.. DO NOT EDIT. .. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY. .. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE: .. "examples/gallery/structure/disulfide_bonds.py" .. LINE NUMBERS ARE GIVEN BELOW. .. only:: html .. note:: :class: sphx-glr-download-link-note :ref:`Go to the end ` to download the full example code .. rst-class:: sphx-glr-example-title .. _sphx_glr_examples_gallery_structure_disulfide_bonds.py: Detection of disulfide bonds ============================ This example presents a function that detects disulfide bridges in protein structures. Then the detected disulfide bonds are visualized and added to the `bonds` attribute of the :class:`AtomArray`. The employed criteria for disulfide bonds are quite simple in this case: the :math:`S_\gamma` atoms of two cystein residues must be in a vicinity of :math:`2.05 \pm 0.05` Å and the dihedral angle of :math:`C_\beta - S_\gamma - S^\prime_\gamma - C^\prime_\beta` must be :math:`90 \pm 10 ^{\circ}`. .. GENERATED FROM PYTHON SOURCE LINES 16-93 .. code-block:: Python # Code source: Patrick Kunzmann # License: BSD 3 clause import io from tempfile import gettempdir import numpy as np import matplotlib.pyplot as plt import matplotlib.patches as patches import biotite.sequence as seq import biotite.structure as struc import biotite.structure.io.pdbx as pdbx import biotite.database.rcsb as rcsb def detect_disulfide_bonds(structure, distance=2.05, distance_tol=0.05, dihedral=90, dihedral_tol=10): # Array where detected disulfide bonds are stored disulfide_bonds = [] # A mask that selects only S-gamma atoms of cysteins sulfide_mask = (structure.res_name == "CYS") & \ (structure.atom_name == "SG") # sulfides in adjacency to other sulfides are detected in an # efficient manner via a cell list cell_list = struc.CellList( structure, cell_size=distance+distance_tol, selection=sulfide_mask ) # Iterate over every index corresponding to an S-gamma atom for sulfide_i in np.where(sulfide_mask)[0]: # Find indices corresponding to other S-gamma atoms, # that are adjacent to the position of structure[sulfide_i] # We use the faster 'get_atoms_in_cells()' instead of # `get_atoms()`, as precise distance measurement is done # afterwards anyway potential_bond_partner_indices = cell_list.get_atoms_in_cells( coord=structure.coord[sulfide_i] ) # Iterate over every index corresponding to an S-gamma atom # as bond partner for sulfide_j in potential_bond_partner_indices: if sulfide_i == sulfide_j: # A sulfide cannot create a bond with itself: continue # Create 'Atom' instances # of the potentially bonds S-gamma atoms sg1 = structure[sulfide_i] sg2 = structure[sulfide_j] # For dihedral angle measurement the corresponding # C-beta atoms are required, too cb1 = structure[ (structure.chain_id == sg1.chain_id) & (structure.res_id == sg1.res_id) & (structure.atom_name == "CB") ] cb2 = structure[ (structure.chain_id == sg2.chain_id) & (structure.res_id == sg2.res_id) & (structure.atom_name == "CB") ] # Measure distance and dihedral angle and check criteria bond_dist = struc.distance(sg1, sg2) bond_dihed = np.abs(np.rad2deg(struc.dihedral(cb1, sg1, sg2, cb2))) if bond_dist > distance - distance_tol and \ bond_dist < distance + distance_tol and \ bond_dihed > dihedral - dihedral_tol and \ bond_dihed < dihedral + dihedral_tol: # Atom meet criteria -> we found a disulfide bond # -> the indices of the bond S-gamma atoms # are put into a tuple with the lower index first bond_tuple = sorted((sulfide_i, sulfide_j)) # Add bond to list of bonds, but each bond only once if bond_tuple not in disulfide_bonds: disulfide_bonds.append(bond_tuple) return np.array(disulfide_bonds, dtype=int) .. GENERATED FROM PYTHON SOURCE LINES 94-106 As test case a structure of a *cysteine knot* protein is used, specifically the squash trypsin inhibitor *EETI-II* (PDB: `2IT7 `_). This motif is famous for its three characteristic disulfide bridges forming a 'knot'. However, the loaded PDBx file already has information about the the disulfide bridges. To have a proper test case, all disulfide bonds are removed from the structure and we pretend that the structure never had information about the disulfide bonds. For later verification that the implemented function works correctly, the disulfide bonds, that are removed, are printed out. .. GENERATED FROM PYTHON SOURCE LINES 106-121 .. code-block:: Python pdbx_file = pdbx.BinaryCIFFile.read( rcsb.fetch("2IT7", "bcif", gettempdir()) ) knottin = pdbx.get_structure(pdbx_file, include_bonds=True, model=1) sulfide_indices = np.where( (knottin.res_name == "CYS") & (knottin.atom_name == "SG") )[0] for i, j, _ in knottin.bonds.as_array(): if i in sulfide_indices and j in sulfide_indices: print(knottin[i]) print(knottin[j]) print() knottin.bonds.remove_bond(i,j) .. rst-class:: sphx-glr-script-out .. code-block:: none A 2 CYS SG S -5.182 1.760 -2.385 A 19 CYS SG S -4.800 -0.230 -2.187 A 9 CYS SG S 4.169 -0.459 -0.194 A 21 CYS SG S 5.347 -0.755 -1.829 A 15 CYS SG S 0.741 -1.839 -1.408 A 27 CYS SG S 0.436 -1.062 0.449 .. GENERATED FROM PYTHON SOURCE LINES 122-126 Now the sanitized structure is put into the disulfide detection function. The detected bonds are printed out and we expect to see the same bonds, that were removed in the code snippet above. .. GENERATED FROM PYTHON SOURCE LINES 126-133 .. code-block:: Python disulfide_bonds = detect_disulfide_bonds(knottin) for sg1_index, sg2_index in disulfide_bonds: print(knottin[sg1_index]) print(knottin[sg2_index]) print() .. rst-class:: sphx-glr-script-out .. code-block:: none A 2 CYS SG S -5.182 1.760 -2.385 A 19 CYS SG S -4.800 -0.230 -2.187 A 9 CYS SG S 4.169 -0.459 -0.194 A 21 CYS SG S 5.347 -0.755 -1.829 A 15 CYS SG S 0.741 -1.839 -1.408 A 27 CYS SG S 0.436 -1.062 0.449 .. GENERATED FROM PYTHON SOURCE LINES 134-138 The found disulfide bonds are visualized with the help of *Matplotlib*: The amino acid sequence is written on the X-axis and the disulfide bonds are depicted by yellow semi-ellipses. .. GENERATED FROM PYTHON SOURCE LINES 138-170 .. code-block:: Python # Create a sequence object for each residue in the structure # As we want each residue to appear only once in the sequence, # we choose an atom that appears in each residue once: the CA sequence = seq.ProteinSequence(knottin.res_name[knottin.atom_name == "CA"]) figure = plt.figure(figsize=(4.0, 1.0)) ax = figure.gca() MARGIN = 0.2 ax.set_xlim(1-MARGIN, len(sequence)+MARGIN) ax.set_ylim(0, 1+MARGIN) ax.set_xticks(np.arange(1, len(sequence)+1)) ax.set_xticklabels(str(sequence)) ax.yaxis.set_tick_params( left=False, right=False, labelleft=False, labelright=False ) ax.xaxis.set_tick_params( bottom=True, top=False, labelbottom=True, labeltop=False, width=0 ) ax.set_frame_on(False) for sg1_index, sg2_index in disulfide_bonds: sg1_res_id = knottin.res_id[sg1_index] sg2_res_id = knottin.res_id[sg2_index] ellipse_center = (sg1_res_id + sg2_res_id) / 2 ellipse_width = sg2_res_id - sg1_res_id # Height is 2 instead of 1, # because only the upper half of the ellipse is visible ax.add_patch(patches.Ellipse( xy=(ellipse_center, 0), width=ellipse_width, height=2, facecolor="None", edgecolor="gold", linewidth=2 )) figure.tight_layout() .. image-sg:: /examples/gallery/structure/images/sphx_glr_disulfide_bonds_001.png :alt: disulfide bonds :srcset: /examples/gallery/structure/images/sphx_glr_disulfide_bonds_001.png :class: sphx-glr-single-img .. GENERATED FROM PYTHON SOURCE LINES 171-173 Finally, the detected bonds are added to the structure. Basically, the removal step above is reversed. .. GENERATED FROM PYTHON SOURCE LINES 173-183 .. code-block:: Python for sg1_index, sg2_index in disulfide_bonds: knottin.bonds.add_bond(sg1_index, sg2_index, struc.BondType.SINGLE) # The structure with added disulfide bonds # could now be written back into a structure file # out_file = pdbx.BinaryCIFFile() pdbx.set_structure(out_file, knottin) out_file.write(io.BytesIO()) plt.show() .. _sphx_glr_download_examples_gallery_structure_disulfide_bonds.py: .. only:: html .. container:: sphx-glr-footer sphx-glr-footer-example .. container:: sphx-glr-download sphx-glr-download-jupyter :download:`Download Jupyter notebook: disulfide_bonds.ipynb ` .. container:: sphx-glr-download sphx-glr-download-python :download:`Download Python source code: disulfide_bonds.py ` .. only:: html .. rst-class:: sphx-glr-signature `Gallery generated by Sphinx-Gallery `_