getting Parent Entity

python
# Navigate up hierarchy
atom = structure[0]['A'][100]['CA']

residue = atom.get_parent()
chain = residue.get_parent()
model = chain.get_parent()
structure = model.get_parent()

Extracting Polypeptide Sequences

python
from Bio.PDB import PDBParser, PPBuilder

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

ppb = PPBuilder()
for pp in ppb.build_peptides(structure):
    seq = pp.get_sequence()
    print(f'Polypeptide: {seq}')
    print(f'Length: {len(seq)}')

# Get all sequences as list
sequences = [pp.get_sequence() for pp in ppb.build_peptides(structure)]

Using CaPPBuilder for Broken Chains

python
from Bio.PDB import CaPPBuilder

# Use when backbone is incomplete
# Connects residues if CA atoms are within 4.3 Angstroms
ppb = CaPPBuilder()
for pp in ppb.build_peptides(structure):
    print(f'Fragment: {pp.get_sequence()}')

Converting Residue Names

python
from Bio.Data.PDBData import protein_letters_3to1

# Three-letter to one-letter conversion
three_letter = 'ALA'
one_letter = protein_letters_3to1.get(three_letter, 'X')
print(f'{three_letter} -> {one_letter}')  # ALA -> A

# Build sequence manually
sequence = ''
for residue in chain:
    if residue.id[0] == ' ':  # Standard residue
        code = protein_letters_3to1.get(residue.resname, 'X')
        sequence += code
print(f'Sequence: {sequence}')

Using Selection.unfold_entities

python
from Bio.PDB import Selection

# Extract entities at specific level
# Codes: S=structure, M=model, C=chain, R=residue, A=atom

# Get all residues from structure
residues = Selection.unfold_entities(structure, 'R')
print(f'Total residues: {len(residues)}')

# Get all atoms from a chain
atoms = Selection.unfold_entities(chain, 'A')
print(f'Atoms in chain: {len(atoms)}')

# Get all chains from model
chains = Selection.unfold_entities(model, 'C')

Handling Disordered Atoms

python
# Check for disorder
if atom.is_disordered():
    print(f'Atom {atom.name} has multiple conformations')
    print(f'Alt locations: {atom.disordered_get_id_list()}')

    # Select specific conformation
    atom.disordered_select('A')
    print(f'Coord for alt A: {atom.coord}')

    # Get all conformations
    for altloc in atom.disordered_get_id_list():
        atom.disordered_select(altloc)
        print(f'  {altloc}: {atom.coord}')

# Get unpacked list (all conformations)
all_atoms = atom.disordered_get_list()

Handling Disordered Residues

python
# Point mutations at same position
if residue.is_disordered():
    print(f'Disordered residue at {residue.id}')
    names = residue.disordered_get_id_list()
    print(f'Alternative residues: {names}')

    # Select specific residue type
    residue.disordered_select('ALA')

Finding Specific Atoms

python
# Get backbone atoms
backbone_names = ['N', 'CA', 'C', 'O']
for residue in chain:
    backbone = [residue[name] for name in backbone_names if residue.has_id(name)]

# Get all C-alpha atoms
ca_atoms = [r['CA'] for r in structure.get_residues() if r.has_id('CA')]
print(f'Found {len(ca_atoms)} CA atoms')

# Get sidechain atoms
for residue in chain:
    sidechain = [a for a in residue if a.name not in ['N', 'CA', 'C', 'O']]

Filtering by Residue Type

python
# Get only amino acids
amino_acids = [r for r in chain if r.id[0] == ' ']

# Get specific amino acid types
arginines = [r for r in chain if r.resname == 'ARG']
charged = [r for r in chain if r.resname in ['ARG', 'LYS', 'ASP', 'GLU']]

# Get hetero atoms
ligands = [r for r in chain if r.id[0].startswith('H_')]
for lig in ligands:
    print(f'Ligand: {lig.resname} at position {lig.id[1]}')

Counting Entities

python
# Count at each level
n_models = len(list(structure.get_models()))
n_chains = len(list(structure.get_chains()))
n_residues = len(list(structure.get_residues()))
n_atoms = len(list(structure.get_atoms()))

print(f'Models: {n_models}, Chains: {n_chains}')
print(f'Residues: {n_residues}, Atoms: {n_atoms}')

# Count per chain
for chain in structure.get_chains():
    n_res = len([r for r in chain if r.id[0] == ' '])
    print(f'Chain {chain.id}: {n_res} amino acids')

Working with NMR Ensembles

python
# NMR structures have multiple models
parser = PDBParser(QUIET=True)
structure = parser.get_structure('nmr', 'nmr_structure.pdb')

n_models = len(list(structure.get_models()))
print(f'NMR ensemble with {n_models} conformers')

# Iterate over models
for model in structure:
    # Each model is a separate conformation
    ca_coords = [r['CA'].coord for r in model.get_residues() if r.has_id('CA')]
    print(f'Model {model.id}: {len(ca_coords)} CA atoms')

Related Skills

• structure-io - Parse and write structure files
• geometric-analysis - Measure distances, angles, RMSD
• structure-modification - Modify coordinates and properties
• sequence-manipulation/seq-objects - Work with extracted sequences