TUTORIAL ON PEPTIDE AND PROTEIN STRUCTURE

IV. QUATERNARY STRUCTURE

                                                Copyright: J. E. Wampler, 1996

Quaternary Structure is the combination of two or more chains, to form a complete unit. The interactions between the chains are not different from those in tertiary structure, but are distinquished only by being interchain rather than intrachain.




Some proteins are composed of identical subunits (chains). A simple example is the dimer of HIV Protease.




Some proteins are composed of non-identical subunits (chains). A simple example is insulin which is made up of two chains, the alpha chain and the beta chain, linked by two disulfide bridges.

Even with identical subunits, their relative positioning controls the symmetry of the complex and can have biochemical implications. Consider two right hands, nearly identical to each other but completely assymetric.



They can be put together to form a "dimer" in many different ways. In some complexes, both "subunits" touch the other one in exactly the same way (Panels A,B,C,E and F) forming an interface with matching surfaces. The result is referred to as an "ISOLOGOUS" association.

In others (Panels (D and G) they each experience a different contact referred to as a "HETEROLOGOUS" association. In these cases, even though they are identical in shape, they are not identical in environment.

These hands also illustrate some other variations seen in protein complexes:

Some complexes involve compact subunits that interact along well defined surfaces (eg. Panels A, E & G).

Some complexes involve more intimate and complex interactions and in a few cases protein chains are interwoven (eg. Panel F).

Some arrangements are ideal for further polymerization. We can imagine as stack of "subunits" of any number built on the basis of the heterologous interactions in panels D and G, or a stack of an even number of subunits based on isologous interactions like say those of Panel C.

With more subunits in the complex, we can imagine some emergent properties that can have biochemical implications. For example, with three identical subunits arranged linearly, the middle subunit experiences a very different environment from the other two.
The possibilities grow with the number of subunits.


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Examples:

The four subunit arrangement of rabbit muscle glycogen phosphorylase is nearly planar (actually, the top two subunits are arranged at a slight angle to the bottom two). Each pair of subunits is a functional unit in this case.

The four subunit arrangement of D-Xylose Isomerase is a more complex, interwoven arrangement.

Bovine hemoglobin is composed of two pairs of non-identical subunits, alpha and beta. Each alpha-beta pair is more closely associated than they are with each other, but the overall arrangement is roughly tetrahedral.

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