Hemoglobin shape:
tetramer or dimer of ab protomers (a2b2 protomer)
arranged in symmetric pairs
The first oligomeric protein structure studied in detail was
hemoglobin
Describe quaternary structure
multiple subunits (>1 polypeptide) combine to form the protein
subunits (individual polypeptides) assemble to make multimers, and form the quaternary structure
The individual subunit of an oligomer is
describe oligomers
protomer, which can be composed of >1 polypeptide
oligomers have repeating structures
associated with a symmetry form (rotation and translation)
What’s a benefit of having multiple subunits
less DNA Req (if it has repeating subunits, cell only needs to encode one smaller gene instead of one massive protein)
reduce impact of error (if one subunit fails, there are still others)
single subunit can be used in a different protein complex (i.e. PDC and a-ketoglutarate dehydrogenate complex)
individual protomers in a complex may be related by
rotational and translational symmetry
Some symmetries are
global (while protein)
local (some portions of the structure contain symmetry)
Pseudosymmetry
non-identical homologous subunits elated by symmetry
What symmetry would you expect for a protein that binds to a palindrome?
two fold
C2
What are the symmetry types
cyclic (CN), 1 rotation axis (n fold)
dihedral (DN), 2N protomers arranged around 2 axes
tetrahedral (12 protomers, 2 and 3 fold axes), octahedral (24 protomers, 2, 3, 4, fold axes)), icosahedral (60 protomers, 2, 3, 5 fold axis) around around multiple axes
helical (protomers related by rotation AND translation. Tobcacco Mosaic Virus)
DNA often contains symmetrical
palindrome sequences (either direction, sequence is the same)
The core E. coli RNA polymerase holoenzyme has the subunit composition a2bb´w. What are possible symmetry forms for this protein?
There is no global symmetry
There is likely to be local symmetry as there are two a subunits,
which will be related to each other spatially by a two-fold axis.
