Data items in the STRUCT_BIOL category record details about
               the structural elements that form each structure of biological
               significance.
               A given crystal structure may contain many different biological
               structures. A given structural component in the asymmetric
               unit may be part of more than one biological unit. A given
               biological structure may involve crystallographic symmetry.
               For instance, in a structure of a lysozyme-FAB structure, the
               light- and heavy-chain components of the FAB could be one
               biological unit, while the two chains of the FAB and the lysozyme
               could constitute a second biological unit.
 
    loop_
    _struct_biol.id
    _struct_biol.details
      1
    ; significant deviations from twofold symmetry exist in this
      dimeric enzyme
    ;
      2
    ; The drug binds to this enzyme in two roughly twofold
      symmetric modes. Hence this biological unit (2) is roughly
      twofold symmetric to biological unit (3). Disorder in the
      protein chain indicated with alternative ID 1 should be
      used with this biological unit.
    ;
      3
    ; The drug binds to this enzyme in two roughly twofold
      symmetric modes. Hence this biological unit (3) is roughly
      twofold symmetric to biological unit (2). Disorder in the
      protein chain indicated with alternative ID 2 should be
      used with this biological unit.
    ;