We'll use
1hho
as our example. The
asymmetric unit
(published PDB file)
of 1hho has 2 chains.
In the Results, the
biological unit(s)
are called biomolecules.
What is BIOMOLECULE 1? As you can see below, for 1hho, it is a TETRAMER
(4 chains).
Sometimes there will be more than 1 biomolecule. Scroll down in the
Results page to see.
Who thinks it is a tetramer? As you can see below,
the AUTHORS believe it is a tetramer.
When the "author determined" biomolecule has a different
number of chains than the asymmetric unit, the biomolecule
is likely to be correct.
When the "author determined" biomolecule has the same
number of chains as the asymmetric unit, the biomolecule
is less likely to be correct. (Authors sometimes forget to
specify the known biomolecule in the PDB file.
See
examples
and
more about this problem.)
When a biomolecule is determined only by SOFTWARE, it is
less likely to be correct.
How was the biomolecule CONSTRUCTED?
The original chain A was duplicated to make 2 chains named A and C.
The original chain B was duplicated to make 2 chains named B and D.
For 1hho, the biological unit has more chains than the asymmetric unit.
In other cases, the biological unit may have the same number of chains,
or fewer chains. When the number is the same, the conformation is
sometimes different! In Proteopedia, you will find
a range of examples.
To try one of those examples, click on the PDB code. At the Proteopedia
page on that PDB code, underneath the molecule, click on FirstGlance
in the Resources line. In FirstGlance, click on Biological Unit.
Credits
How was the biomolecule CONSTRUCTED?