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I've always simply assumed quaternary structure to be characterized by non-covalent interactions such as hydrogen bonding, van der Waals interactions and whatnot. However, if two distinct polypeptides were only connected by one covalent disulfide bridge, would this be considered as quaternary structure, assuming that non-covalent interactions between the subunits are either negligible or even repulsive?

In other words, can a disulfide bridge, on its own, convey quaternary structure?

On a side note, are there any notable examples of this type of interaction?

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I don't know of any examples of this but I would say no doubt, that's quarternary structure. Quarternary isn't so much defined by the kind of interaction but much more the fact that it's between different polypeptides; all lower-level structures are within one polypeptide. (Wikipedia agrees.)

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I've always just gone with the name hetereodimer but there isn't any technical reason why it isn't a quaternary structure. As for an example, I work with antibody fragments or FAbs and cys-diabodies which are exactly that, two distinct polypeptides that are connected by a disulfide bond linkage.

These do have a significant amount of non-covalent interactions. For something with less, maybe ricin?

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