Quaternary structure

First oligomeric protein structure studied?

hemoglobin, tetramer or dimer of aB protomers

C2 symmetry, two a/B pairs related around a 180 degree axis

Multimeric structure

Multiple subunits (>1 polypeptide) combine to form the proteins

Subunits (individual polypeptides) assemble to form quaternary structure

Oligomers

Repeating structure, repeating unit is a protomer, protomers may be composed of >1 polypeptide

Associated with symmetry, rotation (common), translation (rare)

Do we see reflex symmetry?

No proteins are chiral molecules

Why do we have quaternary structure?

• Less DNA required, less synthesis

• Errors in translation are less critical

• May be able to recruit the same function in different contexts- same subunit in different contexts (proteins)- PDH and a-ketoglytarate

Global vs local symmetry

Global- whole protein

Local- some portions contain symmetry

Pseudosymmetry

Non-identical homologous subunits are related by symmetry

Cyclic symmetry

Cn where N is the number of protomers arranged around the rotational axis, fold of axis =N

1 rotation axis

Dihedral symmetry

D(N)- 2N protomers arranged around 2 axes (2xCn), one axis is N fold, the other is two fold, axis are perpendicular

pies on top of each other

eg. Two layers of 3 protomers, D3, 3fold axis in middle, 2fold axis in between layers

Tetrahedral symmetry

12 protomers arranged around multiple axis

Closed roughly spherical

Each face has 2 fold axis, where edges meet is 2 fold

Octahedral (cubic) symmetry

24 protomers arranged around multiple axis

Closed roughly spherical

Faces are 4 fold, where edges meet=2, vertex 9corners meet)=3

Can have 4 protomers per face and 6 faces or 3 and 8- then vertex is 4 and face is 3

Icosahedral symmetry

60 protomers arranged around multiple axis

Closed roughly spherical

3 per face, 20 faces

Edge=2, face=3, vertex =5

Helical symmetry

Related by rotational and translational symmetry

Not closed system- end of top of previous layer

Tobacco mosaic virus, microfilament, spiral staircase

Can you have symmetry with one subunit

No global symmetry but could have local